52 EARTHQUAKES. 



to the rate of change of velocity in the body, and, as the 

 destructive effect will be proportional to the maximum 

 forces, we may consistently indicate the intensity of an 

 earthquake by giving the maximum acceleration to which 

 bodies were subject during the disturbance. On the 

 assumption that the motion of a point on the earth's surface 

 is simple harmonic, the maximum acceleration is directly 

 as the maximum velocity and inversely as the amplitude 



of motion, or as — where v indicates velocity and a 



amplitude. 



The next question of importance is to determine the 

 manner in which earthquake energy becomes dissipated- — 

 that is, to compare together the intensity of an earth- 

 quake as recorded at two or more points at different 

 distances from the origin. First let us imagine the 

 origin of our earthquake to be surrounded by concentric 

 shells, each of which is the breadth of the vibration of a 

 particle. Groing outwards from the centre, each successive 

 shell" will contain a greater number of particles, this 

 number increasing directly as the square of the distance 

 from the origin. Let the blow have its origin at the centre, 

 and give a vibratory movement to the particles in one of 

 the shells near the centre. 



This shell may be supposed to possess a certain amount 

 of energy, which will be measured by its mass and the 

 square of the velocity of its particles. In transferring this 

 energy to the neighbouring shell which surrounds it, 

 because it has to set in motion a greater number of par- 

 ticles than it contains itself, the energy in any one particle 

 of the second layer will be less than the energy in any one 

 particle in the first layer ; the total energy in the second 

 shell, however, will be equal to the total energy in the first 

 shell. Neglecting the energy lost durmg the transfer, if 



