The Life of Isambard Kingdom Brunel

Appendix II

(See Chapter IX. on the ‘Great Britain’ Steam-Ship, p. 254.)

Report to the Directors of the Great Western Steam-Ship Company.

October 1840.

Gentlemen,—I have now the pleasure to lay before you the result of the different experiments which I have made, and of the best consideration I have been enabled to give to the subject of the screw propeller.

The observations which I have to make are naturally divided under two principal heads, namely: first, the simple question of the applicability and efficiency of the screw considered merely as a means of propelling a vessel, compared with the ordinary paddlewheel; and, secondly, the general advantages or disadvantages attending its use.

The consideration of the comparative efficiency of the screw as a means of propelling, of course embraces the whole question, not merely of the effect produced, but also that of the proportionate power absorbed in producing that effect.

With respect to the mere effect of a screw, the performance of the ‘Archimedes’ has proved, in a satisfactory and undeniable manner, that a screw acting against the water with a surface even much smaller than that offered by the paddle-boards of a well-proportioned paddlewheel, will propel the ship at a very fair speed, but at what expense of power this effect has been produced is not so evident.

I shall first examine into the principal cause of what amounts practically to a loss of power, and which is common in a greater or less degree to all modes of propelling a vessel by exerting a pressure against the water as against a fixed point.

The resistance, whether to the surface of a screw, or of a paddle-board, or of the blade of an oar, or any other propelling body, offered by the fluid against which it acts, is of course not perfect, and there is a certain amount of yielding, commonly called the slip, of the paddlewheel; the amount thus slipped causes a considerable waste of power, inasmuch as the full power of the engine is expended through the entire space passed over by the paddles or other propelling surface, while the useful effect produced is only equal to the same power expended over the space through which the vessel passes: this loss frequently amounts to one-quarter, and even one-third, of the whole power employed. To investigate theoretically the amount of slip due to any given form and quantity of surface, involves much more complicated calculations than have generally been applied, and would indeed require data which we hardly possess; but fortunately we have had the means of making experiments, the results of which enable us to determine the comparative slip of the paddle and of the screw, with sufficient accuracy for all practical purposes.

The screw in use on board the ‘Archimedes’ is 5 feet 9 inches diameter, with a pitch of 8 feet—that is to say, in making one revolution the thread of the screw advances 8 feet; the area of the screw, considered as a disc of the same diameter, or the extent of the surface of water which is acted upon in the direction of the axis of the vessel, is therefore about 26 feet, without deducting the section of the shaft-bearing, &c. The midship section of the vessel when I experimented upon her was, according to Mr. Patterson’s estimate, 122 feet; the ratio of the resisting surface to the midship section being therefore as 1 to 4·7, which is a small proportion; and the form of the vessel is by no means peculiarly good as a steamboat. This proportion of propelling surface to midship section is much smaller—that is, the area of the screw is much less in proportion to the size of the vessel than is the area of paddle-boards immersed in steamboats generally.

The average paddle-board immersed and really effective is rather difficult to estimate, as allowances must be made for the disturbance of the water, when the wheel is in motion; but this average in the ‘Great Western’ measured perpendicularly—that is, allowing for the obliquity of the paddle—cannot be less than 180 to 200 feet, say only 180, while the midship section averages about 462 feet; the surface of paddle is therefore about 1/2·56 of the midship section.

I will now give the comparative effects of these different propelling surfaces in these two cases.

I have made very accurate experiments upon the comparative rate of the ‘Archimedes,’ and of the space passed through by the screw, and was enabled to determine this ratio with great certainty.

The average of a number of trials gave the following results:

Rate of ship, 50,867 feet per hour, or about 8⅓ knots.

Space passed through by screw due to the number of revolutions, 65,685 feet.

The average rate of vessel being to that of screw therefore as 1 to 1·2913.

In the performances of the ‘Great Western,’ upon an average of 20 voyages the ratio has been as 1 to 1·2997; but, separating from these 20 such voyages as were unusually short or long, and taking only such as, occupying 14, 15, or 16 days, may be considered as giving a fair average of the speed of the ship when not adversely affected by the wind or heavy seas, the average of these 13 voyages give 1 to 1·283; and leaving out again those of 16 days, and taking only 8 voyages of 14 and 15 days, the average gives a ratio of 1 to 1·27187.

Of these, 5 voyages of 15 days give 1 to 1·29077,
and 3 " 14 " 1 to 1·23901.

The last three, however, were short passages and homeward, when the currents and winds have been in favour, and consequently we may safely say that the ratio must be above 1 to 1·239; and after making every allowance for the effect of swell and other impediments (the experiments upon the ‘Archimedes’ being made in smooth water), the average of the 8 (5 of which were homeward voyages with favourable current and wind and the vessel in good trim), giving a ratio of 1 to 1·27, may be taken as a fair average.

The comparison between the ‘Archimedes’ and the ‘Great Western’ will therefore stand thus—

	Area of Propelling Surface, the Midship Section being 1·0.	Difference of Speed of Vessel and Propelling Surface, or amount of Slip, the ratio of Vessel being 1·0.
‘Archimedes,’ screw	0·203	0·2913
‘Great Western,’ paddle	0·391	0·2708

Showing an amount of slip in the ‘Great Western’ very nearly equal to that of the ‘Archimedes,’ while the ratio of the propelling surface to the midship section in the case of the screw is little more than half that of the paddle-boards in the ‘Great Western.’

In taking the average of the eight voyages of the ‘Great Western’ with favourable winds as I have done, I believe I have made full allowance for the different circumstances of smooth water and sea; but there is ample room in the above comparison to make even greater allowance for these circumstances, and still to leave a result which would prove that with similar areas the screw would meet with at least equal, if not a greater resistance, and consequently will slip as little or less than the ordinary paddle-board.

I subjoin a table also, taken from a well-known work on the steam-engine (Tredgold’s), of the slip of a number of vessels, of which in every case the surface of paddle immersed is far greater in proportion to the midship section than that of the screw in the ‘Archimedes.’

Rate of Paddle, that of Ship being 1.

Medea	1·595
Flamer	1·483
Firebrand	1·501
Columbine	1·529
Salamander	2001·200
Dee	1·366
Firefly	1·364
Firebrand, as altered	1·295
Phito	1·215
Monarch	1·323
Magnet	1·310
Meteor	1·490
Carron	1·287
Average	1·381
Great Western	1·27
Archimedes	1·29

This list shows that the result in the ‘Great Western,’ with which ship I have made the comparison, is in itself a favourable one, and that compared with many others the ‘Archimedes’ would stand much better.

This apparent superiority of the screw over the paddle as regards the resistance offered to it by the water may at first appear startling, but there is a great mistake committed in assuming that the action of the screw is a very oblique action, tending rather to drive the water laterally with a rotatory motion than to push it steadily backwards.

Having witnessed and carefully observed the degree and the nature of the disturbance in the water caused by the screw, and comparing this with the violent displacement of the water by the action of paddle-boards, even under the most favourable circumstances, I no longer feel surprised.

The mass of water pushed backwards by the action of the screw appears to be very large, spreading from the screw probably in the form of an inverted cone, but there is little or no appearance of any rotatory motion, and the surface of the water is not put into rapid motion as in the case of the paddlewheel, which may be observed to impart a considerable velocity to the water, probably for a small depth only, but over a very large space.

As regards the oblique action also, a great mistake appears to have been generally made, and very naturally made, by most persons when first considering the working of the screw. It is generally assumed that the inclined plane formed by the thread of the screw strikes the particles of water at that angle and with the velocity of the revolution of the screw, but it is forgotten that the screw is moving forward with the ship, and therefore that the angle at which the water is struck by the plane is diminished by all that much that the ship with the screw advances—indeed, it is evident that if the ship advanced the whole amount of the pitch of the screw, the screw, oblique as it appears, and rapidly as it revolves, would not strike the water at all, but simply glide through.

The angle at which any given part of the screw does in fact strike the water is only equal to the difference between the angle to which that part of the screw is formed and the angle or direction in which it moves by the compound motion of the revolution of the screw and of the forward motion of the ship and screw; and, contrary to one’s hastily imbibed notions of the action of the screw, this angle at which the plane of the screw is driven against the particles of water, is in such a screw as that of the ‘Archimedes’ very nearly equal over the greater portion of the surface, diminishing to nothing at the centre; and the motion imparted to the water, although perpendicular to the plane of the screw in point of direction, is small in extent or velocity, being also nearly the same over the whole surface of the screw, except close to the centre, where it is infinitely small.

In the ‘Archimedes’ screw, which appears to the eye so oblique, and the centre part of which would appear to act flat against the water, only causing it to revolve, the outer circumference being 18 feet and the slip 1 foot 8 inches, the angle at which this outer edge acts upon the water is only one in 11½.

The total amount of motion imparted to the water at right angles to the plane of the screw by one entire revolution even at the outer edge is not quite equal to the slip, being only 1·67 foot. The rotatory motion is still less, the total distance to which any particle of water is displaced laterally, or at right angles to the axis of the ship, by one entire revolution of the screw being at the outer edge only 0·69 foot, and the maximum distance being in any screw only equal to half the slip, and occurring at that part of the screw where the circumference is equal to the advance of the ship due to one revolution. This maximum of lateral motion is 0·9 foot, and takes place at 0·99 foot, or about 1 foot from the centre. In this mode of considering the direction at which the particles of water are acted upon by the plate of the screw I have taken no notice of the effect of the friction upon the surface of the screw, which, causing to be carried with it a film of water, will modify more or less according to the degree of smoothness of the surface the effect of the screw upon the water; and towards the centre this friction, however smooth the surface may be made, will gradually become equal to, and at last greater than, the propelling effect of that part of the screw; but this defect applies only to a very small portion of the whole area of the screw, and the absence of any very violent impulse to the water in a direction approaching to a right angle with the axis of the vessel, and which has always been assumed as an unavoidable evil in the screw, will account for the absence I have observed upon of any apparent rotatory motion.

I would not pretend, however, to advance these circumstances which I have observed, or these reasonings, as arguments whereon to found an opinion of the action of the screw, the facts as proved by the experiments are what I rely upon; but it is satisfactory to be able to account for the results by circumstances actually observed, and the reasons which suggest themselves.

The effect of a propelling surface in the form of a screw, and moving at a certain velocity, as compared with an equal surface moving at the same velocity but applied in the shape of paddle-boards, having been ascertained, it remains to determine the comparative power required to give motion to that surface.

The difficulty of determining this with any degree of accuracy from any experiments which we could make on board the ‘Archimedes’ was very great, but considering such results as I could obtain in conjunction with experiments which I have since made in our own works, and with the results upon steamboats recorded by others, and of those of experiments made by Colonel Beaufoy on the resistance of bodies in water, I think we may arrive at approximate conclusions sufficiently accurate for our purpose, and which may safely be relied upon.

In the case of the ‘Archimedes’ the engines were certainly not effective well-working engines, the proportions of the gearing or wheel-work between the engine and the screw was bad—such that the engine could not attain its proper speed—the friction of the gearing (which, whether it be a source of resistance necessarily attending the use of the screw or not, I shall consider afterwards) was very great, and the surface of the screw itself, which I had an opportunity of examining out of water, was so rough as necessarily to create very much more friction than would be caused by a tolerably smooth metallic surface. With all these sources of resistance, and under these unfavourable circumstances, the power calculated for the effective pressure on the piston and without deduction for friction or other causes, which, for the sake of distinction hereafter, I shall call the gross power, was about 145 horses, the speed of the vessel being about 8⅓ knots per hour, as actually measured by the land, and full 9 knots as measured with great care by heaving the common log, the midship section being, as before stated, 122 feet, and the lines of the vessel not so good as those of fast boats; comparing this with the gross power of the ‘Great Western’ engines when propelling that vessel at the same velocity, with the advantage of better lines and the other advantages arising from greater dimensions, there does not appear any such discrepancy as to indicate any loss of power by the use of the screw in the ‘Archimedes’; on the contrary, the power expended in the ‘Great Western’ is actually as great as that in the ‘Archimedes,’ as compared with their relative midship sections—and if any great allowance is to be made for the circumstances which I have referred to of larger dimensions and better lines, there would appear to be actually less power expended in proportion to the dimensions and form of the ‘Archimedes’ than in the ‘Great Western.’

