EARTHQUAKE MOTION DEDUCED FEOM OBSERVATION. 79 



upon the focal depth of a shock, the form and position of 

 that focus, the duration of the disturbance, and the nature 

 and arrangement of the materials which, are shaken. 



From observations in Japan, it is clearly shown that 

 massive mountain ranges exert a considerable influence 

 upon the extension of seismal disturbances. On one side 

 of a large range of mountains large cities might be laid in 

 ruins, whilst on the other side the disturbance creating 

 this destruction might not be noticed. 



Velocity and Acceleration of an Earth Particle. — 

 We now pass on to methods of determining the intensity of 

 an earthquake which are less arbitrary than those which 

 have just been discussed. These methods have already 

 been discussed when speaking of artificial disturbances, 

 where it was shown that the intensity of an earthquake as 

 measured by its destructive effects greatly depended upon 

 the suddenness with which the backward and forward 

 motions of the ground were commenced or ended. 



Amongst the earlier investigators of seismic phe- 

 nomena who observed that there existed a connection 

 between the distance to which bodies had been projected 

 during an earthquake and the suddenness or initial 

 velocity with which the ground had been moved beneath 

 them, was Professor Wenthrop of Cambridge, Massachu- 

 setts, who noted that bricks from his chimneys had, by the 

 New England earthquake of 1755, been thrown thirty feet. 

 From this and the known height of the chimney, he calcu- 

 lates that the bricks had been projected with an initial 

 velocity of twenty-one feet per second.* 



The calculations made by Mallet respecting the 

 maximum velocity of an earth particle at the time of the 

 Neapolitan earthquake in 1857 depended upon the over- 

 throw, projection, and fracture of bodies. 



> Pliil. Trans., L., 1755. 

 5 



