made in Pennance Mine. 495 



proceeded having been marked, they were broken away, when 

 iron pyrites was in every instance found imbedded in them ; 

 and there can be no doubt that the smallest branch of copper 

 or lead ore might have been detected in like manner. 



On several occasions the ends of the opposite wires were 

 placed in contact with the rocks near the two veins, when there 

 still appeared to be a tendency in the currents to pass in the 

 same direction, but often they could not be detected, or were 

 too feeble for their direction to be determined with certainty. 

 Pieces of copper pyrites attached to the wires and imbedded 

 in wood, were likewise used instead of the metal for producing 

 contact with the rocks, and with still less effect; and when the 

 contact was made with platinum and zinc in succession, the 

 currents were in opposite directions, and in accordance with 

 the action of those metals respectively ; so that the existence 

 of independent currents under the circumstances described, 

 though more than probable, was not clearly proved. Electri- 

 city, generated by a pair of zinc and copper plates, was trans- 

 mitted through the rocks between the two veins from north to 

 south, and also from south to north, in order to detect any in- 

 dependent currents traversing the rocks by a differential effect 

 on the needle. This method appeared likely to be a very 

 delicate test of electric action in rocks, but no decided results 

 were obtained, the currents passing in opposite directions ap- 

 parently with equal facility, at least the few experiments 

 hitherto made in this way have not led to any satisfactory 

 conclusions relative to the point in question. It should be 

 remarked, however, that the astatic needle employed was in- 

 conveniently sensitive, and was often set in motion when the 

 cause was not very obvious. With needle No. 1 the case 

 was widely different, as it could scarcely be moved by any 

 subterranean currents that were not tolerably energetic, such 

 as were produced when both the wires were in contact with 

 ore-points, and then, as has been stated, it often revolved 

 rapidly. 



It has been long known that electric currents will traverse 

 a very considerable thickness of rock or strata*; but in what 

 degree this property may be modified by the nature or tex- 

 ture of the rocks, the saline contents of the subterranean water, 

 or the proportion of ores included in the circuit, remains to 

 be ascertained. If the influence of these different circum- 



* Many instances of this occur in my paper " on the electro-magnetic 

 properties of metalliferous veins," published in 1830, in the Phil. Tran- 

 sactions, p. 399. I have long ago seen a very feeble current act on a sen- 

 sitive galvanometer after it had traversed nearly a quarter of a mile of strata, 

 and stronger currents would probably be detected in like manner after 

 having passed many times that distance under the surface. 



