Knowledge for the Time / A Manual of Reading, Reference, and Conversation on Subjects of Living Interest, Useful Curiosity, and Amusing Research

by John Timbs

Why the Earth is presumed to be solid.

Besides the confirmation of some of the most material points of the theory of gravitation which results from the experiment of “Weighing the Earth,”[16] it furnishes a presumption of the strongest kind that the earth is solid to the centre, and not, as many have supposed in every age, a hollow shell. The mean density, 5⅔, is very much greater than that of the substances which abound at the surface. All common rocks are under 3, and nothing under the ores of the heaviest metals comes up to 5⅔. The earth is as massive as if it were all composed of silver ore, from the centre to the circumference, so that there must be an increase of density towards the centre. If those who think the earth to be a shell were to presume that its solidity ceased at 500 miles below the surface, they would then be compelled to give to the terrestrial matter, one part with another, a density greater than that of mercury, in order that the whole shell, the hollow part included, might have the mean density which is found by this experiment.—Penny Cyclopædia.

The Centre of the Earth.

Lt.-Col. Sir Henry James writes to the Athenæum as follows:—In verifying on a globe the interesting fact stated by Sir John Herschel, in his Outlines of Astronomy, and by Sir Charles Lyell, in his Principles of Geology, that the central point of the hemisphere which contains the maximum of land, falls very nearly upon London, or more exactly upon Falmouth, our most western port of departure for all parts of the habitable globe, it occurred to me to inquire what would be the central point of that portion of the globe which should include the whole of Europe, Asia, Africa, and America; and I found that the point lies in lat. 23° 3´ on northern tropical line, and in 15° E. long., near a place called Ghad in Africa, about 700 miles south of Tripoli. But the portion of the globe which, from this point as a centre, includes the so-called four quarters of the world is as near as possible two-thirds of the surface of the sphere; and I found that by projecting this portion of the sphere upon a plane drawn parallel to the great circle of which the above defined centre was in the pole and at 20° from it, and from a point in the prolongation of the axis of this great circle distant one-half of the radius from the surface of the sphere, that the whole of the four quarters of the globe could be represented on one strictly geometrical projection. I have had this projection made by Mr. J. O’Farrell, one of the highly intelligent assistants of the Ordnance Survey. I believe this is the first time that two-thirds of a sphere has been presented to the eye at one view.

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.

Identity of Heat and Motion.

George Stephenson’s remark, that the sun is the agent that drives our locomotives, has attained a wider and more definite meaning from modern investigations. It is now known, not only that heat and motion are mysteriously related, but that they are the same thing. From the researches of Mayer and Joule, Thomson and Rankine, it is ascertained that so much heat can be converted into so much motion, and the motion reconverted into the original quantity of heat. Sir William Armstrong says that a degree of Fahrenheit in a pound of water is the same thing as the force required to lift 772 pounds a foot high, thus testifying to the final and exact establishment of the largest generalization which modern science has made; and among the many fruits which cannot but flow from the discovery, one of the earliest is its application, by Sir William Armstrong himself, to test the waste of power in artillery practice, by observing the heat called forth in the shot. Every degree of temperature added to the projectile is part of the force intended to destroy the target; and if it is asked what material makes the most effective cannon-ball, it is only necessary to ascertain what substance will keep coolest when it strikes the mark. It is observable that the convertibility of heat and motion opens up a new light into the ultimate constitution of matter. The marvellous experiments of Professor Tyndall on the power of the minutest films of gas and vapour to absorb heat, as a dark glass stops light, are equally interesting as valuable contributions to meteorology, and as a new mode of probing the molecular condition of the gases themselves. The laws of the variation of atmospheric temperature were unfathomable until it was discovered that the habitable quality of the earth depends on the floating vapour which clothes it, and which keeps it warm in exactly the same way as the coverings by which we protect our bodies from the inclemency of the weather; but the significance of these experiments goes far beyond the limits of a single branch of science, and again we seem to be hovering on the verge of large revelations as to the ultimate arrangement of the particles of matter.

It is in the development of new powers of testing the infinitesimal, and carrying research immeasurably beyond the coarse limits of microscopic vision, that the strength of recent effort has been displayed. The most startling result of this form of investigation is the insight which has been gained into the materials and the condition of the luminous atmosphere of the sun. It could scarcely have been anticipated that the nature of a body separated from us by millions of miles should have been discovered by experiments which deal with qualities hidden in the inconceivably minute dimensions which express the form and distances of what, for want of better knowledge, may still be termed the ultimate atoms of material substances; and yet it was by testing the light-stopping power of thin films of different vapours, that philosophers have felt themselves entitled to say that some of the same substances which we are familiar with on earth have contributed to the atmosphere of the sun.—Saturday Review.