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A ER 
ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA, 89 
towards C, ; and the instant of its departure, by breaking galvanic contact of 
the poles at its stop, marks that of the commencement of the shock. But the 
whole instrument is carried forward by the horizontal component cf the 
shock, and then moves back again; the ball B is therefore carried forward 
also, urged by the block at 7, and is caused to roll up along the inclined 
plane a certain distance, say to C, where it comes to rest, and, reversing its 
motion, rolls back again by gravity, and returns to rest in contact with the 
block and galvanic poles of its own stop. The ball which first moves, which 
we may call the Time Ball (as indicated in time by the pencil trace on 
the clock-cylinder paper), will always be that at the side from which the 
shock arrives. We neglect any account of its subsequent motions. The 
other ball, which we may call the Element Ball, by its movements gives 
us the elements of the wave. The instrument records the whole time that it 
is out of contact with the block g, viz. that of its excursion up and down 
the inclined plane z. If, in place of the wave having emerged at some angle 
to the horizon from S. to N., it had come at the same or at any other angle 
of emergence between vertical and horizontal, in the reverse direction or 
from N. to S., then the action of the balls also would have been reversed, 
B becoming the Time Ball, and being left behind, and thus noting the mo- 
ment of arrival of the wave; and B, being thrown up along the inclined 
plane 2, giving its elements. 
Again (referring to fig. 2), if the wave emerge at some azimuth between 
N. and S. and E.and W., suppose from the $.W., with any angle of emergence, 
then by the vertical component the springs of both the N.S. and E.W. instru- 
ments will be compressed (and nearly alike). The time balls B, of the N.S. and 
B, of the E.W. instruments will be left behind, as before, (and both at the 
same instant will break contact with the block); and the element balls B and 
B will be thrown forward upon their respective inclined planes, as before—to 
equal distances in the case of the exactly intermediate azimuth here supposed, 
but to unequal distances if this azimuth be more to the W. or to the S. 
The instrument records the simultaneous excursions of both balls B and B, 
giving the total time (as before) that each ball is out of contact with its own 
block or stop; and if the direction of the wave-movement be reversed as re- 
spects the instrument (suppose, from some point of N.E. towards S.W.), then 
therespectivemovementsand functions of the balls willalsoreverse themselves, 
B and B being left behind, and B, and B, thrown forward, &c. 
The general size and strength of the instrument must be determined with 
reference to the degree of violence of the earthquake-shocks to be anticipated 
in the seismic region it is intended for. The very greatest, and the very 
smallest perceptible shocks, are alike unsuited for useful measurement. The 
dimensions of the instrument, as shown by the scale of the plate, are such as 
I consider fitted to ensure its functions, under the effects of those shocks of 
mean intensity (such for example, as those common in the Mediterranean 
basin, or in those of Hungary and Austria), and with moderate vertical angles 
of emergence, which are those best to observe in the existing state of our 
knowledge. 
The most important points of precaution of a constructional character to 
be noticed are the following :—The balls should be of lead chiefly (the sur- 
face being formed, for hardness and smoothness, of gun-metal), to reduce 
their proper elasticity as much as possible. The inclination of the planes é, 
must be small, probably never exceeding 15°, and the length and inclination 
so adjusted by experiment, to the maximum time of wave-oscillation in the 
- district of observation, that the whole time of rolling up and down of the ball 
shall be considerably longer in duration. Their bearing-edges must be per- 
