278 Mr Henwood on the Electric Currents 



of the vitreous copper-ore may render it positive to the copper- 

 pyrites. In some cases, it is obvious, differences of depth may 

 compensate for the natural inequality of temperature between 

 the granite and slate, and thus currents may be excited in con- 

 trary directions to those which would obtain, if both ores were 

 equally heated in contact, or if the pyrites were warmer than 

 the vitreous copper. It is evident too, that in many cases " fo- 

 reign ores'" may determine the directions of the currents, and 

 that minor circuits may neutralize each other. 



I think it is hopeless to search for any relation between the 

 directions of the currents and the points of experiments ; for 

 these, although partaking of the natural temperature of the spot, 

 are constantly modified by the combustion of candles and gun- 

 powder, the presence of workmen, and the circulation of air 

 from other parts of the excavations, to all which influences the 

 outer film of ore must be very obedient. 



The exciting differences of temperature must be sought in 

 the mean temperatures of the sums of the masses under experi- 

 ment ; and to them our observations being but approximations , 

 can be but imperfect indices. 



The last division of our subject relates to the opinions which 

 are supposed to be supported by these experiments, and the 

 theory which has been founded on them. 



Taken in conjunction with the beautiful experiment of Mr 

 Barlow, they would, prima facie, be thought confirmatory of 

 the opinion that terrestrial magnetism is of electric origin. But 

 it has been shewn that these currents have no uniformity of di- 

 rection, even in parallel veins; and that they exist of equal force 

 in veins bearing N. W. and S.E. and N. and S. as in those ha- 

 ving an E. and W. direction ; remembering, too, that we are en- 

 titled to speak with but little certainty of Cornwall, whilst of all 

 other parts of the world we can yet but guess. 



If the electrical currents yet discovered have any effect on a 

 magnetized bar freely suspended, they would of course subtend 

 an angle to each other ; in the mines of St Just, however, the 

 currents are parallel to the magnetic meridian. 



But the discovery of electric currents in the present contents 

 of our veins has been made use of as a foundation for a theory 

 of the origin of fhe veins themselves. Professor Sedgwick, in 



