832 
ME. W. HOPKTN^S’S EXPEEniEXTAL EE.SEAECHES 
ra])id increase of temperature than in the latter formation, the increase being in this 
case only 57 feet for 1°. The other cases referred to are more indefinite, on account of 
the omission of any detailed mention of the nature of the beds thi’ough which the heat 
is transmitted. 
20. There are also other observations to which I have not yet alluded, the coiTectness 
of which in their general results may be depended on, though some of them may inevi- 
tably, perhaps, have been made under circumstances calculated to diminish them weight 
when considered separately. I allude to the obseiwations made by Mr. Hexwood, with 
great care and labour, and pubhshed as an Appendix (Xo. 1) to his work ‘ On the 
Metalliferous Deposits of Cornwall and Devon.’ The heat of the mines of that district, 
situated in slate rock, is well known to be greater than that of the mines situated in 
granite. From a number of observations Mr. Hexwood concluded that, on the average, 
and at depths not less than 600 feet, there was an increase of 1° Fahr. for 51 feet in the 
granite, and for 37'2 feet in the slate*. I have not been able to ascertain the conduct- 
ing power of the Cormvall slate, and cannot therefore determine how far these results 
are consistent with each other, according to theory ; but there can be no doubt but that 
the increase ought to be much more rapid, instead of being slower, in chalk than in 
either granite or slate ; for all the older compact rocks, on which I have experimented, 
have been found to have comparatively high conducting powers. This increases stiU 
further the discrepancy between observation and the theory we are discussing. 
21. In the preceding discussion it has been supposed that the isothermal siufaces at 
depths (such, for instance, as 50 or 60 miles) below the influence of local external causes, 
are approximately concentric with the surface of the earth itself, in which case it has 
been shown that the quantity of heat transmitted through the external shell must be 
very nearly the same in every part of it. It may, however, be conceived that these 
deeper isothermal surfaces may deviate from concentric spherical forms much more than 
has been here supposed. It would be difficult, I think, to assign any probable cause 
for such considerable deviations according to our theory ; but admitting, for a moment, 
the hypothesis, let us examine what consequences will flo-w from it, and how far it nlll 
enable us to account for the general uniformity of the rate of increase of terrestrial tem- 
perature as we descend in different localities throughout a region like Western Eiu'ope. 
or for the local variations to which it is subject. 
For this purpose let us take the particular example of Art. 17, except that we here 
suppose the deep isothermal surface LM to be of the form represented in fig. 3, instead 
of being parallel to thn internal surface ; and since the conductive poAver of the unstra- 
tified mass CLMD has been supposed three times as great as that of the sedimentary 
* Since tliis paper M as read, I have been informed by Mr. Fox, that, generally, those Cornish mines M'hich 
present tlie most rapid increase of heat in descending have considerable quantities of water, a portion ol 
whicli may probably ascend from longer depths, and increase the temperature of the liigher portions of the 
mass. The rate of increase of temperature in the dry Cornish mines does not appear to differ much from 
the general rate of 1° Faiir. for about 60 feet of depth. 
