874 
MESSES. J. MILNE AND T. GBAY ON SEISMIC EXPEBIMENTS. 
instrument was, that an observer was enabled to tick off very conveniently intervals 
of time upon the moving strip of paper. When the signal was given for the catch to 
be pulled to allow the ball to fall, an observer started the clockwork, so that before 
the ball reached the ground the carriage was fairly in motion. When the observer 
saw that the pointers of the seismograph had ceased to write, the clock was stopped, 
and the whole arrangement allowed to stand still until the ball could be again wound 
up and a second record obtained. In this way several successive records were obtained 
without the introduction of possible errors due to the resetting of the instruments. 
The seismograph employed was either the small rolling sphere seismograph, or a 
pair of horizontal lever instruments. 
When the horizontal lever seismographs were used—they were so placed that one of 
them could only fully record normal vibrations and the other transversal vibrations— 
it could be seen that the writing of the normal vibrations commenced slightly before 
that of the transversal ones. During these experiments, in order to determine the 
interval of time it had taken for these two sets of vibrations to travel from the fa llin g 
weight to the station at which the instrument was placed, a third pointer was allowed 
to rest upon the smoked glass plate close to the pointers of the horizontal lever 
seismographs. As the plate travelled along, this third pointer, when not interfered 
with, described a straight line. By means of the shock communicated by the ball to 
the ground, a specially contrived instrument, which was placed about 10 feet from the 
point where the ball struck, closed an electric circuit. This closing of a circuit caused 
an electromagnet to suddenly deflect the third pointer and produce a sudden break 
or deviation in the line being drawn by it. The instant when the vibrations reached 
a point 10 feet away from the place where the ball struck the ground was thus very 
clearly marked upon the smoked glass plates. The time at which they reached the 
station where the plate was situated, was indicated by the pointers of the seismograph 
ceasing to draw a straight line and commencing to write the vibrations affecting the 
instrument to which they belonged. The velocities deduced in this manner for the 
transmission of normal and transverse vibrations are discussed under the sixth set of 
experiments. 
