ON THE TRANSIT-VELOCITY OP EARTHaUAKE WAVES. 221 



rocks respectlvelj% We have therefore thus got the means of very simply 

 determining, in a sufficiently approximate manner, the relation between the 

 velocity of impact and that of recoil, a quantity that bears the most intimate 

 relation to the wave-transmissive power of rocks or other like bodies. To 

 conduct this experiment I dropped an ordinary ivory billiard ball upon a 

 number of different masses of the quartz-rock, and also of the slate, both 

 in situ, and upon very large isolated blocks, making the impacts both 

 transverse to the stratifications and foliation and in the same planes as these, 

 in both sorts of rock. The ball was dropped from a constant height of 

 5 feet above the point of impact, and beside a graduated scale held vertically 

 by an assistant, by means of which, after a little practice, and skill in 

 choosing by trial a point of impact, from which the ball shall rebound 

 vertically only, it is easy to observe with considerable accuracy the height 

 to which it recoils, the eye being gradually brought to the same level as that 

 to which the ball rises, so as to read the scale free from parallax. 



If H and h be the height from which the ball has fallen and that to which 

 it rebounds, then 



^^^^^ = -=R, 



which may be viewed as a symbol of the above relation, and closely con- 

 nected with the wave-retardation respectively. In the quartz-rock I obtained 

 the following results : — 



From the hardest and densest blocks or masses, and edgeways to the lami- 

 nation, the ball recoiled 2"33 feet; v is therefore =sVJi=12-25l feet per 

 second. 



From the softer and more earthy masses, and transverse to the planes of 

 lamination, the recoil was 1*50 feet, and 2;=9"822 feet per second. 



And in the slate-rock, — 



From the hardest and densest, edgeways to the foliation, the ball recoiled 

 2*00 feet, or V=11'341 feet per second. 



From the least hard and dense, and transverse to the planes of foliation, 

 the recoil was 1'417 feet, and 2;=9'546 feet per second. 



The mean value for the quartz rock is thus 



12-25H-9-822 „ „,,. ^ , , 



v= ^ =lr036 feet per second ; 



and for the slate rock, 



11-34.1+9-546 ,^,,„^ 

 ^_ _ =10443 feet per second ; 



and as H = 5 feet, V =17*935 feet per second, we have 



and 



1 0*443 

 R^= ,j„ i =0'576 for the slate, 



T? 11 'O^fi 



^=, ^.QQ- =0'553 for the quartz, 



numbers which differ so slightly from equality as to indicate that there is 

 no great difference of transmissive power in the two rocks. Indeed this is 

 rendered certain by consideration of the experiments themselves. Previously 

 to their commencement I expected that in every instance the range in quartz 