The results obtained with the ‘Great Western,’ which as regards speed are similar to those of the ‘Archimedes,’ are necessarily taken from experiments made when she was rather deep, and the speed thereby reduced to 7·9 knots; but I have compared these with results reduced by calculations from experiments at higher speeds, and I find them agree satisfactorily—indeed, at the draft and consequent immersion of paddles when in this state, I consider the ‘Great Western’ as very nearly at her best as regards economy of power and effect produced. I should observe that the particular experiments from which the following calculations are deduced were made with the ‘Great Western’ in smooth water in the Severn. I have added also some calculations deduced from data given by Tredgold as to the performance of the ‘Ruby,’ a good boat with immense surface of paddle-board.

The comparison stands thus:

	Great Western	Archimedes	Ruby
Actual Dimensions:
Midship section	520	122	63
Area of board immersed	230	—	64
Area of a disc of diameter of screw	—	26	—
Relative DIMENSIONS AND Power:
Area of propelling surface, midship section being = 1	0·442	0·213	1·016
Gross power expended for one square foot of midship section	1·023	1·026	0·976

The speed being the same, viz., 7·9 knots, the power expended is as nearly as possible the same in the three, and equal to one horse-power gross to one foot of midship section; while the relative propelling surface in the ‘Archimedes’ is equal to only half that of the ‘Great Western,’ and one-fifth that of the ‘Ruby.’ This gross horse-power, it will be observed, is about equal to one-half a nominal horse-power.

I have made several comparisons with recorded observations made on board the ‘Great Western’ at different times, and with experiments made in other vessels, and I find the same result; in estimating the powers used more particularly in some comparisons with the ‘Great Western,’ I have taken the mean pressure as ascertained on both sides of the piston, while in the ‘Archimedes’ I only obtained that on the top of the piston, which appears generally to be the best, and consequently the estimate is made unfavourably to the ‘Archimedes.’

Such general results are all that I could obtain from the experiments on board the ‘Archimedes,’ but since that time I have made some experiments upon the friction of a plate of metal in water, and have compared these results with the experiments of Colonel Beaufoy, and the conclusion I have come to is that the power absorbed by friction in a well-made screw, apart from all question of the means adopted for working it, would not be such as to interfere with its beneficial application.

The resistance created by the screw itself arises principally from two sources—the resistance to the cutting edge and the tail-edge, and the friction of the surface in contact with the water. The amount of the first may of course be reduced to an unlimited extent by having a fine edge, and practically such edge ought to be much finer than that of the screw of the ‘Archimedes.’

The friction upon the surface will of course materially depend upon the smoothness of that surface, and in the ‘Archimedes’ it was very rough, the iron being corroded at many places, with exfoliations and small holes—the corrosion arising apparently from the galvanic effect produced by the iron and the ship’s copper.

The great number of revolutions required in the screw as compared with those of the paddlewheel, leads a person to assume, without much consideration, that a very high velocity is given to the cutting edges and to the surface of the screw, and consequently that great friction must be produced—this velocity is not, however, nearly so great as it at first appears.

In the present screw of the ‘Archimedes’ the velocity of the extreme point, following its oblique or spiral course, is only about three times that of the vessel, while the average velocity of either of these knife edges or of the surface is not twice that of the vessel.

Now without determining what the actual amount of these resistances may be, we can at once satisfy ourselves that it cannot be very considerable, by comparing it (which we have the means of doing) with the resistance caused by the cutwater and any given portion of the ship’s bottom. The resistance of a knife-edge will be about as the square of the velocity, and if we assume the surface friction to increase in the ratio determined by Colonel Beaufoy—namely, at the 1·75th power, or as the 4th root of the 7th power of the velocity—then the resistance of the knife-edge will be equal to the resistance of a similar edge of about five and a-half times the length of the diameter of the screw moving at the same rate as the vessel, and the surface friction will be equal to that of a piece of the ship’s bottom about six and five-eighth times the area of the screw—or, in the case of the ‘Archimedes,’ the additional power absorbed by the friction of the screw would be about equal to that absorbed by the friction of little more than twice the space of the dead wood which had been cut out to receive the screw—while the knife-edges would be about equivalent to three knife-edges immersed in the water, of the same depth as the ship’s stem.

The actual amount of power absorbed in driving the ‘Archimedes’ screw was probably about twenty horse-power gross, or from ten to twelve nominal horse-power; but I have no doubt that a screw of similar diameter and in good condition would not absorb half that power: and this amount may be still further, and very much reduced, by increasing the relative size of the screw to that of the ship, and thereby reducing the slip, and proportionately reducing the number of revolutions required.

The great extent to which this is capable of being carried will at once be seen when I state that if the ship’s progress were made to be 7 feet instead of 6 feet to each revolution of the screw, which a very slight increase of diameter and pitch of screw would effect, the power absorbed in driving the screw would be diminished in the ratio of the 6²⁴ √6⁷ to 7²⁴ √7⁷—that is, as 6¹⁵/₄ to 7¹⁵/₄, or about as 3 to 2.

I must repeat here the observation I have previously made, and remind you that these calculations are not introduced as proving, but merely as explaining, that which appears to me proved by the general results of the experiments on the ‘Archimedes’—namely, that the effect produced was, considering all the circumstances, fully proportionate to the power expended, while the experiments and calculations which I have since made also satisfy me that these results may be very much improved upon.

As regards the first of the two heads under which I stated that I proposed to consider the subject, namely, the mere efficiency of the screw as a propeller, I think but one conclusion can be drawn from the results of the experiments quoted, and that is, that as compared with the ordinary paddlewheel of sea-going steamers, the screw is, both as regards the effect produced, and the proportionate power required to obtain that effect, an efficient propeller.

I limit the comparison to the ordinary paddlewheels of sea-going steamers, first, because those are the circumstances which we have alone to consider; and, secondly, because it is possible, by increasing the diameter and breadth of the paddles, which, for the attainment of an adequate object is practicable to any extent in a mere river boat, to render the action of the common paddle all but perfect, and probably more effective than any other propeller.

In considering the advantages and disadvantages likely to attend the use of the screw propeller, I will, commencing with the latter, consider such objections as have been advanced by others, as well as those which may have occurred to myself.

The only objections, however, which I think worth consideration are:—

First. The necessity of a peculiar form of vessel.

Secondly. The situation of the screw under water, and consequently to a certain extent unseen and inaccessible, and the liability to injury from its position from grounding or in other ways.

Thirdly. The probability of its being lifted out of water when the ship pitched deep.

Fourthly. The difficulty of getting up the required number of revolutions, and the great defects of the mode employed in the ‘Archimedes,’ and the shaking caused by the machinery.

As regards the form of vessel, undoubtedly a shallow boat, intended for shallow waters, would be very unfit for the application of the screw, which would probably require a greater depth of water than the whole draft of the vessel; but I see no defect or difficulty of this description in the vessel now under consideration, nor can I anticipate any in any vessel this Company is likely to be interested in; a clean run is the most essential condition, and I should suppose no ship was ever built in which this principle of form was carried to a greater extent than in our new iron ship. Her present form I believe to be excellent for the screw, and with a very slight dropping of the keel towards the stern, which can easily be done now without any expense, assisted by the different trim, which, as I shall presently show, will be effected by the use of the screw, the required draft of water will be attained.

It may, perhaps, be as well to mention here, that the diameter of the screw, if in the same ratio to the midship section as in the ‘Archimedes,’ would be only 12 feet 3 inches, my friend Captain Claxton having made a mistake upon this point in calculating it at 16 feet, and that if increased only to 14 feet 4 inches, the diminution referred to in a former part of my Report of one-third in the power lost in working the screw would be effected; considering the speed we wish to attain, probably 15 feet 6 inches would be a good diameter. Upon the whole I think the vessel is as well fitted for a screw as she is for paddles, and much better adapted for either than the ‘Archimedes;’ but if originally intended for a screw, possibly some trifling modification in the form and construction, principally of the keel near the stern, might have been introduced which would have rendered the whole a more perfect job than she would now be if altered—but the absence of this would in no way lessen the efficiency of the screw, and I cannot think that any alteration we might now be obliged to make would exceed in cost the sum of 200l.

Secondly, the inaccessibility of the screw and liability to damage; this appears to me the objection most plausible, but I cannot say that I attach much weight to it, particularly in the case of a vessel intended for long voyages and across the ocean. During the whole passage in deep water I consider the screw far less exposed to injury than a paddlewheel, and that the chances of injury are so remote that even if it were quite inaccessible it would still be altogether safer than paddles, which are so much exposed; but it is by no means inaccessible, the screw may be rendered stationary at any time or during any weather, when it would be barely safe to stop the engines with common paddles, and when it would be very difficult to do anything to the paddles even if the engines were stopped, while the whole of the screw, bearings, &c., may easily be examined and felt from above, and, if necessary, men sent down with common diving jackets and hoods to replace bearings, or attach tackle to move the screw, or clear away any obstacle entangled in it. When in port I still think the chances of injury very remote; an inspection of our model will satisfy you that from the form and size of her midship section the vessel cannot lay in any position in which the screw would touch the ground, while at that time the whole screw may be very easily examined and replaced without any necessity for going into dock.

Thirdly, the probability of its being lifted out of water when the ship pitches deep.

This appears at first to be a very natural and an unavoidable objection, but the result of observations proves that the motion of vessels, of steamers at least, is not such as to cause the apprehended difficulty. Among the observations made on board the ‘Great Western’ steam-ship by Mr. Berkeley Claxton, under my direction, were measurements of the angles of rolling and pitching, and from these it was evident that the vessel never pitches to so great an angle as that to which she rises; such a result might indeed have been anticipated by considering the form of the vessel forward and aft, and the circumstance that a steamer is almost invariably meeting or passing the seas, or, if overtaken by them, is still going at a good rate, which reduces the relative speed of the sea; consequently, although the vessel may be frequently thrown up very violently forward, yet the stern, which has no displacement under water, settles down quietly and heavily upon the surface; or, considering it in another way, the variation of displacement at the stern is very rapid, falling off almost to nothing at a few feet below the water-line, and spreading out to a great extent at a few feet above, whilst forward the difference of displacement is comparatively small, the centre of motion, therefore, is thrown very far aft, and while the bows, which are also opposed to the first shock, are thrown alternately high out of water or plunged deeply into it, the stern floats nearly steady, the vessel resting on its broad counter nearly as the centre of motion: whatever may be the explanation such is the operation, not only as measured by instruments, but more particularly as observed since, practically.

In the ‘Great Western’ the whole cutwater and, it is said, a considerable length of keel, is frequently seen out of water from the bowsprit, while astern it is very doubtful whether more than half the stern part was ever seen; marks have been made by my direction on the rudder to observe this; as yet the 9-foot mark is the lowest seen, and this occurring rarely, and for very short intervals.

In the ‘Archimedes,’ during a voyage performed in her by Mr. Guppy from Bristol to Liverpool, and during which they were exposed on more than one occasion to violent pitching, the screw (which can be watched from the deck) never was uncovered; and Mr. Smith and others on board the ‘Archimedes,’ whose whole conduct was such as to inspire unusual confidence in all information obtained from them, assured me that such was always the case.

In the voyage to Oporto and back, in which I sent Mr. Berkeley Claxton, he made the same observation; these facts, in conjunction with previous and subsequent observations on board the ‘Great Western,’ convince me that nothing is to be feared on this head; but even if the screw were occasionally to be partly exposed, I know of no evil consequences likely to ensue, as I shall clearly point out when referring to the advantages of this propeller over the common paddle.

Fourthly, the difficulty of getting up the required number of revolutions, and the great defects of the mode employed in the ‘Archimedes.’

Upon this point certainly the ‘Archimedes’ offers but a miserable example, and the result is almost enough to prejudice the mind of any person against the whole scheme; the proportions of the gearing, as I have before stated, are so bad that the engines appear, even to the eye, to labour ineffectually to get up their speed. The required speed of the screw is not nearly attained, while the noise and tremor caused by the machinery is such as to render the vessel uninhabitable, and perfectly unfit for passengers, I should almost say for a crew. I never attached much importance to these circumstances, because I felt convinced that such a mere mechanical difficulty would by some means be overcome, if, as I confess I did not then at all anticipate, the screw itself should prove efficient.

The most simple and effectual means of overcoming all objections on these heads always appeared to me to be by the use of straps instead of gearing; and all my experience, and I have seen a great deal of the working of machinery by straps and ropes in the numerous works executed by my father, led me to the conclusion that there existed no difficulty whatever in sending the necessary power through a rope or hemp strap, but I was hardly prepared to find the result so entirely satisfactory as it has proved to be.

