igoS.] THE PHYSICS OF THE EARTH. 179 



Structure. It is evident that at depths such as twenty miles the 

 downward movement of the fluid would continue, though very 

 slowly. Hence the leakage of the oceans is extremely gradual, and 

 the recurrence of earthquakes visibly delayed after relief has once 

 been obtained. Thus while the tightness of the earth's crust due 

 to the grain of the rock and the pressure to which it is subjected in 

 the lower parts does not prevent ocean leakage, it makes the process 

 so slow and gradual as to afford considerable protection to life upon 

 our planet. 



II. On the Physical State of the Earth's Interior, on the 

 Average Rigidity of the Globe as a Whole, and on the 

 Substratum of Plastic Matter Beneath the 

 Crust which in Earthquakes Be- 

 haves AS Fluid. 



§ 12. On the Theory of a Fluid Globe Held by the Older Geolo- 

 gists, and on Hopkins' Argument for Solidity Based on the Phe- 

 nomena of Precession and Nutation. — In the early part of the nine- 

 teenth century it was generally believed by geologists that the earth 

 was a liquid globe covered by a rocky crust much thinner in pro- 

 portion to the diameter than the shell is to that of an egg. This 

 supposed liquid interior had been suggested by the streams of molten 

 lava often observed to issue from volcanoes, and by the igneous 

 rocks so abundantly poured forth in many places. The theory of a 

 fluid globe seemed to be confirmed by the observed increase of 

 temperature downward, which would give rise to molten rock at a 

 depth of some twenty miles. The mountains and other phenomena 

 traceable to dislocations of the crust could all be explained by a solid 

 layer of this thickness, and the natural inference was that the great 

 central nucleus remained liquid. The consolidation of the globe 

 was ascribed to the progress of secular cooling, from the primitive 

 state of high temperature assumed by Laplace in the nebular hy- 

 pothesis postulated for explaining the origin of the solar system. 



The older geologists had not adequately considered the efifects 

 of pressure in augmenting the solidity of the globe as we go down- 

 ward; for since pressure raises the melting point of solids, the 

 matter of the nucleus, though highly heated, might be solid if the 



