664 ME. MALLET ON THE TEANSIT-VELOCITT OF EAETHQUAKE WAVES. 
to devise the following magneto-galvanic arrangement, by which, at a signal given from 
the summit of the quarry cliff (where the firing-battery is usually placed, nearly above 
the mine or heading then to be fired, and at a safe distance back from the edge of the 
cliff, usually a,bout 100 yards) that all was ready, I should myself, stationed at the ob- 
serving-shed (O), be enabled to complete the contact and fire the mine, and do so in 
such a way as to register by means of the chronograph the interval of time that elapsed 
between the moment that I so made contact (or fired) and the arrival of the wave of 
impulse through the rocks of the range or wave-path, when made visible by, and observed 
by me in, the seismoscope. 
For this purpose such an arrangement was required as, upon contact being made by 
me at the observing-shed (O), should set in motion such a contrivance, situated upon 
the quarry cliff, at the remote end of the telegraph wires, as should there instantly close 
the poles of the great (Geove’s) firing-battery and so fire the mine, and in the event 
from any cause of this result not taking place at the preconcerted moment, that then it 
should be free to Mr. Cousens or his assistants to close the poles of the firing-battery 
by hand in the ordinary way. 
In Plate XXIII., in which (fig. 1) this arrangement is figured (without reference to 
scale), A is one of the headings seen in the cliff-face at part of the quarries. Above 
the cliff at B is placed the Geove’s firing-battery ; the conducting wires from its poles 
pass dovm the face of the cliff and into the heading, uniting at the platinum priming- 
wire in the midst of the charge of powder, the further end of the wires terminating 
in mercury-cups at the contact-maker C (about to be described). From the electro- 
magnet of the contact-maker, the two insulated wires are led along upon telegraph 
poles from the summit of the cliff down to the observer’s station at Pen-y-Brin, where 
they terminate also in mercury-cups, one forming the £+ and the other the s— pole 
of the contact-making battery E placed there. This battery consisted of six of the 
usual moistened-sand batteries in use for telegraph purposes. 
The chronograph (D) was placed upon the levelled rock adjacent to this battery, and 
conveniently for its lever (m) being acted on by the left hand of the observer, when lying 
at full length upon the ground, with his eye to the seismoscope based upon the rock at F, 
its optic axis being situated in the vertical plane of the line of wave-path or range F A. 
Close to the seismoscope, and at the same level as the eyepiece of that instrument, a very 
good achromatic telescope was adjusted upon its stand, so as to bring the heading about 
to be experimented on, together with the whole face of the cliff and the firing-battery, 
&c., within its field, — the eyepiece of this telescope being fixed at about a distance of 6 
or 8 inches from that of the seismoscope, and so that the eye of the observer, while lying 
at ease, and with the left hand upon the lever of the chronograph (m), could be instantly 
transferred from the one instrument to the other. In this state of things, when the 
proper signal (by the exhibition of a red flag) was made, and at a preconcerted time as 
nearly as was practicable, by those stationed at the firing-battery at B, that “ all was 
ready,” I applied my eye to the seismoscope and pressed down the lever (m) of the 