In an experiment made in your works at the yard, I have sent through two small whale lines, a power equal to about one-thirtieth of that which would be required in the strap if used in the new ship, and this without any slip or straining of the rope which would be injurious in practice, and without any peculiar means of ensuring adhesion to the drums; so that we have ascertained beyond doubt that sixty such whale lines upon a drum of only 4 feet 3 inches diameter is adequate to our wants, but if we suppose seventy lines of superior manufacture to that used in the experiments with a perfect mode of tightening and working upon a drum of 6 feet diameter, all of which can easily be had, it will ensure the perfect and easy working of a mode of obtaining the required number of revolutions of the screw without noise or tremor. The strap in question would be only about 3 feet or 3 feet 3 inches broad, easily replaced piecemeal, and even, if necessary, without stopping the engines.

All the difficulties enumerated under the fifth head may be considered as entirely overcome, or rather as ceasing to exist; and so far from the working of the screw involving difficulties and unavoidable friction, noise, or tremor, it may be worked with unquestionable and perfect facility, and as compared even with the best-made paddles in smooth water, the whole machine will be noiseless.

It is almost unnecessary that I should say that the screw, apart from the gearing in use on board the ‘Archimedes,’ cannot and does not produce the slightest tremor or noise—it was with some difficulty, and at least only by attentively listening, that the revolutions of the screw could be counted, even when disconnected and free from the noise of the engine or gearing and the vessel being towed, and then only from some defect in the bearings or the shaft of the screw causing a slight beat.

In thus answering the objections supposed to have been urged against the use of the screw, I may probably have appeared to see everything in a favourable light; unhesitatingly I admit that it is so, and that both formerly when I was completely sceptical as to the mere efficiency of the screw as a propeller, and since my doubts on that head have been removed, I always felt that upon all other points the screw possessed every superiority that could be desired over a common paddlewheel for a sea-going vessel.

I shall now proceed to point out the principal advantages peculiar to the use of the screw; they are—

First. A considerable saving of weight, and that principally top weight.

Secondly. The admitting of a better and simpler form of vessel, having greater stiffness with the same quantity of material, and offering less resistance to head wind and seas, and affording more available space within.

Thirdly. The operation of the screw being unaffected by the trim or the rolling of the vessel, and allowing of the free use of sails, with the capability of entirely disconnecting the screw or of varying the multiplying motion so as to adapt the power of the engine to the circumstance either of strong adverse winds or scudding.

Fourthly. Perfect regularity of motion and freedom from the possibility of violent shocks to the engines.

Fifthly. The singularly increased power of steering given to the vessel—and Sixthly. The great reduction in the breadth of beam.

I have gone into some detail in calculating the weights of the parts which are not common to the two systems, and I find that the difference, or actual diminution in weight in favour of the screw as applied to our new ship, is upwards of ninety-five tons; but that a much greater weight even than this is transposed from the top of the ship to the bottom—no less a mass than one hundred and sixty tons is removed from the level of the paddle-shaft or from about 10 feet above the water-line, and replaced by sixty-five tons at about 7 feet below the water-line; not only is buoyancy, and consequently proportionate space for cargo, gained to the extent of the difference, but the relief to the labouring of the vessel in bad weather from the change of position must be immense. If the reverse were under consideration, if in a vessel fitted for sea, however stiff in trim or form, it were suggested to remove sixty-five tons of her ballast, and to place one hundred and sixty tons upon her deck, and thus navigate her across the Atlantic in all weathers, it would probably be considered, not merely as highly dangerous, but as actually impossible. Although such an opinion as that it would be impracticable we now know would be incorrect, yet the extent of the beneficial change is much more striking when considered in this way. As regards the trim of the ship, about one hundred and forty-five tons would be removed from nearly the centre of flotation, and the balance of fifty tons added and distributed over the after part, principally quite aft.

I have not calculated the exact effect of this upon her trim; it would only bring her down by the stern, and this is a defect which there seems, as we too well know, never any difficulty in remedying.

Secondly. The simplifying and improving the form of the ship—both as regards strength and mass exposed to the wind and sea.

The necessity of contracting the midships of a steamer, and making her completely wall-sided, and forming a sort of recess to receive the paddles, interferes considerably with the framing of the ship. In a wooden ship of the size of our new one the whole beam of the ship would have to be contracted in order to carry the planking through in a direct line and obtain the requisite fore and aft tie as has been done in the ‘Great Western’; in the new ship the almost infinite resources afforded by the material used, enabled us to expand the sides and obtain breadth of beam for cabin room, both before and abaft the paddles, and contract the sides at the paddles as seen upon the plan; but in order to strengthen this part, so evidently weak by form, much contrivance and much material was required. By dispensing with paddles, the best form of ship is left free to be adopted; perfect lines may be preserved, more equal strength obtained with increased space, and the whole mass of paddle-boxes and their accompanying sponsons and deck-houses swept away, and the resistance of these huge wings to head winds or seas entirely avoided.

The space gained by avoiding the contraction is calculated by Mr. Patterson to amount to two hundred tons measurement; this would be entirely gained, and would not even involve increased dues or tolls, as it would be added to the engine-room; it would therefore perhaps counterbalance any loss of room caused by the shaft conveying the movement from the engine to the screw, but I believe this nominal increase at one part would not be so great, while in fact the ship would really be more compact, and, though to a very small extent, a smaller ship, as the sponsons would be removed.

The third point of advantage named is perhaps the most important. With paddles, the action is materially affected by the depth of immersion; when the vessel is deep, and consequently the paddles deep, their action is impeded, a greater part of the power of the engine is absorbed in driving the paddle, the speed of the engine is reduced and the effect diminished; when too light also the paddles do not take sufficient hold of the water, the amount of slip increases and power is wasted; in rolling the same effects are produced, and thus at those times when the greatest effect is required, namely, with deep immersion or in bad weather to overcome the increased resistance offered to the vessel, the propelling power is least effective, and Captain Hoskins actually estimates this loss as occasionally equal to two-thirds the whole power.

The bad effects of one paddle being immersed too deeply, and the other not sufficiently, also prevents the free use of the sails; and it must often occur that the impediment thus offered to the working of the paddles more than counterbalances the good effects of a tolerably fair wind. With the screw the effect is constant, at least unaffected by the position or motion of the ship, whether deep or light the screw acts nearly the same, and as to rolling or heeling over, the screw would work equally well (as long as it be immersed) if the vessel were on her beam-ends or bottom upwards.

The screw therefore leaves the ship free to be used as a sailing-vessel to any extent that other circumstances will admit of, and as long as the sails draw there can generally be no doubt that the wind is assisting the ship. The screw may also be thrown in and out of gear at any time and during any weather, either in case of accident to the engines, or in the event of her scudding before a gale of wind, when the engine would be useless; this last, however, I do not consider a probable occurrence, particularly if another arrangement of which the screw is susceptible be taken advantage of.

If the motion be conveyed by a strap, as I have recommended, there is no difficulty in having two or even three drums on the screw-shaft of different diameters, and thus when the resistance to the ship is very much increased by strong head winds, deep draught, and other causes, to use the slow motion and obtain an increased propelling force, or when, on the contrary, the vessel is running before the wind to use the quick motion—by which, in both cases, a great increase of speed would be attained.

This is in fact obtaining at once and by simple means all those advantages, and to a much fuller extent, which are aimed at in the reefing-paddles.

Fourthly, great regularity of motion is naturally consequent upon the screw being unaffected by the rolling of the ship, and upon its being immersed and not exposed therefore to blows from the sea, and except in the case of its being lifted out of water, the resistance is perfectly uniform and perfectly smooth.

An engine could not have a work less capable of causing any jar or shock as to the effect; even if lifted partially out of water the variation of resistance would be as easy or soft, to use a mechanical term, as possible, while the extent of the variation could never approach to that to which paddles continually expose an engine. A heavy sea or a deep plunge will occasionally bring the engines nearly to a stand; while at other moments, if the engineers are to be believed, the paddles are left free and the engines run away at a fearful speed. I am inclined to think this description of the effects somewhat exaggerated; but certainly the screw cannot by possibility be exposed to the same variations as the paddles—it cannot be stopped by the action of the sea, indeed, being wholly immersed, the resistance cannot be increased at all, while under no circumstances can it be relieved to the extent to which paddles are, which may both on some rare occasions be quite out of the water; and therefore whether the resistance of the screw is so constant as I believe it to be, or not, yet as compared with that offered by paddles, it is certainly all but perfectly constant.

Fifthly, the effect upon the steerage is singular, the mass of water put into motion by the thrust of the screw is thrown directly upon the rudder, and the consequence is not only that when the ship is going at any given rate, the rudder is passing through the water at a greater rate, and consequently is more sensible, and acts more powerfully upon the ship; but even when the ship has no way, but the screw is at work, the rudder is acted upon by water moving perhaps at two or three knots per hour, and the vessel is still under command—this must be a most important power to possess in a ship, and must materially diminish many of the greatest dangers arising from a strong head wind and sea, and at the same time and under the same circumstances must increase the speed by improving the steerage.

And lastly, her diminished breadth of beam. Important as this alteration would be to any vessel, it is peculiarly so as connected with Bristol; the total breadth, including paddle-boxes, would be at least 78 feet; with the screw, and taking all the increased beam that might be convenient, it would be under 50—very nearly 30 feet of difference. One of the principal objections to her coming up the river would be removed, and the dock gates might easily be made to receive her.201

There are many other points upon which comparisons may be drawn, but I am not aware that any very important differences exist.

As regards first cost I believe there would be little difference—if any, it would be in favour of the screw; as a reduction of ninety-five tons of iron can hardly fail to cause some saving, although some portion of the substituted machinery may be more costly per ton.

As regards wear and tear I can have no doubt that some considerable saving would be effected; the paddles are a constant source of trouble and expense, and seem never to be capable of being kept in good repair; indeed, they are huge and comparatively light frameworks subjected to extraordinary and constantly repeated shocks, each arm receiving direct about 260,000 very sharp blows per voyage, independently of the more violent shocks from heavy seas, while the screw can be subjected to no such constant source of mischief.

From all that I have said it must be evident to you, gentlemen, that my opinion is strong and decided in favour of the advantage of employing the screw in the new ship; it certainly is so. I am fully aware of the responsibility I take upon myself by giving this advice, I am also fully sensible of the large amount we have at stake, and I have not forgotten the nature and tone of the observations which have on more occasions than one been so freely made by individuals upon the course we have hitherto pursued; although, and I have pleasure in referring to the fact, this course has in every instance where results have been obtained proved successful; but my conviction of the wisdom, I may almost say the necessity, of our adopting the improvement I now recommend is too strong, and I feel it is too well founded, for me to hesitate or to shrink from the responsibility.

I think I have hardly advanced an opinion which I have not supported, and in most cases preceded, by a statement of facts, leaving no doubt as to the correctness and safety of relying on these opinions; still it would be too much to hope that my mode of laying before you these facts which I have collected and the opinions I have formed could produce as strong a conviction in your minds as the consideration of them has in my own; but if you bear in mind that the actual results of the fair and full trial of the ‘Archimedes’ for several months has completely established the fact of the efficiency of the screw as a propeller; that the experiments I have made, as well as the general and apparent results of her working, have equally satisfactorily explained the fact of the power required being no greater in proportion to the effect produced than in the ‘Great Western’ steam-ship, and many other good steamboats; and that these results are satisfactorily explained by theory, you cannot fail to draw the same conclusion that I have done as to the general question of at least the equal efficiency of the screw.

As to the comparisons I have drawn between the general and what I may call the indirect advantages of the one mode of propelling over the other, they seem to me so evident that I am disposed to apologise to you for having occupied your time in pointing them out, and we have the satisfaction of knowing that they are now very generally admitted, particularly by practical men.

In conclusion, I must observe that much more detailed information and recorded results than appear on the face of this Report have been required to enable me to form correct comparisons, and to reduce to calculation and to actual figures and amounts many results observed; and that it would have been impossible for me to have given you such clear and positive facts on many most important points without the very detailed observations made and recorded by Mr. Berkeley Claxton in the several voyages of the ‘Great Western,’ and also in one on board the ‘Archimedes.’

The information obtained from these logs has been, and may still be, of the greatest importance to us in our future working, and I have much pleasure in adding that the manner in which my directions were carried out was highly creditable to Mr. Berkeley Claxton, who, I think, has conferred a great benefit on the Company by his labours. I have to express also my thanks to my friends Captain Claxton and Mr. Guppy for their assistance in the various experiments which have been made, and in working out the results.

