ME. IHALLET ON THE TEANSIT-VELOCITT OE EAETHQEAEH WAVES. 663 
manner that at any point along their length over the quarry cliffs, a pair of branch wires 
(covered udth gutta percha) could be led off, and in like manner another pair to the 
apparatus in the ob serving-shed at Pen-y-Brin, thus giving the means of galvanically 
connecting the extremities of the range, in any way that might be required. 
The mines in use at the quarries frequently consist of two, three, or four separate 
chambers and charges, which are all fired simultaneously (see Plate XXII.) ; but each 
charge is fired by a distinct pair of wires, igniting a fine platinum wire interposed 
in the circuit and immersed in one of the powder-bags. The arrangement of this 
platinum wire in its hollow wooden frame to prevent disturbance, and its connexion with 
the large conducting-Avii’es, are practically the same as those adopted by me in 1849 at 
Killiney, and wiU be found fully described in “ Second Eeport on Earthquakes,” &c., 
Eeport of British Association for 1851, p. 277. 
When several charges are to be fired simultaneously, all the electro-positive wires 
from each chamber are collected into one mercm-y-cup in connexion with one pole of 
the battery, and all the electro-negative wu’es into another mercury-cup. Upon making 
contact between the latter and the second pole of the battery, the current at the same 
moment ignites all the platinum -wires passing through each pair of wires as a separate 
conducting-path. This method requires considerable battery power, but is the only 
certain or reliable one for firing simultaneously a number of separate charges. When 
an attempt is made to pass the current from one pole of the battery through a single pair 
of wires, and through all the fine platinum priming-wires in succession to the return 
pole, there is extreme risk tliat the first or second platinum priming, owing to its 
attenuated section of wire (in virtue of which indeed alone it becomes ignited at all), 
may interpose so much resistance to the current as to prevent the ignition of the third, 
or fom’th, or other subsequent primings, or that the first priming-wire may get abso- 
lutely fused or broken by the first-ignited powder, and so cut off all communication 
with the others before they have been heated sufiiciently. 
A neglect of this obvious consequence of Ohm’s law of resistance appears to have 
been the cause of failure veiy recently, in an attempt to ignite a number of mines of 
demolition, simultaneously, at Chatham. From the great magnitude of the charges 
frequently fired at Holyhead, and the very serious consequences that failure of ignition 
would involve, the battery power habitually employed is wisely of superabundant power. 
It consists of a Gkove’s battery of thirty-two cells, each exposing 96 square inches 
of platinum element. It is but justice to my friend Mr. E. L. Cousins, C.E., to whose 
assistance in these experiments I am so much indebted, to add that during the several 
years he has controlled these vast blasting-operations a single failure of ignition has 
never occurred. 
For the above reasons, and from the necessity that in the event of any failure of such 
apparatus as I might require for experiment, in making contact and firing the mine at 
a given moment, the power should still be reserved to Mr. Cousins to fire it directly 
afterwards in the usual way, so as not to interfere with the works, I was led, finally, 
MDCCCLXI. 4 X 
