CarrutHERs.—Retardation of Earth’s Rotation by Voleanie Action, 855 
tain range at A; we must therefore try and find out its amount. 
If we knew how much the earth masses had to compress before they 
could fall in, and the resistance offered to the compression by the elasticity 
of the material, we could calculate accurately the amount of heat generated; 
these we will now endeavour to ascertain. 
The change which takes place in the shape of the earth in about a 
hundred and fifty thousand years is equal to lessening its equatorial 
diameter one foot, and at the same time lengthening its polar diameter two 
feet ; a reduction of one foot in the diameter at the equator would reduce 
the circumference three feet. The whole compression of the crust in 
Fig. 2 would therefore be three feet, for this amount of change of shape. 
If we assume the masses at A to be each one thousand miles long, or 1-24 
of the whole circumference, each of them would have to compress 4 of a 
foot, which is therefore the amount of the compression at A, 
In order to ascertain the resistance offered to this compression we must 
find what resistance is offered to the compression of ordinary stone under 
different conditions, and for this purpose we will take for our standard 
a pillar one mile long, and for our unit of heat that quantity which would 
raise the temperature of the pillar one degree Fahrenheit. The area of the 
pillar is of no moment, as we want to know the length which would be 
warmed one degree by a given compression of the whole area. 
Stone expands about -000005 of its length when heated one degree; our 
pillar would therefore expand 1-40th of a foot when heated to the same 
extent. If instead of being heated the pillar were compressed 1-40th of a 
foot, an amount of heat would be generated exactly equal to that which 
would have expanded it 1-40th of a foot. Our unit of heat therefore repre- 
sents also a unit of energy sufficient to compress stone 1-40th of a foot 
under ordinary conditions. 
The conditions may be, however, so altered that the same compression 
would require any given number of times more energy to effect it. For 
instance, if we placed the pillar in a great screw press and lowered the upper 
plate until it just touched the top of the pillar; if then we applied one 
degree of heat the pillar would not expand, being prevented by the upper 
plate ; the same would follow at the second and the hundredth application 
of heat. If at last the screw were turned, and the pillar compressed one 
unit, the heat developed would be, not one degree but a hundred degrees ; 
the elasticity of the stone had been increased a hundred-fold by the hundred 
units of heat applied to it, it therefore required a hundred times more force 
to effect the compression, and this is measured by a hundred times more 
heat. 