I am, Gentlemen,
Yours very faithfully,
(Signed) I. K. Brunel.

Footnotes

1 To avoid confusion, Sir Isambard Brunel has been called throughout by that designation, the one by which he is generally known: he was knighted on March 24, 1841.
His Life has been written by Mr. Richard Beamish, F.R.S. (London, 1862.)

2 Lady Brunel survived her husband five years. Of their children, three lived to maturity, one son, Isambard Kingdom, and two daughters, Sophia, wife of the late Sir Benjamin Hawes, K.C.B., Under Secretary of State for War, and Emma, wife of the Rev. George Harrison, Rector of Sutcombe.

3 He was sent to Paris to recover his knowledge of French, which had got rather rusty at school, and also to study mathematics. He retained through life a great admiration of the method of teaching this subject which was adopted in France.
In addition to the time spent in the study of mathematics and languages, Mr. Brunel occupied himself on his holidays in examining the various engineering works going on in Paris, and he used to send his father drawings and descriptions of them.

4 Sir Isambard was also consulted upon a proposed suspension bridge over the Tamar at Saltash, where Mr. Brunel subsequently built the Royal Albert Bridge.

5 This history has been written by Mr. Beamish in his Life of Sir Isambard Brunel, pp. 202-304, and also, up to the year 1828, in the very valuable work by Mr. Henry Law, C.E., entitled ‘A Memoir of the several Operations, and the Construction, of the Thames Tunnel,’ and published by the late Mr. Weale in his Quarterly Papers on Engineering.

6 For an account of these earlier attempts see Law, pp. 3-7.

7 This expectation does not seem to have been realised, as there was never any considerable traffic through the Thames Tunnel. Perhaps, however, it would have been otherwise had the large descents for carriages and horses been constructed.

8 The results obtained by these borings were no doubt fallacious, but not to the extent which has sometimes been imagined. At a meeting of the Institution of Civil Engineers, in November 1849, Dean Buckland called attention to ‘the evils arising from the ignorance of the engineers who reported to Sir Isambard Brunel, previous to the commencement of the Thames Tunnel, that the whole of the bottom of the river at that spot was London clay.’ Whereupon Mr. Brunel rose and said, that he ‘agreed that knowledge of every kind was most desirable, and that it would be well if engineers were generally much better informed on many subjects which would be useful, and more particularly on matters connected with geology; at the same time he could not admit that they were deficient in that knowledge of the surface of the earth which was necessary for the purpose of guiding them in their work. It might be true that many members of the profession were, like himself, not perfectly well acquainted with the minute geological characteristics of the soils they had to deal with, but he thought the education and the practical experience of the profession generally rendered them well acquainted with those features and characteristics which were necessary for their guidance in the design or execution of work. He must also say a few words in defence of those persons (now nearly all dead) who made the borings in the Thames, and were stated to have made so fallacious a report previous to the commencement of the Tunnel. Now, although that statement had by constant repetition become a sort of historical fact, it was really only one of those popular fallacies which obtained too ready credence in the world. The position of the Tunnel was not determined by any report, or by the result of any borings, but with a view to establishing a communication between particular localities for encouraging the traffic which was anticipated from the facility of access to the docks, and for other local reasons, such as the general direction of the roads and streets on both shores. After the position was settled, and not until then, borings were made to ascertain what soils might be expected in that part of the river. It must be remembered that these borings were made full twenty-five years ago, when boring in the bed of a river through a depth of water of nearly thirty feet was not an ordinary occurrence. The tool then generally employed was the worm, and tubes were not even used in such cases. The borings showed the existence in that spot of something which, in the ordinary acceptation of the term, might have been inadvertently called London clay, but he had no recollection of its geological designation having ever been thought of. It was reported and shown to be a very fair clay for working in.... The errors which were made in giving the results of the borings did not, in fact, arise from ignorance, but from mechanical defects in the tools, for it was subsequently discovered that the worm frequently carried a portion of the upper tenacious clay through the softer strata beneath, and brought it up again. The tenacious clay might have been called London clay, but no value was attached to that particular designation; they cared little in engineering for its denomination, provided it was of a good tenacious quality. This mistake in terms (supposing it to have occurred) could not have had any influence on after proceedings; for, before the Tunnel was far advanced, he conducted with great care a series of borings extending across the Thames, and, as he used improved tools and worked through tubes, the holes were kept so dry that a candle was frequently lowered down to the bottom in order to see the amount of infiltration. By this means he was enabled to construct a correct section of the bed of the Thames at that spot, showing every layer of shells and gravel as well as every variation of the surface of the silt, &c. He entered more at length into these details than might perhaps appear necessary, because he felt it was incumbent upon those who had the conduct of works to show that they did not proceed so ignorantly or so recklessly as had been assumed, in the design or execution of large undertakings.’

9 The paragraphs in small type, without any reference, are from Sir Isambard’s journals. The sentences inserted at the side are his marginal summary. Occasionally a few words are added (in square brackets) by way of explanation.

10 The shaft subsequently made on the Wapping shore was sunk to its full depth without any under-pinning.

11 Professor Rankine, in his work on Civil Engineering, p. 599, describes the Thames Tunnel works under the significant heading ‘Tunnelling in Mud.’

12 Proceedings Inst. C. E. i. 34. The circumstances which led Sir Isambard to conceive the idea of a shield, and the earlier designs he made for it, are described, with illustrations, by Mr. Law, pp. 7-10.

13 Mr. Law’s memoir contains a detailed description of every part of the shield, illustrated by careful drawings.

14 Mr. Beamish had joined the works on August 7.

15 On November 20 Mr. Brunel mentions in his diary that he had ‘passed seven days out of the last ten in the Tunnel. For nine days on an average 20⅓ hours per day in the Tunnel and 3⅔ to sleep.’

16 On the previous day Mr. Brunel had been formally appointed resident engineer.

17 Mr. Gravatt had been appointed an assistant engineer six months before.

18 Sir Isambard’s journal of this eventful night consists—as he was not himself present—of Mr. Beamish’s journal, with a few words in warm commendation of that gentleman’s ‘judgment, coolness, and courage,’ followed by observations upon the stability of the shield. He then gives a statement made by Mr. Gravatt, and taken down in shorthand. No extracts are given in the text from Mr. Beamish’s narrative, as he has already inserted it in a condensed form in his Life of Sir Isambard Brunel, pp. 244-248.

19 Mr. Michael Lane, at this time foreman bricklayer, became one of Mr. Brunel’s most valued assistants, and was employed by him on the Monkwearmouth Docks and the Great Western Railway. After filling various posts in the service of that company, he was in 1860 appointed their principal engineer, an office which he held till his death, in February, 1868.

20 On this occasion an amusing incident occurred. Mr. Brunel was exceedingly unwilling to permit his visitors to make this expedition into the arch; but on the assurance that they could all swim perfectly well, he consented to take them, with the understanding that, if he jumped overboard, they were immediately to follow his example, and swim after him to the shaft. While they were in the arch Mr. Brunel (as Sir Isambard mentions) fell overboard. As soon as he recovered himself, and turned to swim back to the boat, he remembered that he had unwittingly given to his companions the signal to jump out into the water. He was much amused, on looking up, to see that they were not swimming after him, but were still sitting in the boat clinging to the gunwale, with faces expressive of blank despair.

21 Mr. Brunel’s comment in his diary is as follows:—‘Without ascribing any particular merit to myself, I cannot help observing, for my future guidance, that being alone, and giving few but clear orders, and those always to the men who were to execute them, I succeeded in an operation not altogether mean, and which a very trifling want of precaution or order might have caused to be a total failure.’

22 On January 15, 1828, the Directors of the Thames Tunnel Company passed the following resolution, which they ordered to be advertised in the Times, New Times, Herald, Ledger, and Courier:—‘That this court, having heard with great admiration of the intrepid courage and presence of mind displayed by Mr. Isambard Brunel, the company’s resident engineer, when the Thames broke into the Tunnel on the morning of the 12th instant, are desirous to give their public testimony to his calm and energetic endeavours, and to that generous principle which induced him to put his own life in more imminent hazard to save the lives of the men under his immediate care.’

23 The Thames Tunnel was not successful as a commercial undertaking; but it has always been considered, especially by foreigners, one of the most interesting sights in London, and has been visited by several millions of persons. In 1865 it was purchased by the East London Railway Company, and trains now (March, 1870) run through it. The possibility of using the Tunnel as a railway had been considered in Mr. Brunel’s lifetime, and the idea was approved of by him.

24 This description is based on the translation given by Mr. Drewry (Suspension Bridges, London, 1832, p. 75), from the Mémoire sur les Ponts Suspendus, by M. Navier (Paris, 1823, p. 49). M. Navier saw the bridges when they were erected at Sheffield in May 1823.

25 The dimensions of these designs were as follows:—

(a.) Length of floor 890 feet. Distance between points of suspension 980 " Length of chain 1,300 " With a capacity to bear excessive load of 650 tons. (b.) Length of floor 916 feet. Distance between points of suspension 1,160 " Length of chain 1,468 " With capacity to bear excessive load of 650 tons.

(a.)	Length of floor	890	feet.
	Distance between points of suspension	980	″
	Length of chain	1,300	″
	With a capacity to bear excessive load of	650	tons.
(b.)	Length of floor	916	feet.
	Distance between points of suspension	1,160	″
	Length of chain	1,468	″
	With capacity to bear excessive load of	650	tons.

26 On plate I. is given (fig. 1) a facsimile on a smaller scale of the drawing sent in by Mr. Brunel for the last-mentioned (b) of these two designs.

27 See below, p. 60.

28 See above, p. 42.

29 The dimensions proposed in this design were as follows:—

Distance between points of suspension	600	feet.
Versed sine	60	″
Width of roadway	32	″

30 A few days before this ceremony, an iron bar, 1½ inch diameter, and about 1,000 feet in length, was hung across the valley from Clifton Rocks to Leigh Down, to facilitate the works. It was traversed by a basket pulled by ropes. The first few journeys of this machine were somewhat perilous. It was intended that Mr. and Mrs. Brunel should be the first passengers; but, when all was ready, one of Mr. Brunel’s assistants started on a clandestine trial trip, and owing to a bend in the bar, the basket stuck half way, and the mast of a passing steamer caught in the rope. The rope was however cut, and he was drawn back. When the apparatus had been put to rights, on another occasion, when Mr. Brunel was in the basket, it got jammed, and he had to climb up the connecting link and get upon the bar, before he could release the basket.

31

Span	702	feet 3 inches.
Versed sine	70	″
Roadway above high-water	248	″

32 Plate I. fig. 2 (p. 49), shows an elevation of the bridge according to the designs on which it was commenced.

33 See Mr. Brunel’s remarks:—Proceedings Inst. C. E. for 1841, pp. 78, 79.

34 Rollers on an arched surface had been used previously in several bridges.

35 The chains were used in the construction of the Saltash bridge.

36 Speech of the Chairman, the late Captain Mark Huish, at the first general meeting, August 2, 1861.

37 Some re-arrangement of Mr. Brunel’s design was rendered necessary in order to adapt the Hungerford bridge chains to the Clifton bridge, and there are three chains instead of two, as in Mr. Brunel’s design. The platform is stiffened by wrought-iron girders instead of by timber trussing, and the whole bridge is stiffened transversely by the wrought-iron girders at the sides, which are connected throughout by diagonal bracing. The clear width of the bridge is 30 feet, 5 feet less than originally intended. It should be added, that no attempt has been made to complete the towers according to Mr. Brunel’s architectural designs.

38 A graphic account of this famous parliamentary contest will be found in the third volume of Mr. Smiles’ Lives of the Engineers, chapter xi.

39 See Mr. Smiles’ Life of George Stephenson, p. 325.

40 See Mr. Smiles’ Lives of the Engineers, vol. iii. chap. xv.

41 By means of the railway (it was said) goods would be conveyed with ease from London to Reading in three or four hours, and from Bath to Bristol in one hour.

42 During Mr. Stephenson’s cross-examination, several questions were put to him as to the dangerous consequences which might be expected to result from travelling through a tunnel a thousand yards long. At length he lost all patience at the ignorance displayed by the questions put to him by counsel, and the following passage of arms took place:—
‘Mr. Stephenson. I wish you had a little engineering knowledge—you would not talk to me so.
Counsel. I feel the disadvantage.
Mr. Stephenson. I am sure you must.’
In other parts of the engineering evidence there are some statements which read strangely enough at the present day, as for example the following: ‘The noise of two trains passing in a tunnel would shake the nerves of this assembly. I do not know such a noise. No passenger would be induced to go twice.’

