ON THE PHYSICAL HTRUCTURE OF THE EARTH. 



205 



The rate v of transmissiou of souud iu solids aud liquids is a fimc- 

 tiou of their compressibilities. In solids, 



V p 



where E is the modulus of elasticity and p the density. In liquids, 



^' /'Pi 

 where /^ is the coefficient of cubic comj^ressibility, H the pressure of 

 the atmosphere, and a the density of mercury. But as in solids the 

 modulus of elasticity is inversely as the compressibility A-, we have 





pHa 



Both in solids and licjuids the velocity of sound is inversely as the 

 square roots of the densities and compressibilities. Although such 

 solids as metals and rocks are denser thiin most liquids, the limits of 

 their elastic compressibility are so much less that sound is propagated 

 far more quickly through such solids than through liquids. In steel 

 and metals generally this has been long since established. In rocks 

 the velocity of sound has been computed from direct experiment by 

 Mallet, and has been found to be greater iu continuous homogeneous 

 rock than the velocities observed in liquids.* 



(4) If we had not the results of direct experiment on the comi)ressi- 

 bilities of liquids and solids to assure us that these properties in liquids 

 are in excess of those obtained for solids we might fairly infer this 

 conclusion from the relative dilatability of such substances under dif- 

 ferences of teraperature.t The construction of our common thermome- 

 ters is based on the greatly superior dilatability of the liquids inclosed 

 in the thermometer-tube over the material of the tube itself. The 

 dynamical theory of heat clearly establishes that the expansion of 

 solids and liquids is a mechanical action as much as their compression 

 under the action of force, and the substances which contract least by 



* See Philosophical Transactions for 1861 and 1862. 



\ Expansions of mctaJs and (jlass for 1" C, accordiruj to Ditlong (i7id relit, at different 



temperatures T. 



