856 Transactions. —Geology. 
If instead of warming the pillar we compressed it by applying equal 
amounts of mechanical energy the result would be the same, The first 
compression would warm the pillar one degree, it would then have double 
the elasticity, and the second amount of energy would effect only half the 
amount of compression, which would, however, be made against double the 
resistance and would therefore develope the same quantity of heat as the 
first, the pillar would therefore be warmed another degree, or two degrees 
in all, and so on with each application of energy. This assumes that all 
the heat generated is retained within the pillar. In experiments with 
short pillars this can never be the case, for the molecular motion is 
carried through the supports to the earth, but when the pillar is the 
earth itself, the heat can only be carried away by the slow process of 
radiation. 
The earth is, in fact, exactly in the position of our supposed pillar. 
Force causing compression has been applied to it by gravity, and is still 
being applied, heat must therefore be generated. When part of this heat 
is radiated into space and lost to the earth, the material loses part of its 
elasticity ; gravity, which before was unable to compress it further, is then 
able to do so, heat is again evolved by the compression, and the elasticity 
of the material nearly maintained, the loss being measured by the radiation, 
less the heat generated by the further compression. 
If our pillar were compressed until it were only half a mile long instead 
of a mile, and at the same time the temperature were maintained at 
100,000 degrees (to which it would have been raised by the compression) ; 
as soon as the constraint was removed the pillar would expand to its full 
length of one mile, as every degree would expand it 1-40th of a foot, and 
1-40 x 100,000 is equal to 2500 feet, or half a mile nearly. In the same 
way, if a portion of the interior of the earth which had been compressed 
until its specific gravity was doubled, were brought to the surface, it would 
expand until it had its original bulk, if it brought with it 100,000 degrees 
of heat; if, however, its heat were less, that is, if any part of it had been 
lost by radiation, it would not expand to the full bulk, and it would, there- 
fore, have less elasticity than would be due to the compression it had un- 
dergone. 
In the case of the earth, some of its heat has certainly been lost by 
radiation. We cannot tell how much, but, for the sake of illustration, and 
to show that on any supposition which can reasonably be made there is 
sufficient left to account for volcanic action, we will assume that it has lost 
by radiation not more than 49 parts out of every 50 of its elasticity. 
On this supposition, the elasticity would be that corresponding to 2000 
instead of 100,000 degrees of heat at that depth where the density is double 
