88 INTERNAL DENSITY OF THE EARTH. [SECT. x. 



the mighty revolutions which have swept innumerable races 

 of organized beings from the earth, which have elevated 

 plains and buried mountains in the ocean, the rotation of 

 the earth and the position of the axes on its surface have 

 undergone but slight variations. 



The strata of the terrestrial spheroid are not only concen- 

 tric and elliptical, but the lunar inequalities show that they 

 increase in density from the surface of the earth to its centre. 

 This would certainly have happened if the earth had origin- 

 ally been fluid, for the denser parts must have subsided 

 towards the centre as it approached a state of equilibrium. 

 But the enormous pressure of the superincumbent mass is a 

 sufficient cause for the phenomenon. Professor Leslie ob- 

 serves that air compressed into the fiftieth part of its volume 

 has its elasticity fifty times augmented. If it continues to 

 contract at that rate, it would, from its own incumbent 

 weight, acquire the density of water at the depth of thirty- 

 four miles. But water itself would have its density doubled 

 at the depth of ninety-three miles, and would even attain 

 the density of quicksilver at a depth of 362 miles. De- 

 scending therefore towards the centre through nearly 4000 

 miles, the condensation of ordinary substances would surpass 

 the utmost powers of conception. Dr. Young says that steel 

 would be compressed into one-fourth and stone into one- 

 eighth of its bulk at the earth's centre. However, we are 

 yet ignorant of the laws of compression of solid bodies be- 

 yond a certain limit ; from the experiments of Mr. Perkins 

 they appear to be capable of a greater degree of compression 

 than has generally been imagined. 



But a density so extreme is not borne out by astronomical 

 observation. It might seem to follow therefore that our 

 planet must have a widely cavernous structure, and that we 

 tread on a crust or shell whose thickness bears a very small pro- 

 portion to the diameter of its sphere. Possibly, too, this great 

 condensation at the central regions may be counterbalanced 

 by the increased elasticity due to a very elevated temperature. 



