316 REPORT—1840. 
that the best experiments on subterranean temperature, in- 
fluenced as it is by so many disturbing and conflicting causes, 
can be regarded as affording only approximations to the truth, 
and, in a greater or less degree, in proportion as they are more 
or less numerous, and made in different localities. 
The second Table exhibits increments of temperature equal 
to 10° each, at intervals of about 47, 79, and 125 fathoms of 
descent. ‘The comparative augmentation of temperature at 
small depths exhibited in this Table, and its reduction at greater 
ones, may perhaps be more or less attributed to the ascent of 
warm air and vapour from the deeper galleries of the mines, 
and the descent of colder currents into these parts. 
I have taken the mean temperature at 50° Fahr., and it is, 
I think, clearly not more than this in the mining districts of 
Cornwall and Devon, judging from the experiments I have 
instituted on the temperature of the ground at three different 
stations, at the depth of three feet, which give a mean of 
49°°86* for the year, at a mean elevation of about 240 feet 
above the level of the sea; and also from the meteorological 
registers kept in this neighbourhood, some of which appear in 
the Cornwall Polytechnic Society’s Reports and the Annals 
of Philosophy tf. 
IT add a diagram or section, by way of illustration of the 
first Table. 
* See Transactions of the Cornwall Geolog. Society, vol. iii. pp. 326-328. 
+ In the Annals of Philosophy, vol. xvi. p. 371, 1820, the mean tempera- 
ture of eleven years at Penzance is stated to have been only 49°. The eleva- 
tion of this town above the sea-level is inconsiderable, and perhaps 100 to 200 _ 
feet below the average of the mining districts where the experiments were 
made. 
