1864. | Matter on Larthquakes. 65 
shattered. When first it was pointed out that an Earthquake shock 
was an elastic wave, if appeared, upon physical grounds, that the 
rate at which the shock having reached one place on the earth’s sur- 
face, would pass on to another beyond, must be something nearly 
as great as that theoretically due to the elasticity and density of 
the rocks beneath, that is to say, often as much as 8,000 or 10,000 
feet per second. This was submitted to experiment; granite rock, 
highly elastic and dense, ought to transmit a shock wave nearly as 
fast as any rocky or other material forming part of our globe, and wet 
sand ought to transmit it almost at the extreme limit of slowness. 
More than a mile of wet uniform sand was measured carefully upon 
the shore of Killiney Bay, in Ireland, and several hundred feet in the 
granite of Dalkey Island adjacent. At one end of each of these 
ranges, respectively, small Earthquakes were made by exploding 
galvanically, casks of gunpowder buried in the sand, and blasts sunk 
in large cylindrical holes sprung in the granite, special means 
being devised for determining the time of transit, and accurate 
enough to measure time to less than the five-thousandth of a second ; 
the time-measuring apparatus being set in motion, and stopped by 
the same galvanic apparatus that fired the powder a mile or more 
away. An instrument, called a Seismoscope, was also devised and 
employed, by which the arrival of the wave of impulse transmitted 
from the powder exploded at the remote end, should be rendered 
visible to the eye, through the disturbance of a telescopic image, 
reflected in the liquid mirror of a small trough full of quicksilver, 
which was caused to undulate and flicker by the momentary tremor 
of the ground beneath it. The sensibility of this instrument was 
so great that a horse trotting on the sand half-a-mile away was visibly 
seen to shake the ground, and a stamp of the foot or tap of a hammer 
on a large stone several hundred feet away, produced visible dis- 
turbance. ‘This instrument was also employed at Holyhead. 
The results of these experiments caused some surprise amongst 
physical philosophers, for in place of the cnormous rates of transit 
that were expected, it was ascertained that the mean rates of wave pro- 
pagation were only as follows in the respective media, viz. : 
In the most solid Granite . 6 4 . 1664574 feet per second. 
In shattered-like Granite . - 13067425 zi a 
In contorted and stratified Rock (Quartz and 
Slate) . . : - 1088°559 . * 
In wet sand A * 5 : c ; 824°915 3 a 
The retardation is due to the discontinuity of the rocks, the mass of 
every known rock being broken up by joints and fissures, at each of 
which there is a loss of vis viva, and a loss of time in the transmis- 
sion of the wave. The accuracy of these results, at first received 
with some just reserve, has since been amply confirmed by observations 
and calculations of the actual transit periods of Natural Earthquake 
waves, occurring in the Rhine Provinces, Hungary, and Southern Italy, 
which are found closely to co-ordinate with those of experiment. 
It was ascertained that in the contorted heterogeneous and shat- 
tered rocks of Holyhead, no less than seven-eighths of the total theoretic 
VOL. I. F 
