ME. MALLET OX THE TEAXSIT-VELOCITY OE EAETHQIJAKE WAVES. 665 
chronograph with a sharp rapid movement ; this instantly closed the poles of the con- 
tact-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 
Gkove’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 seis- 
moscope at F, I withdrew my hand from the lever of the chronograph (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 briefiy to describe the several instruments separately. The seis- 
moscope and chronograph have been already fully described in the account of the expe- 
riments made in 1849 at Killiney and Dalkey (Second Report on Earthquakes, &c.. Re- 
port of Brit. Assoc. 1851), to which reference may be made. 
Briefiy, the seismoscope (fig. 3*, Plate XXIII.) consists of a cast-iron base-plate, on 
the centre of the surface of which 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 direction 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 achro- 
matic object-glass at its lower end, and a pair of cross wires (horizontal 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 an achromatic telescope [a) with a single 
wire is similarly adjusted, so that when the moveable blackened cover {ee) is placed 
over the trough, &c., no light can reach the surface of the mercury except through the 
tube c. The image of the cross wires in the latter is therefore seen through the tele- 
scope a, clearly reflected and defined in the surface of the mercury, so long as the 
fluid metal remains absolutely at rest ; but the moment the slightest vibration or dis- 
turbance is by any means communicated to the instrument, the surface of the fluid 
miiTor is disturbed and the image is distorted, or generally disappears totally. The 
telescope magnifies II ‘39 times linearly, and the total magnifying power of the instru- 
ment, to exalt the manifestation to the eye of any slight disturbance of the mercurial 
miiTor, is nearly twenty-three times. Its actual sensibility is extremely great. In the 
present case, however, this was not needful, as the impulse transmitted from these 
powerful explosions produced in all cases the most complete obliteration of the image, 
and in those of the most powerful mines experimented on caused a movement in the 
mercury of the trough that would have been visible to the naked eye. Indeed in that 
of the 24th of November, 1860, the amplitude of the wave that reached the seismoscope 
was so great as to cause the mercury to sway forwards and backwards in the trough to 
a depth that might have been measured. 
After the earth-wave has reached this instrument, a certain interval of time is neces- 
sary for the production of the wave in the mercury, and for its transit from the end of 
4x2 
