ON THE PHYSICAL STRUCTURE OF THE EARTH. 



205 



The rate v of trausmisnion of soiuul iu solids aud liquids is a fuuc- 

 tiou of tbeir compressibilities. Iu solids, 



V p 



P 



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



^1 



=^^ 



MPi 



where ju is the coefficient of cubic compressibility, H the pressure of 

 the atmosphere, aud a the density of mercury. But as iu solids the 

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





kpHa 



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

 square roots of the densities and compressibilities. Although such 

 solids as metals and rocifs are denser than 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 fouud to be greater in coutinuous homogeneous 

 rock than the velocities observed in liquids.* 



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

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

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

 couclusion from the relative dilatability of such substances under dif- 

 ferences of temperature.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 Philosophioal Transactions for 1861 and 1862. 



\ Expansions of metals and glass for 1° C, according to Dulong and Petit, at different 



temperatures T. 