43 At this time the Lords’ committees were open to all peers who chose to sit on them, and it was not considered indecorous for peers who had not attended any of the previous sittings to vote on the division.

44 The Great Western Railway was constructed with but few deviations from the line sanctioned in 1835. The only alteration of any importance was at the London end, where, by an Act passed in 1836, the line was taken to Paddington, instead of joining the London and Birmingham Railway near Kensal Green. This change of plan was rendered necessary by reason of a difficulty having arisen between the two companies as to the terms of their agreement, and not, as has been often stated, in consequence of the adoption of the broad gauge on the Great Western line.

45 Sir William Armstrong’s hydraulic machinery at Paddington is described by him in a communication printed in the Report of the British Association for 1854, p. 418: ‘I have also applied it [water pressure machinery] extensively to railway purposes chiefly under the direction of Mr. Brunel, who has found a multitude of cases involving lifting or traction power in which it may be made available. Most of these applications are well exemplified at the new station of the Great Western Railway Company in London, where the loading and unloading of trucks, the hoisting into warehouses, the lifting of loaded trucks from one level to another, the moving of turn-tables, and the hauling of trucks and traversing machines are all performed, or about to be so, by means of hydraulic pressure supplied by one central steam engine with connected accumulators.’

46 See p. 104.

47 No copy of this report can be found; but documents of subsequent date sufficiently indicate the nature of the arguments Mr. Brunel used in it.
Mr. Brunel had about this time given much attention to the principles of wheel carriages, as is manifested by an interesting article ‘On Draught’ written by him for the work on ‘The Horse,’ published by the Society for the Diffusion of Useful Knowledge.

48 With regard to this point, Mr. Brunel afterwards admitted that he had held a mistaken opinion. In speaking of his reasons for adopting the narrow gauge on the Taff Vale Railway in 1838, he said before the Gauge Commission:—‘One of the reasons, I remember, was one which would not influence me now; but at that time I certainly assumed that the effect of curves was such, that the radius of the curve might be measured in units of the gauge, in which I have since found myself to have been mistaken.’

49 See Mr. Brunel’s report of August 1838, printed in Appendix I. p. 528.
This plan was never adopted, as it was found desirable upon the broad gauge to use still wider carriages overhanging the wheels; but advantage was taken of the broader base to use wheels of greater diameter. However, in the saloon carriages, where ease of travelling was the chief object aimed at, the bodies were placed within the wheels.

50 In the course of constructing the earth-works of a railway, the contractors were accustomed to lay down temporary ways or lines of rail, for the earth waggons to travel upon. When these were done with, the proper road for the trains was laid down; and this, to distinguish it from the former one, was called the permanent way.

51 See Wood On Railways, 3rd. edit. 1838, p. 151.

52 A full description of the original road of the Great Western Railway, communicated by Mr. Brunel, will be found in Wood’s Treatise on Railroads, 3rd edit. 1838, p. 708.

53 At this time Mr. Brunel was confined to the house by the effects of his accident on board the ‘Great Western’ steam-ship (see p. 242). Had he been on the spot, he would have been able to give the work careful consideration during its progress, and to judge of the expediency of proceeding with the plan.

54 The continuity of the timbers diminishes the risk of trains leaving the line from small imperfections in the permanent way. And, should a train leave the rails, the injury to the carriages and to the road is generally less serious than it is when the wheels of a carriage off the rails come into repeated and violent contact with the cross sleepers. Instances have frequently occurred where carriages which have left the rails have run considerable distances on the longitudinal timbers without injury.

55 This experiment excited the greatest interest, and it was long afterwards related how Mr. Brunel, by the stroke of a hammer, had knocked to pieces the scientific deductions of Dr. Lardner, who, as was well known, had prompted Mr. Wood’s decision in this matter.
Mr. Brunel was so much impressed with the great influence which the operation of the blast-pipe had on the working of the locomotive that he afterwards investigated the whole subject, and made further experiments to determine whether or not it might be expedient to abandon the steam blast, and to maintain the draught in the chimney with a fan worked by a rotary steam jet.

56 The inconveniences of a break of gauge had already been brought into notice. One of the narrow-gauge companies, the Midland, worked two existing lines of railway, one between Birmingham and Gloucester, laid on the narrow gauge, and another between Bristol and Gloucester, on the broad gauge; and thus there was a break of gauge at Gloucester.

57 It should, however, be added, that the Commissioners had stated in the body of their report: ‘We feel it a duty to observe here, that the public are mainly indebted for the present rate of speed, and the increased accommodation of the railway carriages, to the genius of Mr. Brunel and the liberality of the Great Western Company.’

58 These experiments will be found in the Appendix to the Report of the Commissioners of Railways, respecting railway communication between London and Birmingham (ordered to be printed May 22, 1848).

59 This was fully borne out afterwards, the express trains running in the same time, 3 hours, over both routes, though the length of the broad-gauge line was 129 miles, as against 113 of the narrow. Similar favourable results have been since exhibited in the competition between the broad and narrow gauge lines to Exeter.

60 Among many important advances in railway travelling made on the Great Western Railway, it may be mentioned that it was on this line that express trains running long distances without stopping were first introduced; and that, in 1845, within about a year of the completion of the line to Exeter, express trains ran from London to Exeter, 194 miles, in 4½ hours. This rate of travelling, which was accomplished without difficulty by the broad gauge in its early days, has scarcely been exceeded since on any railway.

61 Even in the locomotives when of equal power, Mr. Brunel calculated that the extra weight was not more than about 500 lbs.
The extra cost of the Great Western Railway was only, including land, from 300l. to 500l. per mile, or less than 10 per cent. of the whole; although Mr. Brunel had taken advantage of the broad gauge to get carriage bodies 2 feet wider than was then usual.
The wide carriages and waggons were found less costly than the narrow ones in proportion to the load they carried.

62 The apparatus patented in 1839 by Mr. Samuel Clegg and Messrs. Jacob and Joseph Samuda, and improved from time to time by them, was that adopted in almost all the attempts made in this country to introduce the Atmospheric System. In reckoning up the force which was available for mastering the practical difficulties of the undertaking, the death, in 1844, of Mr. Jacob Samuda must be considered to have been to his brother, and to all others concerned, a great and irreparable loss.

63 A considerable amount of engine power was necessarily consumed in exhausting the tube before the passage of the train commenced; and it might at first sight appear that this work was wasted, and that it was only the work which the engine performed during the passage of the train which was useful in traction. This, however, was not the case; for, as was admitted by the more scientific of the opponents of the Atmospheric System, the power employed in anticipatory pumping was work legitimately stored up and re-delivered in relief of the engines during the passage of the train. A waste of power incidental to the Atmospheric System was indicated by the heat of the air which was delivered by the exhausting pumps. This waste, however, amounted on the average to only 10 per cent. of the total work done. A further source of waste of power was the friction of the air passing along the tube to the exhausting pumps; this waste was found to amount, on the average, to from 10 to 15 per cent. of the total work done.

64 This was almost the only case in which Mr. Stephenson approved of the application of the System.
Before the Croydon and Epsom Committee, in answer to the question, ‘Does the Atmospheric railway give you any power of using practically and usefully steeper inclines than the locomotive railways?’ Mr. Stephenson said, ‘Yes, I think it does, but still at a very inordinate loss of power; still it is within the scope of the Atmospheric System under particular circumstances. I remember a case where it might be advantageous. Mr. Brunel went to Italy for the purpose of laying out a line there, and from Genoa over the Apennines he had to form a line; it would probably rise 15 or 20 miles at 1 in 100 or 1 in 60 or 70. Where there is that continuous line of ascent, where no stoppages are required, where the locomotive is totally inapplicable, there I can conceive nothing more eligible than the Atmospheric plan’ (p. 80).

65 The length of the line was 52 miles, but as it was considered that auxiliary stationary power would in any case be necessary on the 10½ miles of very steep inclines, the cost of the Atmospheric apparatus is taken on 41½ miles.

66 It must be remembered that beyond the South Devon Railway was the projected railway through Cornwall, which, with its long and heavy gradients, was, in all its features, even more suitable than the South Devon for the application of the Atmospheric System. Had that system succeeded, and been introduced on the Cornwall Railway, a very great saving might have been made in the cost of the works of this line.

67 I.e. before the Croydon and Epsom Committee. See above, p. 138.

68 It will be found on pp. 35-52 of the Minutes of Evidence taken before the Atmospheric Committee (ordered to be printed April 24, 1845).

69 This appeared with sufficient clearness from the general comparison between vacuum, weight of train, and speed. The exact appropriation of the force employed was shown by some dynamometric experiments made on the line.
The highest speed recorded was 68 miles per hour, with a train of 28 tons, the speed averaging 64 miles per hour for four level miles of the line, the vacuum being 16 inches. This speed should have exhibited a resistance of about 21 lbs. per ton, or 588 lbs., as the running resistance or friction, and 645 lbs. for the resistance of the air; in all 1,233 lbs. Now, the pressure due to 16 inches of vacuum on the piston is 1,390 lbs., which gives 157 lbs. as the friction of the piston; a result which corresponds sufficiently well with a direct dynamometric experiment.
Going to the other extreme, there are numerous records of trains of 100 tons which attained, on a level of four miles in length, average speeds of from 30 to 35 miles per hour, with 16·5 inches of vacuum, one train of 103 tons going 32·4 miles per hour with 16·9 inches of vacuum.

70 This valve consisted of a number of long delicate blades of spring steel, arranged parallel to each other, as in a musical box, but with wider intervals. These plates rested on a series of truly faced bars, which crossed the end of the air-passage. The slightest pressure outwards lifted the springs; and as the area of opening was large, a very free passage was given to the air. On the current ceasing, the blades instantly, yet without shock, replaced themselves in contact with the bars, clipping them tightly under a very small reverse pressure, and effectually closing the passage. Their merit consisted in their being almost without weight, and thus promptly re-closing the aperture by a delicate elastic reaction.

71 Trains frequently arrived late on the Atmospheric portion of the South Devon Railway, owing to its being at the end of the long trunk line from London to Exeter, and having at its other end a locomotive line contending with very heavy gradients.

72 It may be mentioned that, from the date of the abandonment of the Atmospheric System, he refused to receive any remuneration for his professional services as engineer beyond a nominal retaining fee.

73 See above, p. 138.

74 These quantities are the result of the experiments made in September 1847. They agree with what is now the received opinion of authorities on train resistances, and represent favourably the case for the locomotives at the time of Mr. Brunel’s report in August 1844. At the time when Mr. Brunel wrote his report of August 1844, the weight of a locomotive, as has been said, bore a higher ratio to its power.

75 It must be borne in mind that all the inconveniences attending the use of auxiliary locomotives must be encountered, or else the excessive dead weight of an engine powerful enough to take a train up the steepest gradient in a hilly district must accompany it for the whole length of that part of the line.

76 No dynamometer was used in these experiments, but all other requisite data were recorded with the greatest exactness, and the horse-power employed may be deduced by means of the scale of resistance which the subsequent dynamomotric trials supply. Moreover, the result above arrived at for the consumption of coke is verified by an examination of published indicator diagrams taken off the same engine on another occasion.

77 It would of course be impossible here to give a description of all Mr. Brunel’s bridges, or even to refer to the most important of them with that minuteness which would be required if this were a book written for professional use. The following publications may be consulted:—Bourne’s History and Description of the Great Western Railway, 1846; Brees’ Railway Practice, 1837; Simms’ Public Works of Great Britain, 1838; Proceedings of Institution of Civil Engineers, vols. 14, 25, and generally; Molinos et Pronnier, Construction des Ponts Métalliques, 1857; Humber’s Cast and Wrought-Iron Bridge Construction, 1861; and Humber’s Record of Engineering for 1866. At the end of the description of many of the bridges in this chapter a note has been given of publications in which the bridge has been referred to.

78 In the early days of the Great Western Railway special designs were made for every one of the ordinary bridges over and under the railway; but when, in consequence of the rapid extension of the Great Western system, the number of bridges to be designed became very large, Mr. Brunel had a set of ‘standard drawings’ prepared and engraved, which embodied the experience gained, and contained designs suitable for various situations. The contract drawings were made by adapting to the particular circumstances of each case the standard drawing which was most applicable to it. This system, besides securing uniformity of construction, introduced a considerable amount of economy; since, the standard drawings being based upon the results arrived at in an extensive practice, the proper structural arrangements and dimensions were indicated with far greater accuracy than could be attained in a reasonable time by an independent calculation in each individual case.

