﻿Vol. 66. ,] AN EARTHQUAKE MODEL. 347 



movement will proceed with increasing velocity, until it meets with 

 an obstacle or the position of equilibrium is passed. In the former 

 case it will be brought to a standstill with a sudden jar made up 

 of numerous short-period vibrations. In the latter it will either 

 continue with diminishing velocity, until it is arrested by an obstacle 

 in the manner already described ; or, what is probably a rare occur- 

 rence, the swing will be prolonged until the accumulation of negative 

 acceleration due to the elasticity of the rock exactly cancels the 

 velocity at the position of equilibrium. In either event it will 

 return to this position, about which it will swing to and fro until 

 it comes to rest under the influence of friction, which will of course 

 profoundly affect all the movements that have been described. 



It is, I believe, the short-period vibrations due to the 

 sudden check of the fault-movement when an obstacle 

 is encountered, which constitute the earthquake pro- 

 perly so called, and are the cause of its destructive 

 action. The fracture which initiates the movement doubtless 

 gives rise to minor vibrations, and the same is the case with the 

 friction between the rocks as they brush past each other. To 

 the latter action must also be attributed the grating or rumbling 

 sounds which are often described as heralding a violent shock. 



There still remains to be considered the question of the long- 

 period vibrations which form the more important portion of the 

 records of distant earthquakes, and are separated from the shorter 

 vibrations that precede them by their inability to traverse the 

 highly-compressed deeper portions of the earth's interior. They 

 may very well be attributed to the backward and forward swing 

 of the severed rocks about their position of equilibrium, although 

 it is possible that in some cases they may represent beats due to the 

 interference of vibrations of shorter period. 



The strength of the earthquake will evidently depend largely 

 on the degree in which the rock has retained its elasticity. If 

 the creep be very slow, the rock may be able to adjust itself to 

 the new conditions, and there will be no fracture and no shock. 

 A similar result may ensue, if the rocks are at a high temperature 

 or under great pressure. If, on the other hand, they are more 

 or less incoherent, they will possess no elasticity, and fracture may 

 occur without a shock ensuing. In the same manner movement 

 may occur without shock along a pre-existing fault, the sides 

 of which do not adhere to each other. In these cases, however, the 

 movement will probably be of the same slow character as that of 

 the main rock-masses on each side of the fault. 



The model which I have designed to illustrate these principles 

 has been constructed by my cousin, Mr. Frederic J. Bakewell, 

 electrical and mechanical engineer, to whom I am much indebted 

 for his ingenuity and resource in overcoming the difficulties that 

 presented themselves. 



It consists of two vertical rectangular frames (f t & / a in the photo- 

 graphs — figs. 1, 2, & 3, p. 318) mounted side by side on a common 



2b 2 





