46 EARTHQUAKES. 



which we may liken to the transmission of an earthquake 

 wave between two distant localities on the earth's surface, 

 and the other being the up and down motion of our 

 weight, which we may compare to the backward and for- 

 ward swinging which we experience at the time of an 

 earthquake. 



These two motions — namely, the pulse-like wave pro- 

 duced by the transmission of motion, and the backward 

 and forward oscillation of the weight or of any point on the 

 spring — must be carefully distinguished from each other. 



First, we will consider the backward and forward motion 

 of the weight. The distance through which the weight 

 moves depends upon the force of the blow. The number 

 of up and down oscillations it makes, say in a second, 

 depends upon the stiffness of the spring. The weight, 

 supposing it to be always the same, will move more quickly 

 at the end of a stiff spring than at the end of a flaccid 

 one ; that is to say, its velocity is quicker. As in any 

 given spring the number of up and down oscillations are 

 always the same in a given interval of time, if these 

 oscillations are of great extent, the weight must move 

 more quickly with large than with small oscillations. 



At the time of an earthquake the manner in which 

 we are moved backward and forward is very similar to the 

 manner in which the weight is moved. If we stand on a 

 hard rock- like granite, we are to a great extent placed as 

 if we were attached to a stiff quickly-vibrating spring. If, 

 however, we are on a soft rock, it is more like being on a 

 loose flaccid spring. 



All that has thus far been considered has been a back- 

 ward and forward kind of motion, where there is a recti- 

 linear covipression and extension amor gst the particles on 

 which we stand. 



We might, however, imagine oar rock, which for the 