79 It was called the ‘Wharncliffe Viaduct,’ in acknowledgment of the services rendered to the Company by the late Lord Wharncliffe as Chairman of the Committee in the House of Lords. Drawings of this bridge are given in Simms’ Public Works of Great Britain, 1838, pl. 54, 55, and 56; and in Bourne’s History and Description of the Great Western Railway, 1846.

80 At the back of retaining walls, such as the abutments and wing walls of bridges which were subject to the pressure of earth behind them, Mr. Brunel introduced what were termed ‘sailing courses,’ projecting shelves corbelled out at the back of the wall. The weight of earth resting on these shelves virtually increased the weight of the back of the wall, and assisted it in resisting the forward pressure of the earth.

81 During the construction of the bridge a part of the crown of the eastern arch proved defective, in consequence of the cement in the middle of the brickwork not having set sufficiently at the time when the centering was eased. Apprehensions which had been entertained by some as to the safety of the structure were groundless, for when the defective part was taken out and replaced, no further trouble was experienced. The bridge has stood well, and has shown none of those symptoms which an overstrained structure exhibits.

82 Simms’ Public Works of Great Britain, pl. 57, 58; Bourne’s History and Description of the Great Western Railway, p. 36.

83 When Mr. Brunel for architectural effect employed Gothic or pointed arches, he occasionally made the main part of the arch of a form different from the curve visible on the face, but he more frequently made it of the same pointed form throughout. In this case he did not obtain equilibrium by loading the crown, but he kept the line of pressure sufficiently within the thickness of the arch by strengthening the haunches.

84 Brees’ Railway Practice, pl. 42.

85 There is an illustration of this bridge in Bourne’s History and Description of the Great Western Railway.

86 Joggles are small pieces of hard wood or cast-iron of rectangular cross section, placed between two beams, and fitted carefully into notches cut across them. The beams are then bolted firmly together. The object is to stop the slipping which would occur between the two surfaces if the beams were merely laid one on the other and loaded, and so to make two pieces of equal size act as one of double the depth, and therefore of four times the strength of a single piece. In 1841 Mr. Brunel made experiments to satisfy himself of the strength gained by this method; and he afterwards perfected the arrangement by ensuring an exact fit in the notches by tightening up the joggles with wrought-iron wedges.

87 A drawing of this bridge is appended to the Report of the Commission on the Application of Iron to Railway Structures, 1849.

88 Mr. Brunel, by taking care in his timber structures to distribute the load uniformly, was frequently able to dispense with costly foundations. On one occasion a timber viaduct 30 feet high was placed upon a broad platform resting on an old embankment upwards of 50 feet in height. A similar arrangement was adopted by him in some cases on slips in embankments which it would have been uncertain and expensive to make up with earthwork.

89 For a description of this viaduct, see Proceedings Inst. C. E., vol. xiv. for 1854-5, p. 492.

90 Mr. Brunel about this time introduced a great improvement in the manufacture of wrought-iron bolts for bridges, when these, as is usually the case are screwed at the ends. A screw cannot be made on a bolt without the metal being cut into to the extent of the depth of the thread, and the strength thereby considerably reduced. The section, taken at the bottom of the thread, is much smaller than that of the bolt; and, moreover, owing to the abrupt change at the commencement of the thread, the strength is not so great as that due to the reduced sectional area. The improvement consisted in swelling out the iron of the end of the bolt where the thread was to be made, so that the diameter, at the deepest part of the thread, should be fully equal to that of the bolt. The saving of metal by this improvement, especially in the case of long bolts, is very considerable.

91 An arrangement was introduced by Mr. Brunel to prevent any disturbance in the permanent way by a settlement of the embankments at the ends of the viaducts. There were no large abutments and wing walls, but the end of the viaduct was formed with a queen truss in the parapet, which rested on a platform on the top of the embankment slope. In any slight settlement of the earth, the end of the viaduct sunk with it, and the permanent way was not disturbed. Wedges were provided to raise the ends of the trusses and readjust their level. This was an important provision, as it applied to about 60 cases on the Cornwall Railway.

92 The simple arrangement of the timber-work was specially arranged with a view to giving facility for replacing portions of it, should they decay.

93 The viaducts on the Cornwall Railway between Plymouth and Truro are thirty-four in number. Of these there are nineteen which have from six to twenty openings, and are from 80 to 153 feet in height. The aggregate length of these nineteen viaducts is nearly 2¾ miles.

94 A drawing showing this form of section is appended to the Report of the Commission on the Application of Iron to Railway Structures, 1849.

95 The remainder of this letter is printed in Chapter XVI. p. 486.

96 Mr. Brunel was thoroughly conversant with the principles of mathematical analysis, and was able with great readiness to apply it in practice; but at the same time he preferred, when it was possible, to use geometrical methods of solution for engineering problems.

97 In the note at the end of this chapter mention is made of some experiments made by Mr. Brunel on riveted joints.

98 See above, p. 185.

99 A description of this bridge is given in Humber’s Bridge Construction, vol. i. p. 228, and vol. ii. pl. 70, 71, 72; Molinos et Pronnier, Construction des Ponts Métalliques, p. 328, pl. 20, 21, 22.

100 See Proceedings Inst. C.E., for 1847-8, vol. vii. p. 138.

101 After some perseverance, Mr. Brunel succeeded in getting these unusually large links, which were 20 feet long, rolled in a single piece without welding on the eyes. He had to go down himself to the manufactory in order to get the men into the way of doing the work.

102 See note at the end of this chapter for experiments on ropes and chains. The crabs were designed and made specially for this duty. Each had two barrels, grooved to receive the chain, which was passed several times round both barrels, so as to get sufficient grip; and it was in this way possible to wind in with the crabs any length of chain without having to stop to fleet, as would have been the case had a single-barrelled crab been used.
These crabs were subsequently used at the floating of the Saltash trusses, and at the launch of the ‘Great Eastern.’
A similar arrangement was applied in the paying-out machinery of the Atlantic cable, and is still used for the picking-up gear in the ‘Great Eastern.’

103 See Encyclopædia Britannica, ‘Iron Bridges,’ vol. xii. p. 601; Molinos et Pronnier, Construction des Ponts Métalliques, p. 320, pl. 23, 24, 25.

104 These would have been magnificent specimens of timber-work, and a design for somewhat similar trusses had at one time been prepared for the Chepstow Bridge. It is worthy of mention that Sir Isambard Brunel at one time designed a timber arched bridge of 800 feet span to cross the Neva at St. Petersburg.

105 Before the work was begun Mr. Brunel made calculations to determine whether or not it might be desirable to cross the river with one span of 850 feet, in order to avoid the great depth at the centre pier.

A few extracts from letters relating to bridges of large span will be interesting:

‘January 31, 1852.

‘I have revised my calculation as to a span of 1,000 feet, and find that even with the loads and limitations of strains which I adopt—namely a proper thickness of ballast, and a possible load of a train of engines without tenders, and a limitation under such a load of 5 tons’ strain per square inch, that a span of 1,000 feet may be made in England of the very best workmanship, and sent out and erected for I should say safely 250,000l., of course a single way—another 250,000l. ought, I should think, to cover the rest of the bridge.

I should like to explain to you the mode I should propose for raising such a bridge, weighing 7,000 tons.

‘December 1, 1852.

As you ask me my opinion of the advisability of patenting your bridge, I give it you; though you will probably be the first person who will have followed such advice if you do so, and might safely patent such a novel mode of using advice.

In my opinion you cannot patent the bridge. Without detracting in the least from your merit of invention, the form has been so frequently and exactly applied that no patent could hold. The Saltash bridge now just advertised for letting is exactly on the same principle as regards form; and this is so old to my knowledge that I can claim no invention, and the use of cast iron for such purpose is also incapable of being patented. There is much that is good in your bridge, and you deserve credit, but you would find innumerable claimants to dispute, and successfully, your attempt to claim a monopoly by a patent—I myself for one. Besides, I see it published in a book.

‘May 30, 1854.

’As to your present enquiry, I do not think that what I am doing at Saltash would be applicable in this case; but without being guilty of great presumption, I think I may say that if the same plan will not do, it is fair to assume that the same brains which concocted the plan to suit the difficulties of the Tamar might very likely find the means of overcoming those of the Severn.

If I should be able to suggest a feasible plan, and there should be found people ready to make it, I shall have the satisfaction of bridging the Severn as well as the Tamar.’

106 It has of course been impossible to refer to the points on which Mr. Brunel was aided, in his different works, by the suggestions of his assistants; but it may be mentioned here that, as appears from one of Mr. Brunel’s letters, the plan of working under a diving-bell had been proposed by Mr. William Glennie, his assistant, before he knew that Mr. Brunel had decided to adopt it.
Mr. Brunel also mentions in another letter that the ‘very simple and effectual manner’ of applying the pneumatic apparatus, by forming the annular space round the circumference of the bottom of the cylinder, was suggested to him by Mr. Brereton, when the method of constructing the cylinder was being finally settled.

107 In the Proceedings of the Institution of Civil Engineers, vol. xxi. 1861-2, will be found a paper by Mr. Brereton, giving a detailed description of the means employed for the construction of the central pier.

108 Shortly after Mr. Brunel’s death some of his friends on the Board of the Cornwall Railway placed the following inscription, in raised letters, over the lard archways—I. K. BRUNEL, ENGINEER, 1859.

109 A portion of the chains used were those which had been made for the Clifton Suspension Bridge, Mr. Brunel’s earliest design.

110 Though it is convenient to explain the nature of the strains in the Saltash bridge as an arch and suspension bridge combined, it is not intended to imply that there is any virtual difference between this truss and the one at Chepstow, for in both the strain on the tube counteracts the strain on the chains, though the one tube is curved and the other straight.

111 Humber’s Bridge Construction, vol. i. p. 231; vol. ii. pl. 78, 79, 80.

112 This load amounted to two and three quarter tons per foot run, in addition to the weight of the truss. Under this load the central deflection was about 5 inches.
The strain on the iron of the tube and of the chains with a load of one ton per foot run, in addition to the weight of the truss, flooring, and ballast, is under four tons per square inch.

113 The difficult operations of floating and lifting the superstructures of the Chepstow and Saltash bridges were carried out entirely by Mr. Brunel and his assistants, there being no contractor engaged in, or responsible for, the work in either case.

114 A photograph taken shortly after the floating of the second tube forms the frontispiece of the first volume of Humber’s Bridge Construction.

115 See above pp. 190 and 193.

116 It has been observed with much truth that full justice has not been done to Mr. Brunel’s exertions in this department of practical science.—See ‘Address of George Parker Bidder, Esq., on his election as President of the Institution of Civil Engineers, January 10, 1860.’

117 Captain Claxton died on March 27, 1868, in his 79th year. The manuscript of this and the three following chapters was fortunately completed in time to be submitted to him. He spared no trouble either in giving or procuring original documents and other materials for all parts of this book, in the preparation of which he took the liveliest interest.

118 To enable the ship to resist the action of the heavy Atlantic waves, especial pains were taken to give her great longitudinal strength. The ribs were of oak, of scantling equal to that of line-of-battle ships. They were placed close together, and caulked within and without before the planking was put on. They were dowelled and bolted in pairs; and there were also four rows of 1½-inch iron bolts, 24 feet long, and scarfing about 4 feet, which ran longitudinally through the whole length of the bottom frames of the ship. She was closely trussed with iron and wooden diagonals and shelf pieces, which, with the whole of her upper works, were fastened with bolts and nuts to a much greater extent than had hitherto been the practice.
The principal dimensions of the hull and engines are given in the note to this chapter (p. 245).

119 The engines as designed by Messrs. Maudslay were beam engines, although Mr. Brunel had strongly urged them to adopt the more compact form of direct-acting engines. They, however, thought it better not to depart from what was then the usual form.

120 Steam and its Uses, by Dr. Lardner, 1856. Chapter on ‘Steam Navigation,’ section 10.

121 ‘An exposition of the advantages of the proposed Railway from Limerick to Waterford.’

122 The Steam Engine: its Application to Navigation and Railways, with Plain Maxims for Railway Speculators, 5th edition, 1836, p. 307.

