Instruments
During his preparations for his earlier flight Roald Amundsen was struck with the idea of using a sun-compass, and arranged with “Goerz Optische Werke” to construct such an instrument. The firm met his suggestions in the most friendly manner, and the result was our invaluable solar-compasses. The principle of these is as follows:
The sun’s reflection is cast through a periscope down onto a dull disc directly in front of the pilot. By the side of the instrument there is a clock which can be coupled to a cogwheel on the periscope. The clock is constructed so that it can swing the periscope round 360° in the average time that it takes the sun to perform a similar movement. By the aid of a graduated scale on the periscope, which can be placed at a certain angle, one can set it in agreement with the flying-boat’s nose. Should I, for example, start exactly at midday, I should set the periscope so that it points direct astern. Exactly at twelve o’clock I attach the clock to the instrument. Should the seaplane now by chance face the north, I would see a little reflection of the sun in the center of the dull disc which is marked by a cross. The periscope will now follow the sun’s course so that the reflection will always be in the center of the disc as long as the seaplane continues the same course.
Should it be set working at another time, it would be calculated from the angle of the sun, at that moment when the clock is set going. The clock is always regulated according to Greenwich time (or any other recognized time), but the longitudinal distance must be taken into account, and in the same manner the angle must deviate away from the meridian beneath if one does not desire to steer parallel with it. On the top of the periscope there is a screw with an inner part, where an adjustment can be made according to the declination on that day. The solar-compass is mounted on a base on which can be made corrections for eventual latitudinal changes. The periscope’s axis must always stand parallel with the earth’s axis. A change in the upward tilt of the machine must also be reckoned with.
The lenses in the periscope are constructed to give a radius of 10°; that is to say, if the sun’s reflection appears in the disc’s outer edge, one can allow 10° before it disappears in the other outer edge. If one has set the solar-compass for a flight directly north, one will continue in the right direction so long as the flying machine has no deflection. In order to detect such deflections we had a combined speedometer and deviation measure which was also given to us free of charge by Goerz. Amundsen attended to these on the northward journey—Dietrichson on the southward. They both speak of them with high praise. Their uses are shortly as follows: Inside the instrument, on a move-able ring, is fastened a diametrical wire. One looks through the instrument down to the ground below or to the ice, and adjusts the wire the longitudinal way of the ship, then pays attention to the objects passing aftwards under the plane (icebergs, for example), noting whether they follow the direct line of the wire or deviate to the side. Should there be a deviation, one knows that they are not following the direct course in which the nose is pointing, so it has to be set at an angle allowing for the deviation. The wire must be drawn to the side quite slowly until one finds that the objects which one can notice now follow the line of the wire exactly. This points now in the direction one comes from, and the wire’s angle, compared with the boat’s nose, can be read directly in the instrument. That gives the angle of deviation.
One can also leave the wire as it is, and turn the whole instrument instead. The angle of deviation is to be read on the instrument’s base. This is the easiest way, as it allows one to get on quickly with measuring the speed. Having calculated the deviation, it is not correct to steer against the wind allowing only a corresponding number of degrees, or it will be found that there is still a deviation, though not so great as before. To correct it it would have to be measured, then some steering would have to be done, then it would have to be measured again and so on, before it could be regulated. It is, therefore, better to come to a quick and exact result by quickly taking the speed measure. This is done with the same instrument, by watching an object pass between four points of the scale, as the machine goes over it. The pilot continues to fly in a steady course during the entire observations. The navigator sets a stop clock going when an object passes the scale at an angle of 45°, and he stops the clock when the object passes zero, as it will then be centrally under the machine. The altitude above the under-lying territory is read on the altimeter, and by aid of this and the stop clock’s indications it is possible to calculate correctly the speed over the ground-distance covered. We have now got the following particulars: The speed through the air which the speedometer shows and which is called the air-speed,—the steering course through the air which we will call the air-course,—the speed over the ground which we will call the ground-speed and last the deviation’s angle. These calculations have to be worked out in conjunction with each other on a calculating machine, showing in a second what steering-course shall be adopted under the existing wind conditions, to carry the plane in the desired direction. In addition to this there is a gratis enlightenment, showing the exact direction and strength of the wind at that altitude.
The pilot announces if a new course shall be steered. If he steers according to the solar-compass, the navigator adjusts the solar-compass by turning the periscope a corresponding number of degrees.
