218 EAETHQUAKES. 



employed, somewhat differently, when speaking of the 

 effects produced on buildings. 



For places, like s and p, situated at equal distances 

 from the seismic vertical, it is evident that the intensity 

 of the shock will be different, and also its time of arrival. 

 It will also be observed that the isoseismals will take the 

 form of ovals or distorted ellipses, the larger or fuller end 

 of which being to the left of the fissure. 



Other cases, like those just given, which are dis- 

 cussed by Mallet in his account of the Neapolitan earth- 

 quake, are where the fissure forms the division be- 

 tween materials of different elasticities. In the hard and 

 more elastic material the waves will be more crowded, the 

 velocity of a wave particle will be greater, and the transit 

 will be quicker than in the less elastic medium. 



The result is that the distance of equal effect from the 

 seismic vertical will be greatest in the direction of the 

 more compressible material. 



Unless these considerations are kept carefully before 

 the mind when investigating the depth and, we may add, 

 the position and form of the centrum of an earthquake, 

 serious errors may arise. 



Greatest depth of an earthquake origin. — A curious 

 but instructive calculation which Mallet made was a 

 determination of the greatest possible depth at which an 

 earthquake may occur. This calculation is based upon 

 the idea that the impulsive effect of an earthquake has 

 an intimate relationship with the height of neighbouring 

 volcanoes, the column of lava supported on a volcanic 

 cone being a measure of the internal pressure tending to 

 rupture the adjacent crust of the earth. 



Michell, in 1700, virtually propounded this idea, when 

 he suggested that the velocity of propagation of an earth- 

 quake was related to the height of such a column.^ 

 ' Phil. Trans, vol. li. 



