1 877.] On Underground Temperature . 173 
in which, be it observed, we have not pressure alone, but 
motion destroyed and converted into heat. 
Sir Humphry Davy ascribed the volcanic fires to the 
oxidation of earthy and metallic bases, supposed by him to 
exist low down in the interior of the earth. Since water 
is everywhere present in the crust, if it be absorbed by the 
abstraction of its oxygen when it encounters the bases, and 
by the blowing off from volcanic vents of the equivalent 
hydrogen, then its place must be continuously supplied by 
the percolation downwards of fresh accessions ; and thus a 
generally diffused increase of temperature was supposed to 
arise, and to manifest itself by its escape towards the 
surface. 
A very fascinating theory, which has met with support 
among many scientific men, has of late years been promul- 
gated by Mr. Mallet, who considers the heat of volcanic 
aCtion to be derived from the transformed work of crushing 
the rocks of the earth’s crust, owing to the contraction of 
its interior through long-sustained cooling. The cause of 
evolution of the volcanic heat under this theory is of the 
same kind as that alluded to above, in the case of creeps ; 
while the ubiquitous increase of temperature in descending 
into the earth is looked upon as chiefly a manifestation of 
the generally diffused heat of the interior, by the escape of 
which the contraction in volume is produced. Volcanic 
aCtion, Mr. Mallet tells us, is caused by this contraction 
manifesting itself locally and paroxysmally. 
In order to decide what theory best accounts for the phe- 
nomena, it is obvious that the first step is to make sure of 
the faCts themselves. In other words — What is the law 
regulating this increase of heat ? At what rate does the 
temperature augment ? This might be supposed to be a 
point easily settled. It might be supposed that nothing 
would be easier than to insert thermometers into the rock 
at different depths in a mine, and to read off their indica- 
tions ; or to lower them into a bore-hole for the same 
purpose. But there are many difficulties to be overcome, 
and a host of disturbing causes present. The rock, or the 
face of the coal in a mine, is affeCted by the temperature of 
the air. The air is warmed by the presence of men and 
horses ; it is cooled by the ventilation carefully kept up. 
Hence the surface of the working is not at the true temper- 
ature of the rock. The result of careful experiments made 
by Sir G. Elliot showed that when thermometers were 
inserted in the coal in a long-wall working, at distances of 
3, 6, and 12 feet from the face, no alteration could be 
