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ONS eS 
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ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 91] 
Unless, possibly, in the case of nearly vertical emergence, and from the most 
solid, and elastic crystalline rock, an ample latitude, ¢, is secured by the ver- 
tical spring. 
We will now consider the movements of the element balls B and B, along 
the planes 2, ¢, due to the horizontal component of motion, taking the two in- 
struments (viz. the N. S. and E. W. seismometers) together, and assuming the 
horizontal component in any azimuth 0. 
The blocks gv (N.S.) and g7 (E. W.) move forward horizontally, and 
force on the balls B and B, before them until the instant, . when the blocks 
have acquired their maximum velocities, with that of the wave,v; the balls then 
part company from the blocks, and continue to move up along the respective 
inclined planes 2, i, sliding for the first indefinitely short moment, and then, 
with a certain reduction of velocity due to the friction of the planes which 
produce the change of motion, rolling up along them. This initial sliding 
velocity will be 
For the ball B ... V=v sin 0; 
For the ball B,... V=vw cos @. 
As soon as the sliding is converted into rolling motion by friction, these 
velocities will become 
Bie at. 5 
—v sin 0, and — v cos @. 
7 UNO 
Assuming that the change takes place almost instantly after the balls have 
begun to move from the blocks, i.e. that gravity has not had time perceptibly 
to alter the velocity up the plane, and neglecting the small effects, due to the 
elastic compression of the -balls and blocks themselves, and also supposing 
that the loss of velocity of the ball, by conversion of its sliding into rolling 
motion by friction, is less than the diminution of velocity of the block (in the 
same short time), in returning from its maximum velocity to rest, the balls 
B and B, will be retarded by forces— 
i being the common inclination of the planes. 
The ball B will therefore ascend upon its plane to a vertical height 
we have therefore 
So also the ball B, will ascend to the height 
v cos = J ont: 
tan a=, /H 
HY 
therefore 
