42 



NATURE 



\_May 9, 1878 



been made for this purpose. From these measure- 

 ments, there was no doubt that the earth is very 

 nearly a sphere of 8,000 miles in diameter, or 25,000 

 miles in circumference. When he spoke of the sur- 

 face of the earth, it must be understood that he 

 spoke of the sea-level. Above that level stand moun- 

 tains, and below are the depths of the sea. But 

 although these inequalities of surface are taken into 

 consideration by those who go accurately into calcula- 

 tions, they are comparatively very small. Suppose he 

 were to make a sphere twenty-five feet in diameter, re- 

 presenting the earth, how much did they think the moun- 

 tains would rise above the level ? One-fifth of an inch. 

 Well, of course that never could be seen ; and it was a 

 thing that in all ordinary calculations might be neglected. 

 So that they might say that the earth was a sphere, with 

 an exception he would mention presently. Then there 

 was another thing which was important to their present 

 subject, and that was the density of the matter of which 

 the earth is formed ; and this was a matter which had 

 engaged the best experimenters in two or three ways. 

 The first of the experiments of this kind is a very cele- 

 brated one known as the Schihallien experiment, so 

 called from its being an experiment on a mountain 

 in the Scottish Highlands (Perthshire) particularly 

 adapted to these measurements, which are most favour- 

 ably carried on in the north and south direction. It was 

 found that the mountain Schihallien disturbed the plumb- 

 line, causing a deviation from 'the vertical of 11" or 12". 

 Then if that mountain, whose dimensions we can measure, 

 turns the plumb-line so far, what is the proportion of its 

 attracting mass to the attracting mass of the earth ? And 

 as we know. the size of the mountain and the size of the 

 earth, we can compare the density of the mountain and 

 that of the earth. This process was gone through with 

 great care, and it was found that, taking the density of 

 the mountain as we could trace it by its constituent rocks, 

 the density of the earth would be about four and a half 

 iimes that of water, or about twice the average density of 

 the surface rocks. The earth had density everywhere, 

 but was more dense towards the centre than the outside. 

 The next experiment is known as the Cavendish experi- 

 ment. Here was a very light rod of deal, six feet long, 

 suspended by a fine copper or silver wire (which is the most 

 delicate suspension we can have) forty inches long, within 

 a wooden case to defend it from currents of air. At 

 each end of the lever was hung a ball two inches in 

 diameter, and by a simple contrivance a pair of leaden 

 spheres, weighing together perhaps 3oolbs., were brought 

 simultaneously into the neighbourhood of the balls (but 

 outside the case), on opposite sides, so that they might 

 attract the small balls ; and the experiment was varied 

 until, by a series of calculations, the density of the earth 

 was ascertained, and gave a greater result than before, 

 naniely, that the average density of the earth was about 

 5| times that of water. Then the third experiment was 

 one which he made himself in the Harton colliery, near 

 South Shields. That was by seeing how much the force 

 of gravity was altered by going to a great depth, the force 

 of gravity being ascertained and compared at the top 

 and bottom by the swinging of a pendulum. From that 

 a calculation was made, and it gave the density of the 

 earth as six times that of water. He believed the best 

 calculation was that founded upon the Cavendish expe- 

 riment, and was quite willing to take something like 5^ 

 times the density of water as the average density of the 

 earth, including every part of it. There were conse- 

 quences which followed from that which were certainly very 

 striking. As this density was rather more than double that 

 of the surface rocks, it showed that towards the centre 

 the earth was more condensed than at the outside. But 

 there was one result of the calculation which rather 

 startled hini when he made his own experiment on the 

 subject. Since these rocks press upon each other more 



and more the further you go down, what is the pressure 

 upon the square inch when you approach the centre of 

 the earth ? Many gentlemen there would have heard of a 

 pressure of 50 lbs. or 100 lbs. on the square inch, and per- 

 haps the greatest pressure we know is that by which tough 

 Aberdeen granite is crushed — 10,000 lbs. to the square 

 inch. But it must be 30,000,000 lbs. to the square inch 

 in the centre of the earth ; and it is an astounding thing 

 to imagine what consequences may follow. We have no 

 idea of any such degree of pressure, and cannot there- 

 fore conceive what its consequences may be. Perhaps 

 thereby gas may be squeezed into gold or platinum, and 

 powder to solid, or solid to powder — we cannot tell what 

 it does. That enormous pressure, and our total ignorance 

 of it, is one of the difficulties and troubles of this case. 

 He thought the general state of the earth would be under- 

 stood from what he had said, and now he came to the 

 rotation of the earth. The earth revolves, as everybody 

 knows, in the course of a day ; and everybody knows also, 

 from the housemaid who whirls her mop to the greatest 

 philosopher, that rotation will swell out the middle of the 

 earth. Calculations have been made upon that, and the 

 result is that the diameter of the earth in the equatorial 

 direction is greater by about i -300th part than the diameter 

 in the polar direction. When they found that the mea-. 

 surement of the dimensions of the earth agreed so well 

 with that conclusion, it led them to the further conclusion 

 that the earth is, or has been, in a fluid state. In corro- 

 boration of this, he would mention a singular circum- 

 stance which occurred in our Indian Survey. In pro- 

 ceeding northward from Cape Comorin, the curvature of 

 the earth agreed very well for many hundreds of miles 

 with that found in other parts of the earth (with due 

 reference to the elliptic form of the earth). On approach- 

 ing the Himalaya Mountains, the plumb-line was sensibly 

 attracted by the mountains. The late Archdeacon Pratt 

 investigated, from the form of the mountains and the 

 density of the rocks, the disturbance of the plumb-line, 

 and found that it ought to be much greater than it really 

 is. Sir George explained this by supposing that the whole 

 of that country is floating upon a dense fluid, and that 

 the thick mass of the lighter mountain-matter sinks deep 

 in the fluid, and that the displacement of denser matter 

 neutralises almost entirely the attraction of the lofty 

 mountains. The form of the earth is not such as would 

 be taken by a solid structure, but such as would be taken 

 by a fluid mass with solids floating upon it. 



In the second part of his address. Sir George Airy 

 referred to what is known about the temperatures. 

 They knew something of the rate at which temperature 

 travels through the earth. The experiments on this point 

 had begun, as many good experiments have begun, with 

 the French, who fixed thermometers with very long stalks 

 to the depth of twenty-five feet in the ground. These 

 experiments were followed up, after some time, with 

 similar thermometers at the Observatory at Edinburgh, 

 and about the same time at the Observatory at Green- 

 wich, and there the deeper thermometers were read every 

 day. The first and most conspicuous result of these ex- 

 periments is the retardation of the seasons. At the depth 

 of twenty-five feet, high midsummer heat occurs at 

 December, which shows that it takes five months for the 

 heat to travel down that depth. If you compute it 

 further, it takes 100 years to travel a mile; so that if the 

 crust of the earth is 100 miles thick, it will take 10,000 

 years for the transmission of heat through it. This 

 showed that really, after all, we may have a great deal 

 of heat below us, and that it will not come to us for a 

 very long time. It will come at last, but it will come 

 travelling up slowly, and in the meantime the radiation 

 from the surface of the earth will carry it off very rapidly. 

 So that it is quite possible that with a cool surface there 

 may be a great deal of heat below. In every part of the 

 earth there is evidence of intense heat in former times. 



