656 ME. MALLET ON THE TEANSIT-VELOCITY OF EAETHQHAKE ‘WAVES. 
stratified rock formations of that locality, by taking advantage of the powerful explosions 
necessary at the quarries. These quarries are situated (see Map, Plate XX.) on Holy- 
head Mountain on its N.E. flank, in metamorphic quartz rock, and in 1852 (a vast mass 
of material having been already removed) presented a lofty, irregular, and nearly vertical 
scarp, reaching to 150 feet in height above the floor of the quarry in some places. 
From this wall of solid rock the process of dislodgement was continued, not by the 
usual method of blasting, by means of small charges fired in jumper holes bored into 
the rock, but by the occasional explosion of large mines, containing at times as much as 
nine tons of gunpowder lodged in one or in three or more separate foci, deep within the 
face of the cliff, and formed by driving “headings” or galleries from the base of the 
mural face into the rock. From the charges of powder placed in bags at the innermost 
extremities of these headings, which were stopped up by several feet of “tamping” of 
stone, rubbish, and clay, conducting- wires were led out to a suitable and safe distance; 
so that on making by these the circuit complete between the poles of a powerful Sjiee’s 
galvanic batteiy, a small piece of thin platinum wire adjusted within the charge of gun- 
powder became heated, and ignited the powder. The explosion thus followed instan- 
taneously the making contact between the poles of the batteiy. 
Experience has enabled the engineers charged with the work so exactly to proportion 
the charge of powder to the work it is intended to perform in each case, that no rock is 
thrown to any distance ; the whole force is consumed in dislocating and droppmg dovvai 
to its base as a vast sloping talus of disrupted rock and stone the portion of the cliff 
operated on ; in fact, at the moment of explosion the mass of previously solid rock seems 
to fall to pieces like a lump of suddenly slacked quicklime. The shock or impulse, 
however, delivered by the explosion upon the remaining solid rock, behind and around 
the focus, and propagated through it in all directions outwards, as an elastic wav^e of 
impulse, was at an early stage of the operations remarked to be so powerful, that it 
could he felt distinctly in the quaking of the ground at distances of several hundred 
yards, and was sufficient even to shake down articles of delf ware from the shelves of 
cottages a long way off from the quarries. 
Early in 1853 I visited those quarries, and examined generally the adjacent locality 
and rock formations, and having satisfied myself that these operations could be made 
available, I applied to my distinguished friend, the late lamented Mr. Rendell, C.E., 
the engineer in chief of the Asylum Harbour, and readily obtained from him permission 
to make such experiments as should not interfere with the progress of the works. 
The prosecution of these experiments having been favourably represented to the 
British Association for the Advancement of Science, and to the Council of the Royal 
Society, a sum of money was voted by each of these bodies respectively, and placed at 
the author’s disposal, with the desire that he should undertake and conduct the experi- 
ments. 
It was not, however, until the summer of 1856 that my own avocations, and various 
preliminaries, allowed any progress to be made with the experiments themselves. Ne- 
