58 



EARTHQUAKES. 



The object of tbe experiment was to find the nature 

 of the vibrations produced in the crust of the earth by 

 such a blow, the velocity of transmission through this 

 comparatively soft material, the effect of hills and excava- 

 tions in cutting off such disturbances, and the law accord- 

 ing to which the amplitude of the vibrations diminishes 

 with the distance from the source. 



A considerable variety of apparatus was used during 

 these experiments, but the most reliable results were 

 obtained from the records of a rolling sphere seismograph, 

 which wrote the vibrations on a stationary plate, and from 

 the records' of two bracket seismographs, similar to Pro- 

 fessor Ewing's horizontal lever seismographs, which gave 

 a record of the vibrations as two rectangular components 

 on a moving plate of smoked glass. 



Fia. 10. 



The general result as to the nature of the disturbance 

 was that two distinct sets of vibrations were set up by the 

 blow. In one set the direction of motion was along a 

 line joining the point of observation with the point from 

 which the disturbance emanated ; in the other set the 



