EARTHQUAKE MOTION DEDUCED FROM OBSERVATION. 83 



pedestal, in consequence of its inertia it would be left 

 behind by the forward motion of the shock, and simply 

 drop at the foot of its support. In the case of frictional 

 adherence it would be carried forward by the velocity ac- 

 quired before this adherence was broken, and thrown in a 

 direction opposite to that given in the figure — that is to 

 say, in the direction of the shock.' 



The Absolute Intensity of the Force exerted by an 

 Earthquake, — No doubt it has occurred to many who have 

 experienced an earthquake that the power which gave 

 birth to such a disturbance must have been enormously 

 great. The estimates which we shall make of the abso- 

 lute amount of energy represented by an earthquake 

 cannot, on account of the nature of the factors with 

 which we deal, be regarded as accurate. They may, how- 

 ever, be of assistance in forming estimates of quantities 

 about which we have at present no conception. One 

 method of obtaining the result we seek is that which was 

 employed by Mallet in his calculations respecting the 

 Neapolitan earthquake. Although disbelieving in the 

 general increment of temperature as we descend in the 

 earth at an average rate of 1° F. for every fifty or sixty 

 feet of descent, for want of better means. Mallet assumes 

 this law to be true, and, knowing from a variety of 

 observations the depth of various parts of the cavity from 

 which the disturbance sprang, he calculates the tem- 

 peratures of this cavity in various parts as due to its 

 depth beneath the surface. Next, it is assumed that 

 steam was suddenly admitted into this cavity, which might 

 exert the greatest possible pressure due to the maximum 

 temperature. This was calculated as being about 684 

 atmospheres. 



* The solution is taken from Mallet's Account of the Neapolitan 

 Earthqualte^ vol. 1. p. 155. 



