MESSRS. J. MILNE AND T. GRAY ON SEISMIC EXPERIMENTS. 
879 
not act until the circuit became permanently closed. The result of this was evidently 
to reduce the apparent time of transit and hence increase the velocity. 
The second cause of error above indicated was rendered very likely by the fact that 
the motion of the machinery in the works kept up a continual vibration in the ground. 
An automatic circuit-breaker was next introduced into this arrangement and the 
distance reduced to 200 feet. A correction for instrumental error was in this case 
determined from observations at 100 feet. With this arrangement a mean velocity of 
transit equal to 568 feet per second was obtained. 
The state of the ground was in this case affected to some extent by frost, the 
temperature the previous night being 9° Fahr. below freezing. 
The circuit-closer was next altered and three more determinations made, the mean 
result of which gave 379 feet per second. In this case, however, the previously 
determined correction had to be used owing to the fact that a new correction could 
not be made that day, and was not afterwards made. 
This result is somewhat uncertain, because a new circuit-breaker was here used and 
more improvements introduced in the closer. 
Neither of these methods having given results with which we felt at all satisfied, we 
turned our attention to the registration of the motion of the ground in conjunction 
with the time. We were led to this method of experiment through some trials of a 
seismograph, designed by one of us, which we had taken advantage of these experi¬ 
ments to make. This instrument (Gray’s rolling sphere seismograph) proved so sen¬ 
sitive that the motion of the ground could be plainly written at a distance of about 
400 feet. The first trial of this method was made as follows:—A glass plate mounted 
on three wheels was arranged in such a way that it could be pulled forward uniformly 
by clockwork under the writing point of the seismograph. A separate arrangement 
consisting of an electromagnet and writing levers was fitted to write on the same 
plate. This electromagnet was placed in circuit with the circuit-closer previously 
used, and hence when the circuit was closed a mark was made on the plate; a short 
time after this the seismograph began to write, and the interval between gave the 
time of transit subject to the error of the circuit-closer. The error of the circuit-closer 
was again determined by bringing the recording apparatus 150 nearer to it. This, 
however, also proved inconvenient and on the whole unsatisfactory, as it caused a 
great number of somewhat laborious experiments to be made which could be obviously 
avoided by doubling our apparatus and taking the difference of time between two 
stations. The propriety of using an instrument which would only record a component 
of the earth’s motion soon suggested itself when we came to consider the question of 
the relative velocities of the direct and transverse vibrations. A very approximate 
estimate of this could be got by the instrument just mentioned, but as it wrote the 
resultant motion there was a little difficulty in determining the exact point at which 
the transverse wave became felt. To get over the difficulty we had recourse to a pair 
of horizontal lever seismographs in our possession. These instruments have already 
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