123 It is right to add that, according to the report given in the Athenæum newspaper of the meetings of the British Association at Liverpool in September 1837, ‘Dr. Lardner addressed the section (Mechanical) on his old subject, the application of steam to long voyages. His remarks and calculations were to a great extent identical with those brought forward by him last year at Bristol, and published long since in his work on the steam-engine, but the conclusions were somewhat varied. The Doctor did not now deny that the voyage might be practicable, but he did not believe that it would be profitable’ (Athenæum, September 23, 1837). Dr. Lardner was answered by several speakers, and among them by Mr. Guppy, who pointed out in much detail the unreliable character of Dr. Lardner’s data; while nothing was suggested about commercial profits or subsidies. It may therefore be inferred that Dr. Lardner’s arguments as to the consumption of fuel remained the same, although he may have abandoned the conclusion which legitimately followed from them—namely, that the long voyages were practically impossible.
The Report in the Bristol Mirror newspaper of the same date (copied from the Liverpool Standard) is as follows:—‘Dr. Lardner’s speech was little beyond a repetition of his discourse last year in Bristol, re-published by him in the Edinburgh Review. The voyage to America by steam he treated as practicable, but so uncertain as to render a profitable result hopeless.... During nearly all the year there was an adverse west wind, and the Gulf Stream was to be avoided.’

124 She had only seven passengers on board; fifty, it is stated, were deterred from going in her by hearing of the fire.

125 These engines were to have had two cylinders of 88 inches diameter.

126 It is interesting, in connection with this subject, to mention the following circumstance. At Mr. Brunel’s recommendation, Mr. Humphrys consulted Mr. James Nasmyth as to the best means of forging the large paddle-shaft; as they could not get any manufacturer to undertake it. To accomplish this forging Mr. Nasmyth designed his steam-hammer, and though it was not then erected in Bristol, in consequence of the alteration of the form of the engines of the ‘Great Britain,’ it soon afterwards came into general use.

127 In the Minute Book of this date it is mentioned as a reason for postponing any decision on the subject, that Mr. Brunel was making ‘final,’ and afterwards ‘further,’ experiments.

128 See Mr. Guppy’s paper, printed in the Proceedings of the Institution of Civil Engineers for 1845, p. 151.

129 This report is printed in Appendix II. p. 539. The paragraphs in which Mr. Brunel describes the advantages of the screw propeller will be found at page 552.

130 A description of Mr. Humphrys’ trunk engines is given in Tredgold’s work on the steam engine (ed. 1838), p. 390.

131 The balanced rudder, which is peculiarly applicable to screw ships, has encountered much opposition; but it has lately been successfully introduced by Mr. E. J. Reed, C.B., into vessels designed by him for the Royal Navy.

132 It maybe convenient here to state that the dock in which the ‘Great Britain’ was built led into the Floating Harbour, which is a portion of the channel of the river Avon closed in. The Floating Harbour communicates through the Cumberland Basin with the river.

133 One of the consequences of the publication of this report was, that Mr. Brunel received so many letters containing suggestions for lifting and floating the ship, that he was obliged to have a circular letter printed declining assistance; and more than four hundred letters were also received by the Company’s secretary.

134 ‘The “Great Britain” Steam-Ship. Extracts from the letters of Captain Claxton, R.N., to I. K. Brunel, Esq., and the Directors. Bristol, 1847.’
Mr. Bremner’s apparatus is also described in a paper read by him at the Institution of Civil Engineers, and printed in the twenty-first volume of the Transactions, p. 160.
Accurate illustrations of the break-water and floating apparatus will be found in the Illustrated London News of August 21, 1847.
In publishing the correspondence to which reference has been made, the Directors acknowledge their obligations to all concerned in the arduous task of saving the ‘Great Britain;’ and they add—‘to Mr. Brunel above all their thanks are most due, for opening their eyes to what might be accomplished, and for taking upon himself the responsibility of her release, provided that Captain Claxton was employed to carry out his views.’

135 Sir E. Parry was at that time Controller of Steam Machinery, and a warm supporter of those who desired to make a fair trial of the screw propeller.

136 Bourne, on the Screw Propeller, ed. 1867, p. 263.

137 The records of these trials, found among Mr. Brunel’s papers, show results which coincide in all material points with those given by Mr. Bourne at p. 284 of his work on the Screw Propeller, ed. 1867.

138 Bourne, Appendix, p. xxxiii.

139 The letter to Mr. Guppy of August 1843 (given above, p. 259) contains a reference to some of Mr. Brunel’s ideas upon iron shipbuilding, subsequent to the date of the design of the ‘Great Britain.’

140 Extract from a memorandum by Mr. Brunel (dated February 25, 1854) on the early history of the great ship.

141 So Mr. Brunel wrote to the secretary when excusing his own absence from the meeting on account of illness.

142 Mr. Brunel concludes this report by indicating the best mode of contracting for the construction of the ship and engines, and suggests that an examination should be made of the river Hooghly.

143 From the great length of this report it has been necessary to omit the less important paragraphs.

144 The passages which are omitted refer to the relative cost and merits of the different tenders.

145 Two years later Mr. Brunel spoke of the success of the screw engines as ‘a most grave question upon which hangs I fear to think how much.’

146 Mr. Geach was one of the most zealous friends of the great ship. His death, within a year after the commencement of the work, was the first of her many misfortunes.

147 Stringent conditions as to jacketing were introduced into the contracts; but they were not enforced by Mr. Brunel, in deference to the strong objections urged by the makers. He much regretted this concession.

148 The destination of the ship as proposed at this time was Australia.

149 The power of blowing off the steam without noise is of great importance, and Mr. Brunel made many experiments on the point. The noise of the steam blowing off, when the engines of a ship are stopped for fear of an impending collision or other accident, is so great, that it is almost impossible to hear any orders that may be given.

150 The screw propeller of ‘Agamemnon’ was broken in a trial trip outside Plymouth Sound, on November 9, 1853; and the result was that, from not having efficient governors, the engines flew round at an alarming speed.

151 Mr. Russell’s works. The ship was built in a yard immediately adjoining them.

152 See, for example, an article in the Quarterly Review of March 1856, entitled ‘The Triton and the Minnows.’

153 It must be remembered that this report describes the ship as she was at that time designed by Mr. Brunel.

154 Extract from memoranda of October 5, 1855:—
‘By constant observation, to lay down position and course of ship, and correct compasses.
‘To record speed of ship through water by logs.
‘Revolution of engines.
‘Force and direction of wind.
‘Draught and trim of ship, sails carried, &c.
‘Temperature and peculiarities of sea water.
’As the result of these observations, to plot down hour by hour the position of the ship, to compute the speed, variations of compass, the direction and force of current, and true direction and force of the wind.’

155 Mr. Brunel also proposed to have charts prepared for the route of the great ship to Australia and Calcutta, similar in character to some which were made under his directions for the route of the ‘Great Western’ to New York, which he described as follows:—
‘When we started the “Great Western” to New York, I had a chart drawn and engraved of the sea (that is, the lines of latitude and longitude, and the bearings of the compass, and the coast and soundings) on a cylindrical projection of a great circle from Bristol to New York; and we found it very useful for the captain to see his great circle sailing, and to see how much he was deviating from it.’

156 Captain Harrison’s command of the ‘Great Eastern’ after she left the river was unhappily of but short duration, as he was drowned by the accidental upsetting of his boat in Southampton Water on January 21, 1860, four months after Mr. Brunel’s death.

157 With this paper Mr. Brunel enclosed a letter, in which he points out with great earnestness the responsibility of his own position.
After calling the Directors’ attention to the important step they were about to take, he proceeds:—
‘I am not pointing out a danger without being prepared to propose a remedy. The same man who, after he has been selected and appointed on account of his previous character as a sailor, and as an experienced naval man, would probably feel disposed to reject advice coming from those who do not profess to be sailors, and to resist directions which might appear to him as trenching upon his authority, or as implying doubts of his ability, could have no such feeling (if he is a sensible man and fit for the position) if his attention had been drawn to these views before his appointment, and if he accepts the trust reposed in him on the understanding that he is expected to pay attention to these opinions, and if, as I shall urge upon the Directors to ascertain, he entertains no objection to the adoption of them, and agrees to follow the principles of action which I hope to induce the Directors to adopt as rules in the navigation of the ship.
‘I propose therefore to lay before the Directors the result of my anxious consideration of this subject, to urge upon them the adoption of my views, and, if they adopt them, to urge that they should make it a condition in the selection and appointment of a commander that such views are approved of and adopted by him.
‘This is a strong and plainly stated request, but not more strongly put than I feel that the occasion requires.
‘I have an immense stake in the success of this enterprise. I do not refer merely to my pecuniary investment; but, as affecting my professional reputation, my stake is much deeper; as, although I was accidentally led by circumstances into proposing the plan we have adopted, and the Company was not originally formed to carry it out, and although the plan when proposed was well weighed and considered by men competent to judge, at all events, upon the prospects as a commercial speculation, and although it was adopted by them, and therefore they must share in the responsibility, and although many may share with me in the credit of our success; yet there is no doubt that I should have to bear solely and very heavily the blame of a failure. On this ground alone, therefore, the Directors, I am sure, will willingly allow me to urge my views strongly, and will excuse the length at which I do so. But I shall rely upon satisfying them that my views and opinion should command their concurrence on their own merits; and with this preface, which has already reached an undue length, I will lay before them a paper on the subject, most of which has been written for some time in anticipation of the present circumstances, and having been thus written at different periods is, I fear, somewhat disjointed, and wanting in arrangement, and therefore much longer than it might have been.’

158 Shortly after the publication of this report, Mr. Brunel received a letter from Mr. G. W. Bull, of Buffalo, U.S.A., encouraging him to adopt the plan of launching sideways, as that was the way in which the large steamboats of the American lakes were launched. In the course of a correspondence which ensued, Mr. Bull gave much information as to the manner in which these launches were effected. He advocated a free launch for the ‘Great Eastern.’

159 For an account of this and other experiments and observations on friction, see note A at the end of this chapter.

[160] The power mentioned here as applied for starting the ship was two hydraulic presses, to overcome adhesion at the first start. The river tackle was relied on to overcome the friction.

161 See below, note A, p. 389.

162 The reason for stopping was lest the ship should float at high water during the night.

163 It remained a doubtful point whether or not the bow cradle was stopped by the brakes. Subsequent experience favoured the opinion that it had stopped of itself.

164 The ship had always been spoken of as the ‘Great Eastern,’ and this name was specially agreeable to Mr. Brunel. It was not a point on which he set much store, but his views were pretty well known; and, as the name had not been of his choosing, but had rather grown out of the name of the Company, and the natural association of ideas with his first ship, the ‘Great Western,’ it might reasonably have been supposed that it would have been adopted. But some fastidious person suggested that the name was objectionable, as consisting of two adjectives. A list of names was prepared and submitted to Mr. Brunel on the day of the launch at the moment when he was busiest. He said off-hand that they might call it ‘Tom Thumb’ if they liked. The Directors, however, selected the name ‘Leviathan,’ and so the ship was christened. The new name never stuck to the ship, and she was registered as the ‘Great Eastern.’

165 The action of an hydraulic press is limited to the length of stroke of the ram. Hence, as soon as the ship moved a distance equal to the length of the stroke, it became necessary to relieve the water pressure in the cylinder, to pull back the ram into it, and to insert between the ram of the press and the cradle an additional length of timber. This operation of pulling back the ram and putting in another length of timber was called ‘fleeting’ the press, a term used in many mechanical operations when the motive arrangements are reinstated in their primary position ready for a further advance in the work.

166 On this occasion Mr. Brunel had his way, and there were not a dozen spectators in the yard.

167 See note A at the end of the chapter.

[168] At this time the friction when the ship was in motion was not much greater than it had been on previous occasions; but, the adhesion being greatly increased, a large excess of power became stored up in the elasticity of the abutments and the chains. This excess of power, on the ship commencing to move, discharged itself in giving her velocity, and therefore she moved by short slips, instead of by a gradual motion, as had been the case previously, when the resistance due to friction and that due to adhesion had been more nearly equal. For observations on the friction, see note A at the end of the chapter.

