ON THE TRANSIT-VELOCITY OP EARTHQUAKE WAVES. 209 



In PI. IV., 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 Grove's firing-battery; the conducting 

 wires from its poles pass down the face of the cliff and into the heading, 

 uniting at the platinum priming-wire in the midst of the charge of powder, 

 tlie further end of tlie 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 cliflT down to the observer's station at Pcn-y-Brin, where 

 they terminate also in mercury-cups, one forming the e+ and the other 

 the e— 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-patli 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 ray eye to the seismoscope, 

 and pressed down the lever (m) of the chronograph with a sharp rapid move- 

 ment ; this instantly closed the poles of the contact-making battery C, causing 

 the galvanic current to pass through the electro-magnets of the contact-maker 

 away at the quarries at C. This directly closed the poles of the Grove's firing- 

 battery at B, and fired the mine. The moment I observed the arrival of 

 the wave of impulse propagated through the range from the explosion at A 

 in the seismoscope at F, I withdrew my hand from the lever of the chro- 

 nograph (m), and thus stopped the instrument, the interval of time between 

 its having been started and stopped thus registering the (uncorrected) time 

 of transit of the wave for the distance A F. It will now be necessary briefly 

 to describe the several instruments separately. The seismoscope and chro- 

 nograph have been already fully described in the account of the experi- 

 ments made in 18i9 at Killiney and Dalkey (Second Report on Earthquakes, 

 &c.. Report of Brit. Assoc. 1851), to which reference may be made. 



Briefly, the seismoscope (fig. 3*, PI. IV.) consists of a cast-iron base-plate, 

 on the centre ofthe surface ofwhich is placed an accurately formed trough (b), 

 12 inches long, 4 inches wide, and 2 inches deep, containing an inch in 

 depth of pure mercury, with its surface free from oxide or dust, so as to 

 reflect properly. The longer axis of this trough is placed in the direc- 

 tion of the wave-path, the base of the instrument being level. At the 

 opposite end of the trough are placed standards with suitable adjustments : 

 that at the end next the centre of impulse carries a tube (c), provided with 

 an achromatic object-glass at its lower end, and a pair of cross wires (hori- 

 zontal and vertical); its optic axis is adjusted to 45° incidence with the 

 reflecting surface of mercury in the trough. At the other end of the trough 



1861. P 



