Address to the Royal Geological Society. 21 
of the materials of the earth, considered solely with reference to 
their heightened temperature. 
Now, on the other hand, there are two circumstances which 
tend to lessen further the importance of increase of pressure. 
The first is that, although small diminutions of the volume of 
solids under pressure and consequent increments of density are 
sensibly proportional to the pressures, yet the rate of compression 
will fall off with a very great augmentation of pressure. The 
second is, that the very increase of density actually helps the 
thermal expansion to work against itself; for if the density of a 
body has been increased by some means, the general rule is, that 
the rate at which it expands with rise of temperature is also in- 
creased. So here are two circumstances which go to lessen the 
tendency which the pressure at considerable depths has to in- 
crease the heat-transmitting powers of the materials of the 
cooling globe. 
Therefore, then, besides the fact that laboratory experiments 
are already encouraging to us, there is the great probability that 
the power that the high temperature of the interior of the globe 
has to promote our interests as geologists is greater, and the 
opposing power of the high pressure is less, than what might be 
concluded, on first thoughts, from such experiments, carried out 
under necessarily limited variations of condition. 
But these considerations, though auxiliary, are only such— 
Can we form any rough conjecture, on other grounds, as to the 
actual relations of the two contending influences within the 
body of the earth? When we consider the enormous pressure 
that must exist within the globe, it is surprising to find that the 
mean density of the globe is only about double that of the mean 
density of its superficial parts (disregarding the thin film of 
ocean.) This formerly induced some persons, who were neglect- 
ing all other considerations, to believe that the inner part of the 
earth must be hollow. The difficulty felt by those persons would 
have been greatly increased on being informed by the astrono- 
mers, that certain peculiarities in the motion of the moon, both 
in latitude and longitude, shew that the attracting spheroidal 
earth must be much denser in the central parts than elsewhere. 
This is confirmed by other considerations. This goes to keep 
down the density of the less inward parts (the mean density being 
