208 REPORT— 1861. 



cally the same as those adopted by me in 1849 at Killiney, and will be 

 found fully described in " Second Report on Earthquakes," &c., Report 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 mercury-cup in 

 connexion with one pole of the batlery, and all the electro-negative wires 

 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 con- 

 ducting 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 that 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 

 fourth, or other subsequent primings, or that the first priming-wire may 

 get absolutely fused or broken by the first-ignited powder, and so cut off all 

 communication with the others before they have been heated sufficiently. 



A neglect of this obvious consequence of Ohm's law of resistance 

 appears to have been the cause of failure very 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 Grove's battery of thirty-two cells, each exposing ninety-six square 

 inches of platinum element. It is but justice to my friend Mr. R. L. Cou- 

 sens, 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, to devise the following 

 magneto-galvanic arrangement, by which, at a signal given from tlie sum- 

 mit of the quarry cliff (where the firing-battery is usually placed, nearly 

 above the mine or heading then to be fired, and at a safe distance back 

 from the edge of the cliff, usually about 100 yards) that all was ready, I 

 should myself, stationed at the observing-shed (O), be enabled to com- 

 plete the contact and fire the mine, and do so in such a way as to register 

 by means of the chronograph the interval of time that elapsed between 

 the moment that I so made contact (or fired) and the arrival of the wave 

 of impulse through the rocks of the range or wave-path, when made visible 

 by, and observed by me in, the seismoscope. 



For this purpose such an arrangement was required as, upon contact 

 being made by me at the observing-shed (O), should set in motion such a 

 contrivance, situated upon the quarry cliff, at the remote end of the tele- 

 graph wires, as should there instantly close the poles of the great (Grove's) 

 firing-battery and so fire the mine, and in the event from any cause of this 

 result not taking place at the preconcerted moment, that then it should be 

 free to Mr. Cousens or his assistants to close the poles of the firing-battery by 

 baud in the ordinary way. 



