OF METALLIFEROUS VEINS IN THE MINES OF CORNWALL. 
407 
refer to the new mines which have been discovered in various parts of North 
and South America, Siberia, Ireland, See. or to the mining county of Cornwall, 
in which whole districts have comparatively of late been found abounding 
with mineral treasure, where none had been formerly suspected to exist,) 
it may I think be presumed, that the electrical currents, which so affect 
the needle in the galvanometer, may likewise influence the direction of the 
magnetic needle on the surface of the earth : at least no explanation of this 
phenomenon appears to be so plausible, or so well connected with ascertained 
facts. Even the cause of the variations of the needle, mysterious as it has 
hitherto appeared to be, may probably be referred to the relative energies of 
the opposing electrical currents, which are perhaps subject to occasional modi- 
fications ; and the appearance of earthquakes and volcanic action, from time 
to time, seems to countenance the probability of such changes. 
Nor should it be overlooked in reference to this view of the subject, that the 
oblique bearing which is generally observable in the strata and veins, with 
respect to the equator, causes them, as it were, to cross at opposite sides of the 
globe in the same parallels of latitude, so that their tendency, if any, must 
necessarily be to produce more than one magnetic pole in each hemisphere. 
Thus, in this respect also, the hypothesis accords with the interesting fact 
lately announced ; — of Professor Hansteen having ascertained the existence of 
a second magnetic pole within the arctic circle. The revolution of the earth 
on its axis from west to east seems moreover to harmonize with the idea of 
oblique electrical currents ; since rotation in the same direction may be pro- 
duced by corresponding electro-magnetic arrangements. 
Before I conclude, I will briefly mention a few facts relative to the tempera- 
ture of some of the mines in Cornwall. 
At Tingtang copper mine, in the parish of Gwennap, at the bottom of the 
engine shaft, which is in killas, and 178 fathoms deep, the water about two 
months ago was at the temperature of 82°. In 1820, when the same shaft was 
1 05 fathoms deep, the temperature of the water was 68° : thus an increase of 1 4° 
has been observed in sinking 73 fathoms, which is equal to 1° in 5 fathoms. 
At Huel Vor tin mine, near Helston, the water was 69° at the bottom of a 
shaft 139 fathoms deep, in the year 1819. It is now 209 fathoms deep, and 
3 a 
MDCCCXXX. 