[169] Mr. Stephenson went with Mr. Brunel into all the facts relative to the operations and all the experience gained therefrom, and examined carefully the investigations which had been made by means of self-recording apparatus, and which showed that the friction diminished as the velocity increased. So struck was Mr. Stephenson at these results that he urged Mr. Brunel on no account to neglect having the brakes properly attended to; lest the great adhesion should cause a large amount of stored-up power, which might give the ship such a velocity that the force of gravity might exceed that of the friction. The brakes had not been hitherto neglected; but, in conformity with Mr. Stephenson’s suggestion, it was thenceforward made a rule that, when a moderate amount of slack had been overhauled from the drums, the brakes should be put down, on a signal being given that the pressure in the presses was rising.

170 Mr. Brunel received a great many suggestions for launching the ship. Puncheons, gas, artillery, and especially levers, were among the appliances recommended. With the desire of behaving civilly to well-meaning persons who were wishing to do him a service, Mr. Brunel adopted a plan similar to that which he had found useful when the ‘Great Britain’ was on shore. He had a circular letter printed, thanking his correspondents for their suggestions, but saying that the number of such communications had become so great that it was impossible for him to do more than cause the receipt to be acknowledged, with thanks for the intentions of the parties writing.
This plan had, however, the effect of increasing the number of his correspondents, as several of them wrote a second time to express their regret that their letters had had no better effect than to be classed with the numerous communications which Mr. Brunel said he had received on the same subject, which had seemed unworthy of his notice; and they explained that though other people’s schemes were, no doubt, worthless, still that their own, if adopted, would launch the ship.

171 The number of the presses being almost doubled, but the resistance of the ship not being much greater than before, the elastic compression of the abutments was less than it had been previously, so that when the ship moved there was much less work stored up to give it velocity; therefore the slides were shorter than they had been before.

172 Among the many congratulations which Mr. Brunel received on the completion of the launch, there was perhaps none which pleased him more than the following letter from Mr. Robert Stephenson, who had been prevented by illness from being present at the concluding operations, the critical character of which he had fully appreciated:—

February 1, 1858.

My dear Brunel,—I slept last night like a top, after I received your message. I got desperately anxious all day, but my doctor would not permit me to venture so far away as Millwall.

I do, my good friend, most sincerely congratulate you on the arrival of the conclusion of your anxiety.

Yours sincerely,
Robert Stephenson.

A letter from Mr. Brunel to his friend Mr. Froude, describing the floating, is given in note B to this chapter.

[173] The principal dimensions of the ship and engines are given in the note to this chapter, p. 416.

174 During the last-mentioned voyage, returning from Quebec, a north-easterly wind blew with a velocity of from 30 to 40 miles an hour. For the first 24 hours the ship paid no attention to the sea; the next day, the wind remaining the same, but the sea having got more swell on, the ship began to roll slowly and sedately, the rolls gradually increasing up to about 9 degrees on either side of the perpendicular, and dying out again, and then recommencing.

175 Between the screw and the rudder was what was termed the after stern-post, which had no duty to perform except to steady the heel of the ship, into which the rudder was stepped.

176 Before the ship was finished, Mr. Brunel had her stability and rolling carefully investigated. Calculations were made to ascertain the position of her centre of gravity and the other necessary elements for determining her stability and period of rolling.
He also had a model made, with arrangements for altering the levels of weights placed inside. By this means the results of the calculations were verified. It was determined that the ship would make a single roll from one side to the other in about six seconds. While she was in the Thames a steamer struck the hulk alongside and gave the great ship a slight impulse. Mr. Brunel, who was on board, took advantage of the opportunity to observe the period of the roll she made, and was pleased at finding it agree with the calculated period. It is to the investigations initiated at that time by Mr. Brunel that are due the great steps since made in the knowledge of the laws which govern the rolling of ships. Had Mr. Brunel lived he would no doubt have taken the same pains to record the rolling of the ‘Great Eastern’ as he had in the case of the ‘Great Western’ when, in 1839, he sent an assistant to America and back, who took observations of the rolling and pitching of that vessel in several voyages. These observations were made by a simple angle-measuring instrument, adjusted by the visible level line of the horizon, and not by the fallacious method of noting down the swings of a pendulum.
The ‘Great Eastern’ remains almost perfectly steady in ordinary rough seas. When the seas become very long, so that their period is nearly the same as that of the ship, she rolls; though the number of degrees on either side of the perpendicular is not large. By stowing the weights of cargo high in the ship, the tendency to roll has been much diminished, and when engaged in cable laying, with the enormous weights in the cable tanks all placed above the lower deck, she is remarkably steady.

177 History of the Atlantic Telegraph: New York, 1866.

178 It may be desirable to give a short explanation of these works, which consisted principally of a sluice, a trunk, and a drag-boat.
The Sluice was made in the abutment of Prince’s Street Bridge, and was intended to create a scour after the water had been let off from either side of the float, the opening at the bridge being closed by a caisson which had long been in use when (as was formerly the case) the upper part of the float was scoured through Bathurst Basin.
The Trunk, near the entrance from Cumberland Basin, is a wooden culvert between the floating harbour and the river. As much of the mud as could be dragged there was deposited at the entrance of the trunk, and, when the tide was low in the new channel of the Avon, sluices were opened, and the water rushed through from the floating harbour, carrying the mud with it.
The Drag-Boat was fitted with a steam-engine which worked a large windlass with three compartments, round two of which chains were passed and fixed to posts on the quays, and the boat was dragged backwards and forwards. The third compartment of the windlass worked a chain which elevated or depressed a scraper, attached to a long pole at the stern, and secured from swerving by a chain-bridle which passed under the boat. The scraper stirred up the mud, and deposited the more solid parts at the entrance of the trunk.
Mr. Brunel also desired that the float boards of the Neetham Dam should be put into proper working order; and that they should be altered so that, in times of land floods, the whole or a considerable portion of the excess of water should be retained, and passed through the feeder; and that even the water of spring tides should be allowed to pass the dam, and then be stopped back for the same purpose.

179 The advantage of forming an air-chamber in the lower part of the gate, and allowing the water to enter above it, is that when the size of the air-chamber is properly adjusted to the weight of the gate, there need not come on the wheels, while moving, more than a trifling amount of weight.

180 The arrangements of a buoyant gate have been explained above, p. 429.

181 In the year 1856 and afterwards, Mr. Brunel was engaged in improving the navigable channel of the river Neath, at its embouchure into the Bristol Channel. A bank of furnace slag, for directing the course of the river, had been made previously by Mr. Palmer, and continued as far seawards as it was then thought could be done with safety. Mr. Brunel carried a training bank still farther, and succeeded in cutting off a bend of the river; Mr. Brereton has since extended the navigable channel in a straight line to low-water mark, a distance of two miles; and the bar has been lowered to within 1 foot of the level of the dock sill.

182 Reports on the Paris Universal Exhibition 1867 vol. i., p. xxiv. 194

183 When, at the close of the Exhibition, Mr. Brunel was compelled, much against his will, to accept a pecuniary acknowledgment of his services, he spent the money in the erection of model cottages at Barton, a village near his property in Devonshire.

184 See Report of the Select Committee of the House of Commons on Ordnance, 1863, Minutes of Evidence and Appendix (ordered to be printed July 23, 1863):—p. 306, Report of Ordnance Select Committee; p. 44, Statement of Mr. Whitworth; p. 56, Evidence of Mr. Whitworth (Q. 1329-1337); pp. 58, 59, the same (Q. 1385-1410); pp. 402, 403, Papers delivered in by Mr. Isambard Brunel; p. 110, Further Evidence of Mr. Whitworth (Q. 2545-2551); p. 112, the same (Q. 2602-2610); p. 548, Letter from Mr. Westley Richards to Mr. Isambard Brunel.

185 Captain Claxton, at Mr. Brunel’s desire, went for a voyage in Messrs. Ruthven’s vessel, the ‘Enterprise,’ in order to test her performances.

186 This appears to be the only instance in which Mr. Brunel printed an account of any of his works.

187 On this application of tin for the covering of the roof, Dr. Parkes observes (Manual of Practical Hygiene, London, 1869, p. 317):—‘In the Crimean War the roofs of Renkioi Hospital, on the Dardanelles, were covered with polished tin; it was found, however, somewhat difficult to place it so as to exclude rain, and the surface soon became tarnished. The thermometric experiments did not show a greater lessening of heat than 3° Fahr. below houses not tin-coated.’

188 ‘At Renkioi, in Turkey, Mr. Brunel supplied square wooden sewers about 15 inches to the side; they were tarred inside, and acted most admirably without leakage for fifteen months, till the end of the war. The water-closets (Jennings’s simple syphon), arranged with a small water-box below the cistern, to economise water, never got out of order, and in fact the drainage of the hospital was literally perfect. I have little doubt such well-tarred wooden sewers would last two or three years’ (Parkes’s Manual of Practical Hygiene, p. 635).

189 The woodcut, fig. 22, is a reduction of part of this drawing.

190 The case of Ranger v. the Great Western Railway Company.

191 See above, p. 192.

192 The remainder of this letter is printed above, p. 192.

193 Although Mr. Brunel endeavoured, as far as possible, to lay aside work during his visits to Watcombe, he found occasions for the use of his mechanical knowledge. He prepared tools for transplanting young trees; he did not, however, succeed in making them flourish as well as before they were transplanted, though he attended to the work himself, and took care that a large amount of earth was moved with the tree. The bridge of rough poles across the turnpike road between Teignmouth and Torquay is as good of its kind as his larger works.

194 On June 10, 1830, when he was twenty-four years old, Mr. Brunel was elected a Fellow of the Royal Society. He subsequently became a member of most of the other scientific societies; but he rarely attended any meetings, except those of the Institution of Civil Engineers.

195 At the beginning of 1858, on the expiration of Mr. Robert Stephenson’s term of office as President of the Institution of Civil Engineers, Mr. Brunel came next in rotation for election; but his failing health, and the pressure of his professional duties, led him to request that he might not then be put in nomination.—- See Inaugural Address of John Fowler, Esq., January 9, 1866.

196 A drawing of their Nile boat, the ‘Florence,’ which he made for his daughter, exhibits the same beautiful minuteness which appears in all his early sketches for the Clifton bridge and Great Western Railway.

197 Mr. Brunel’s Nile boat, being of iron, could not safely go up the Cataracts.

198 A few weeks after Mr. Brunel’s death, a meeting of his friends was held, when it was determined to raise some memorial to him. A statue was made by the late Baron Marochetti, and a site for it promised by the First Commissioner of Works; but it has not yet been erected.
Mr. Brunel’s family, by the permission of the Dean of Westminster, have placed a memorial window in the north aisle of the nave of Westminster Abbey.
Along the bottom of the window (which consists of two lights, each 23 feet 6 inches high and 4 feet wide, surmounted by a quatrefoil opening, 6 feet 6 inches across) is the Inscription, ‘IN MEMORY OF ISAMBARD KINGDOM BRUNEL, CIVIL ENGINEER. BORN APRIL 9, 1806. DEPARTED THIS LIFE, SEPTEMBER 15, 1859.’ Over this are four allegorical figures (two in each light): Fortitude, Justice, Faith, and Charity. The upper part of the window consists of six panels, divided by a pattern work of lilies and pomegranates. The panels contain subjects from the history of the Temple. The three subjects in the western light represent scenes from the Old Testament—viz. the Dedication of the Temple by Solomon, the Finding of the Book of the Law by Hilkiah, and the Laying the Foundations of the Second Temple. The subjects in the eastern light are from the New Testament—viz. Simeon Blessing the Infant Saviour, Christ Disputing with the Doctors, and The Disciples pointing out to Christ the Buildings of the Temple. In the heads of each light are angels kneeling, and in the quatrefoil is a representation of Our Lord in Glory, surrounded by angels.
The work was placed in the hands of Mr. R. Norman Shaw, architect, who prepared the general design, arranged the scale of the various figures, and designed the ornamental pattern work. The figure subjects were drawn by Mr. Henry Holiday, and the whole design was executed in glass by Messrs. Heaton, Butler, & Bayne.

199 Mr. Locke here spoke in feeling language of Mr. Robert Stephenson.

200 There appears, from the table, to be an immense extent of paddle-board to this vessel, if the table be correct—greater than the midship section.

201 In a Report by Mr. Brunel on improvements in the port of Bristol, dated December 26, 1839, the following passage occurs: ‘A great change is unquestionably about to take place in the carriage of merchandise by sea, a change similar, though possibly not so striking, as that which has so suddenly been effected by railways in land carriage. To ensure the speed which the passenger traffic demands, great size in the vessels is required; in the course of a very few years we shall find Atlantic steamers desirous of taking 500 or 600 tons of cargo to make up their draught.’