The Evolution of the Sciences 



three and four seconds after its origin. If the 

 centre of disturbance were on the surface, and 

 if the ground were homogeneous, these positions 

 of the surface wave would describe round the 

 epicentre concentric and equidistant circles. 

 If, on the contrary, the origin of the disturbance 

 were at the very centre of the earth, the 

 wave would reach simultaneously every point of 

 the surface. In reality all cases occur between 

 these two extremes and much nearer the 

 first than the second. The distance be- 

 tween the circles of disturbance decreases 

 as they are removed from the epicentre; from 

 their positions it is possible to estimate 

 with some accuracy the original depth of 

 the disturbance. The method just mentioned 

 is, however, not the only one, and the ingen- 

 uity of geologists has multiplied methods and 

 observations concerning this very important 

 question^ 



Now, all observers are agreed regarding one 

 point, viz., that the centre of disturbance is 

 invariably situated within a short distance from 

 the surface. This is the result of one hundred 

 and fifty observations, made with the greatest 



134 



