yo sae ss ee 
ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 77 
this stroke will give an approximation to the strength of the shocks. The 
middle of the stroke, or, if they are vertical, the end of the uninterrupted 
line, gives the time of the commencement of the shock. The strength and 
direction of the shocks may also be approximated if the (as respects rota- 
tion) fixed plate hikl have an annular recess, filled with quicksilver until 
its surface reaches the holes sss, made in the cylindrical sides. At the 
first motion of the pendulum, the quicksilver will be shed out through these 
holes into a dish divided into the same number of compartments as there are 
holes, like those already in use in many existing instruments of this kind 
(Cacciatores)”. 
Such are the chief seismometers hitherto proposed. They all involve in 
some form the principle either of the solid or of the fluid pendulum, the 
latter term being applied to the oscillations of liquids in tubes or other such 
vessels ; and have disadvantages, both theoretic and practical or constructive, 
which render their indications inaccurate. 
Every pendulum seismometer has a time of oscillation due to its length, 
which in the case of the solid pendulum is 
t=2/ u 
g 
and in the case of the oscillating liquid 
ey ay ic a 
? 
l being the length of the pendulum and of the oscillating column of liquid 
respectively ; but if P = the period of the earth-wave or shock, then when- 
P Heer Oe : J P 
ever T=P, or x P, or —, the indication of the instrument will be in excess 
n 
of the horizontal component of the wave’s motion ; when, on the contrary, T 
represents no function of P, it may be much less than it. 
The amount of error depends also upon the velocity of movement of the 
horizontal component of the wave. If this be considerable, the solid pen- 
dulum, whether hanging or inverted, acted on by gravity or elasticity, is at 
the first moment left behind; as the rod becomes more oblique, the pen- 
dulum is dragged along, and acquires a velocity (in a direction which ap- 
proaches to horizontal) greater than that due to the are through which the 
pendulum has fallen in the time. At the end of the wave’s forward move- 
ment, then, the pendulum is thrown forward too far; and at the end of the 
return movement of the wave, it moves beyond the range of the latter, by a 
small are due to its proper motion. This objection applies, though with less 
cogency, to the fluid pendula, and in their case to both the vertical and hori- 
zontal components of the wave. 
These discrepancies of indication will vary whenever the velocity and di- 
mensions of the earth-wave become altered ; and as, for the same instrument, 
T varies with sin’ \ (A being the latitude), it is obvious that even two per- 
fectly similar instruments at stations north and south of each other, will not 
give strictly comparable results for the same earth-wave. 
These are but examples of one or two points of theoretic difficulty, to 
which others might be added, and which affect these instruments prin- 
cipally as indicators of the dimensions of the earth-wave. Some of these 
theoretic disturbances may be eliminated by calculation from the results ; 
but there are also some apparently insuperable difficulties, of a practical or 
constructive nature, which affect all solid pendula as reliable indicators even 
