78 INTERNAL DENSITY OF THE EARTH. SECT. X. 



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 observes that air compressed into 

 the fiftieth part of its volume has its elasticity fifty times aug- 

 mented. 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. 

 Descending 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 beyond 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 proportion to 

 the diameter of its sphere. Possibly, too, this great condensa- 

 tion at the central regions may be counterbalanced by. the in- 

 creased elasticity due to a very elevated temperature. 



