Xl INTRODUCTION. 



solution of such questions as may engage the attention of our 

 readers. 



LESLIE computes that air would become as dense as water 

 at the depth of 33 j miles ; it would even acquire the density 

 of quicksilver at a further depth of 163| miles ; and he hence 

 concludes with the probability that the ocean may rest on a 

 bed of compressed air. Water at the depth of 93 miles would 

 be compressed into half its bulk ; at the depth of 362.5 miles 

 it w r ould acquire the ordinary density of quicksilver. Even 

 marble itself, subjected to its own pressure, would become twice 

 as dense as before, at the enormous depth of 283.6 miles. But 

 air, from its rapid compressibility, would sooner acquire the 

 same density with water, than this fluid would reach the con- 

 densation of marble. 



For the coincidence of air and water the depth is 35.5 miles ; 

 for equal densities of water and marble 172.9 miles. At the 

 depth of 395.6 miles, or one-tenth the radius of the earth, air 

 would attain the density of 101960 billions ; while at the same 

 depth water would acquire but the density of 4.3492, and marble 

 only 3.8095. At the centre of the earth, the density of air would 

 be expressed by 764 with 166 ciphers annexed ; while water 

 would be condensed three millions nine thousand times its bulk 

 at the surface of the ocean ; and marble would acquire the 

 density of 119. The inference is, that if the structure of our 

 globe were uniform, and its mass consisted of such materials as 

 we are acquainted with, its mean density would far surpass the 

 limits assigned by Astronomy. Now both Dr. Maskelyne and 

 Cavendish nearly concur in representing the mean density at 

 only about five times greater than that of water. Leslie is 

 thence of opinion, that our planet must have a vast cavernous 

 structure, the crust of which, for aught we know to the 

 contrary, may cover some very diffusive medium of astonishing 

 elasticity, as light, which when embodied constitutes elemental 

 heat or fire.* 



* Elements of Natural Philosophy, vol. i. pp. 447457, second edition. 



