The Cooling of the Earth
It is a generally received belief among geologists, that the centre of the earth is occupied by incandescent fluid matter, which is gradually but constantly losing its heat. Adopting this theory, which rests on mere conjecture, Professor William Thomson, in a paper published in the Transactions of the Royal Society of Edinburgh, endeavours to fix the date of the first consolidation of the globe, supposed to have been once in a state of perfect liquefaction. It is estimated that the temperature increases as we descend towards the centre of the earth, at the average rate of one degree of Fahrenheit per 50 British feet, or 105 degrees per mile. Our author admits the temperature of melting rock to be 7000 degrees; supposing, therefore, the surface of the earth to have been in a fluid state, its consolidation, he thinks, cannot have taken place less than 20,000,000 years ago, since we should otherwise have more underground heat than we actually have; nor more than 400,000,000 years ago, because in that case we should have much less. This, it must be allowed, is rather a wide range, and is a curious instance of the strange results which calculation affords when applied to a gratuitous hypothesis. Compared with the earth’s radius, which is 3958 miles, the depths to which we have been able to penetrate are utterly insignificant, and can afford no reliable data whatever; the more so, as by Professor Thomson’s own admission, the rate of increase of temperature decreases progressively.
Our author, moreover, in the course of his arguments, meets with difficulties, the importance of which does not seem to have escaped him, since he endeavours to remove them by some rather doubtful assertions. To those, for instance, who would object to the supposition that any natural action could possibly produce at one instant, and maintain for ever after, a 7000 degrees’ lowering of the surface temperature of the earth, he replies:—“I answer by saying, what I think cannot be denied, that a large mass of rock exposed freely to our air and sky will, after it once becomes crusted over, present in a few hours, or a few days, or at the most a few weeks, a surface so cool that it can be walked over with impunity.” Now we do confess ourselves very much inclined to deny such a proposition. What kind of mass does our author mean? Is it a small mass? then he need but visit a gun foundry, where he will find pieces of ordnance still hot though cast several days before. Or is it a large mass, like a mountain? The nearest approach to it would be lava, which remains hot for weeks after the eruption, and for any larger mass there is no evidence either in existence or possible. But the immense difficulty of the subject may be inferred from the fact, that Professor Thomson himself further down makes an admission which is fatal to his own view, viz., that “if at any time the earth were in the condition of a thin solid shell of, suppose, 50 or 100 feet thick of granite, enclosing a continuous melted mass of 20 per cent. less specific gravity in its upper parts, where the pressure is small, this condition cannot have lasted many minutes, since the rigidity of a solid shell of superficial extent so vast in comparison with its thickness must be as nothing, and the slightest disturbance would cause some part to bend down, crack, and allow the liquid to run out over the solid.” What then, we may ask, becomes of the liquid theory altogether?—Galignani’s Messenger.