418 
Proceedings of Royal Society of Edinburgh . [sess. 
expended on every mass of 172J lbs. near the centre of the earth. 
If the capacity of rock for heat be taken at *21 that of water, then 
‘21 x 9 *76 = 96 > 6QQ ° Fahr. 1S ^ ie r ^ se temperature corresponding. 
with the energy expended in compressing the rock. 
It is not meant to be implied that the temperature at any given 
time can be deduced from the above figures, for considering the 
extreme slowness with which rock yields to pressure — as shown 
by the slowness of all earth movements — the action must have 
continued for an indefinite time, and it would tend to cause a 
continual flow of heat from the centre outwards, which being of 
necessity excessively slow, a very high temperature may be safely 
inferred as obtaining from this cause at the centre of the earth. 
A similar calculation as to the average pressure in the earth 
20,900,000 x 345 92 ^ „ 
gives — 0 x 253 =: 2161 x 10° lbs. per square foot, 
and the energy expended in doubling the density of each cubic 
2161 xlO 6 1 
foot comes out ~ x ^ = 540‘25 x 10 b foot-pounds, or about 
one-fifth as much as may have been expended on an equal mass 
at the centre. 
As to the effect of compression in heating, there can be no 
doubt ; but such a mode of heating could scarcely cause liquefaction 
— after all cavities were filled up — for it consists of packing the 
molecules more closely together, which in the case of rock is the 
reverse of what is required to enable it to liquefy. However, 
owing to the flow of heat from the centre, the parts nearer the 
surface ought to be hotter than would be inferred from considering 
them in the same way as has been done above for the central 
parts. 
As bearing on the above views, the following may be considered. 
The contortions and heating of the metamorphic rocks are 
probably both simultaneous results of a gradual yielding to 
pressure under circumstances which prevented the heat getting 
away except very slowly. Although the heating of these rocks 
has been very great, they do not appear to have been liquefied, 
possibly from not having been able to escape from under the 
pressure when at their hottest. 
