36 SEVENTH REPORT—1837. 
On the higher Temperature which prevails in the Slate than in the 
Granite of Cornwall. By W.J. Henwoop, F’.G.S., Member of the 
Geological Society of France, H.M. Assay-Master of Tin in the 
Duchy of Cornwall. 
It is not very easy to devise an unexceptionable mode of ascertain- 
ing the temperature natural to any given spot under-ground. 
In the experiments (about the earliest) of Trebra*, and in the later 
and most valuable ones of Cordier, the thermometer was inserted in a 
hole in the rock ; to which it may be objected, that if it be exposed to 
the action of a stream of water, it will indicate the temperature of the 
liquid ; and if but a little ooze out of the rock, its evaporation will re- 
duce the heat; whilst at all times the influence of the air with which 
the gallery (level) is filled will affect it to some depth, and this is some- 
times perfectly still, and at other times in rapid motion, often coming 
from parts of the mine where workmen are numerous, and frequently 
from the surface, depending on the direction of the wind, which very 
often in its changes reverses the direction of the subterranean cur- 
rents. 
The same reasons have long induced observers to abandon the tem- 
peratures of air and of stagnant pools of water in mines. 
Streams of water issuing from the unbroken rock are less liable to 
be affected by several of these influences, as they are not likely to be 
much disturbed by the few last beds of rock through which they per- 
colate ; but whether they indicate the temperature of the level where 
they appear, or of some higher or lower spot, is not so readily ascer- 
tained, and can after all be but suspected from their coincidence with 
the prevailing tenor of other observations. This last is, however, Mr. 
Henwood thinks, less objectionable than the other modes, and is that 
which he has himself pursued in the observations, of which an abstract 
is annexed. 
SLATE GRANITE. 
Si hae woh ek q|sil| 2 
f2 | se | 2 26 | as | 2 
E | ss & 2 
Depth (fathoms.) | $3 | BE | & Depth (fathoms.) | 83 | gE | 8 
Sey 2a)|. 8 <8) 231 & 
oOo} a A Sophie 
Surface to 50 3h eZ 57° Surface to 50 31 7 | 51°6 
EO); 25) 100°} 73.|19,c} 61°38 50 , 100} 79] 17 | 55°8 
100 4, 150] 127] 29 | 68° 100 ~=,,_-150 | 183] 12 | 655 
150 7s, BOO F170). eT | 28° 150 ,, 200 
200 and beyond} 221 5 | 85°-6 || 200 and beyond | 237 3 a sie3 
* Annales des Mines, i. 377, for the year 1817. 
