196 



) 



[Sect. VII. 



Section VII, 



ON OBSERVATION OF 



EAETHQUAKE PHENOMENA 



By 11. MALLET, A.B., Mem. Ins. C.E., M.R.LA. 



Whenever a blow or pressure of any sort is suddenly 

 applied, or tlie passive force of a previously steady or 

 slowly variable pressure is suddenly either increased or 

 diminislied, as these affect material substances, all of 

 which, whether solid, liquid, or gaseous, are more or less 

 elastic, then a pulse or wave of force, originated by such 

 an impuhe. is transferred, through the materials acted 

 on, in all directions from the centre of impuhe^ or in such 

 directions as the limits of the materials permit. The 

 transfer of such an elastic rvave is merely the continuous 

 forward movement, of a change in the relative positions, 

 of the Integrant molecules or particles, of a determinate 

 volume, of the whole mass of material. 



Ordinary sounds are waves of this sort in air. The 

 shaking of the ground felt at the passage of a neighbour- 

 ing railway-train is an instance of such waves in solid 



round or rock. A sound heard by a person under water, 



or th^^ ^hock felt in a boat lying near a blast exploded 

 under water, are examples of an elastic wave in a \u\md. 



The velocitv with which such a wave traverses, varies in 

 difterent materials, and depends principally upon the de- 





tib 