So long as one need not fly over clouds or fog all goes well. With steadiness it is possible to control the course over the ground and steer the plane straight to the Pole by territorial navigation. During the two first hours, after we had passed Spitzbergen’s north coast, we had thick fog under us and got no drift observations. As soon as we could get these the solar-compass was corrected. We had, however, in the meantime deviated so far westwards that the indicator pointed well over to the west side of the Pole. One must pay particular attention to the fact that the solar-compass only indicates a northward direction so long as one is on the same meridian which the compass was adjusted to. If one has deviated to the side and continues to steer according to the solar-compass, one will set a course directly parallel with the meridian for which the compass was adjusted when starting. For a new adjustment of the compass, so that it points towards the Pole, one must in every case take the bearings. Both during the northward journey, and during the homeward flight, the solar-compasses were of the utmost benefit to us. Without these and depending only on the magnetic compasses we should have been very much less confident. The selection of our magnetic compasses was only settled after we had studied the various types most analytically, paying particular attention to the conditions which they would have to answer to in the Arctic Ocean.
I should like here to mention a common mistake founded on a popular idea, that the Magnetic Pole lies at the North Pole. The globe is a great magnet which has two magnetic points, a North Pole and a South Pole, and fortunately the Magnetic Poles do not lie in the same places as the geographical poles. The earth’s magnetic North Pole, which draws towards itself the compasses’ North, lies on the north coast of Canada about 70° N. and 95° W. long. In general this is called for convenience the magnetic North Pole. Its position, as is well known, was verified by Roald Amundsen during the Gjoa expedition.
Looking at the map, it will be discovered that the magnetic pole lies about an equal length from the geographical North Pole as from Spitzbergen. Therefore it stands to reason that the compass which can be used in Spitzbergen can therefore be used in the fairway from there to the Pole. The one thing which might cause us moments of misgiving was the magnitude of the compass’s variations in the district we wished to reach. (There is little data resulting from exact observation to give us the reason of these variations.)
During a visit to Bedford, Dietrichson and I discussed this part of the enterprise with one of my English airman friends, Captain Johnstone, and we are most grateful for the assistance he gave us. The result of the discussion was that we chose a steering compass as well as a standard compass of an up-to-date type made by the firm of Hughes & Son, London. These compasses are made to repel movement, and to bring the needle slowly back to its correct position without the slightest oscillation either to the right or left. In the Arctic Sea, where the horizontal component of the earth’s magnetism is proportionately weak, it must always take time for the needle to swing back into position as it is so strongly repelled by existing conditions. But we preferred this to one with a lengthy oscillation and a big swing backwards and forwards. Steering compasses of the above kind are eminently suitable on account of a special construction which it will take too long to describe here. The standard compass was excellent. The magnetic condition in the navigation compartment was also ideal. The deviation’s coefficiency was shown by the readings we took to be so trifling that we could consider our compasses free from deviation. Just before leaving Spitzbergen we had one of the German Ludoph-compasses sent to us, with a request for us to give it a trial. I placed it in the pilot’s compartment of N 25, where it proved itself to be an excellent compass. If the machine heeled over the dial also took a certain tilt and the vertical component of the earth’s magnetism caused considerable oscillation as the natural result of its great attraction. Whilst the Ludoph-compass oscillated somewhat, the other took some time to swing back, making it impossible for me to say which I preferred. I steered with both of them, controlling the one by the other. During the homeward flight I continually steered by the magnetic compasses, and had no difficulty so long as I could have a “Landmark” ahead. During the fog it was not such an easy matter.
A/G Gyrorector, Berlin, kindly placed at our disposal a gyroscopic apparatus for each machine—as a loan. This instrument commended itself to me and is the best I have seen hitherto for flying in fog or darkness. The rising and tilting indicator was of use to me during the whole flight. The conditions, however, were such that I did not have to make great use of the direction indicator, beyond the fact that on the northward flight I experimented with it in case we should find it necessary at some time to make a forced landing in the fog. The arrangement between the two planes was that at all costs, if we should pass through fog, not to get separated from each other. At the close of the homeward journey, as mentioned elsewhere, we flew into such thick fog that I could have made use of the direction indicator. We flew, however, so low there that the whole time I had to keep my eye glued to the ice beneath and in front of us.
We had ordered a wireless installation for N 24, but went without it as it was not ready in time. It was the only thing we went off without. We never missed it. I might mention here that we had laid down a principle not to wait at all for any belated goods.
After seeing that many different suppliers, at home as well as abroad, should despatch the goods in time to reach Tromsö, to be loaded by a certain date, I got endless notices to say the goods would be belated and that we must put off our flight some days. The answer was always the same: “We shall go without goods if they have not arrived.” The result was, except in the case of the wireless, that everything was delivered in good time. Had we once started to put off our departure we should have had constant delays.