408 
MR. R. W. FOX ON THE ELECTRO-MAGNETIC PROPERTIES 
the temperature is 79°, which gives a mean increase of 1° in sinking 7 fathoms. 
This part of the mine is in killas. 
The highest temperature of the water at the bottom of Poldice copper and 
tin mine in the parish of Gwennap, in 1820, which was then 144 fathoms under 
the surface, was 80°. It is now 176 fathoms deep, and the temperature is 99°; 
and in a cross level 20 fathoms further north, the water is 100°*. 
The two last-mentioned temperatures are the highest hitherto observed in 
any of the mines of this county; and the increase is equal to 19° in one case, 
and 20° in another, in sinking 32 fathoms, or 1° for 1^ fathom. Three persons 
only were employed at a time near each of these stations, and the water 
pumped up from this part of the mine was estimated at 1,800,000 gallons in 
twenty-four hours ; and I found on examination that it contained a considera- 
ble quantity of common salt in solution. 
* The thermometer used on these occasions was compared with others, and corrected one degree. 
