462 
ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 
then, as a circumstance nearly completely proved, that the earth possesses a heat 
of its own, independent of what it receives from the sun, and being the remains 
of its original heat. This return to ideas long since put forth by our greatest 
men, shews that it is never safe to slight even the boldest conjectures of men of 
genius; it is one of their privileges that truth often appears to them in their 
dreams. ’ Since we then have a central temperature sufficient to fuse all the 
materials we meet with on the earth’s surface, it must necessarily follow that the 
mass of heated matter in the centre is in a fluid or incandescent state—unless 
we can shew that the enormous pressure which can be demonstrated to exist on 
the matter in the interior of our planet retards or prevents its fusion. There is 
no chance of our being able to experiment with matter under a pressure any 
thing at all approaching that to which it must be subjected at depths consider¬ 
ably below the earth’s surface. No experiments, that I am aware of, go to prove 
solid matter to be of more difficult fussion when under a heavy superincumbent 
weight; but the certain knowledge of the existence of these great pressures 
inclines me to the belief that matter may there possess properties of which we 
know nothing. 
It can be demonstrated that the attraction at any point within a sphere of 
equal density, or of successive concentric strata of uniform but unequal densities, 
is always as the distance from the centre. Therefore the whole pressure of a 
column of homogeneous matter from the circumference of the earth to its centre, 
will be half what the same pressure w T ould be if the attractive force were uniform 
throughout. This will give for matter of the mean density of our planet a force 
equal to a million and a half atmospheres exercised by the superincumbent mass 
at the earth’s centre. Water at the centre would be under a pressure of 330,000 
atmospheres. Such a force might retain it as a solid, though even at a high 
temperature ; but here, also, w r e are left to conjecture, as we have no experiments 
made on fluids under great pressures to guide us. 
Arago and Dulong have given an empirical formula for ascertaining the 
temperature at which water begins to assume a gaseous state under pressure. 
This is verified by experiment only so far as 5^ atmospheres ; there is therefore 
a difficulty in extending it to great weights; the furthest it has been attempted 
by its authors is to 1,000 atmospheres, where it may not greatly err. Water at 
the depth of six miles below the level of the sea is nearly under this pressure, 
though Arago’s formula shews that it will require a temperature above a red 
heat,, and higher than that of melted Zinc, to bring it to the boiling point. This 
sufficiently demonstrates how different may be the state of matter at depths in 
the earth from what we know of it on the surface. - 
The late Sir John Leslie concluded, from an inquiry into the elasticity of solid 
bodies, that at a distance of a few miles below the earth’s surface, a specific 
