90 REPORT—1858, 
fectly parallel and smooth; and the length of the planes must be such, as to 
make it highly improbable that any ball, in its excursion under shock, can 
reach the upper end. A wood stop is fixed at this point to arrest the ball, 
should it ever chance to reach it; and beyond this a stout net (like the purse of 
a billiard-table) may be fixed to a separate support (from the floor), to receive 
the ball, if upon an extraordinary occasion thrown out of the instrument. 
It is assumed that any alternate alteration of the inclination, of the inclined 
planes 7, i, by actual swrface-wndulation, carrying the whole instrument with 
it at the passage of the earth-wave, may be neglected, z.e. that, for example, 
a wave passing in a direction from 8. to N. will not sensibly lift up the S. end 
(of the N.S. instrument) first, and then the N, end, and so first increase the 
inclination of the plane of B, and reduce that of B, and then vice versd; 
and that whatever amount of ¢é/ting may thus occur will so momentarily 
affect the inclined planes, and in opposite directions, as not to interfere 
with the proposed movements of the balls. 
This assumption is justified by the fact that the value of A, the amplitude 
of the earth-wave in the normal, is always great in relation to its altitude, 
and in the case of oblique surface-emergence its horizontal component is of 
still greater length; so that the angle of slope of either face of the emergent 
wave with the horizon, is practically imperceptible in moderate shocks; and, 
further, any tilting that can occur takes place in opposite directions suc- 
cessively, so as nearly to compensate. 
The vertical spring e must be delicate and sensitive, at the first instant of 
its compression, in proportion to the movement by inertia of the large mass 
that it carries, and its range, proportioned to the degree of steepness of 
emergence to be expected in the region of observation. 
The whoie vertical component is absorbed by this spring, and may be mea- 
sured by its compression ; but it is important that it shall give way sensitively, 
at the first moment of shock, in order that neither of the balls shall have any 
tendency to rise from the inclined planes that support them, and that its resili- 
ence shall not be too lively, so as not to produce rebound upon the restoration 
from compression. In certain seismic regions, where great steepness of 
emergence may be looked for, the vertical component will probably be best 
met by the depression of a conical float with the apex downward, fixed to 
the lower end of the bar 04, into a cylindrical vessel of water placed beneath 
the instrument; but this must be matter of experiment in such regions. 
Were the whole instrument rigidly fixed to the ground, the latter as well 
as the materials of the instrument and ball highly elastic, and the velocity 
of emergence of the wave, in its vertical component, very great, it is obvious 
that time would not be afforded to the ball B, merely to roll up along the 
plane; it would be ¢hrown up obliquely from it, aud, describing a short trajec- 
tory, would fall back again upon the plane a little higher up, and then re- 
peat a still shorter trajectory, or begin to roll upwards. But the ball is very 
inelastic, the rate of emergence of the wave is not ‘very great in its vertical 
component; and the effect of this upon the instrument is spread over a still 
longer time by the interposition of the spring e. 
If =the time of the wave in seconds, : will be nearly the instant of its 
maximum velocity v, in feet per second ; thus the condition that shall ensure 
the ball B rolling only, and not being projected, is that the vertical compo- 
nent of v shall be less than 
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a ee ee ee ee 
