3IE. IHALLET THE TEAJTSIT-YELOCITY OE EABTHQTJAKE WAVES. 677 
intimate composition. It remains to show, experimentally, that they do not differ in thesQ 
conditions of transmissive power, to such an extent as materially to affect the results. 
If a perfectly elastic ball be dropped upon a mass of perfectly elastic rock, whose 
volume may be considered as infinite with respect to that of the ball, the latter will 
rebound to the height from which it descended ; and if the same ball, though not per- 
fectly elastic, be dropped in succession npon like masses of two different rocks, it will 
rebound from each to a height less than that from which it fell, and the value of which 
will depend mainly upon the elasticity, the depth of the impression, and the degree of 
discontinuity of the rocks respectively. We have therefore thus got the means of very 
simply determining, in a sufficiently approximate manner, the relation between the velo- 
city 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 biUiard 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 It be the height from which the ball has fallen and that to which it 
rebounds, tlien 
which may be \iewed as a symbol of the above relation, and closely connected with the 
wave-retardations respectively. In the quartz rock I obtained the following results : — 
From the hardest and densest blocks or masses, and edgeways to the lamination, the 
ball recoiled 2‘33feet; v is therefore =S\//i=12‘25I feet per second. 
From the softer and more earthy masses, and transverse to the planes of lamination, 
the recoil was 1’50 feet, and ?;=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 
^J=1I’341 feet per second. 
From the least hard and dense, and transverse to the planes of foliation, the recoil was 
1’417 feet, and v=9’546 feet per second. 
The mean value for the quartz rock is thus 
v= 2 =11-036 feet per second; 
and for the slate rock, 
1 1 + ^ ^•^*’ — 20-443 feet per second; 
