174 
ME. KOBERT MALLET ON VOLCANIC ENERGY. 
radii) and scarcely sensibly affect the curvature of the surface within limited areas, we 
may consider our globe as spherical and §i=§ 2 , whence equation I. becomes 
and 
T=P X f III. 
If the curved surface be very largely elliptical, then the value of T given by this 
equation is not quite exact, as may happen in cases of local elevation or depression 
from local causes; but we are not here concerned with this further, as we shall view 
the globe as spherical without sensible error. 
Now, to apply this to our case, let us suppose that we have a spherical shell of only 
one mile thick forming the surface of our globe, that it preserves its form by equilibrium, 
and that it is wholly unsupported by the nucleus below it, by the attraction of which, 
resulting in its own weight, it is urged towards the centre of the sphere. 
84. Let the shell be of the most resistant rocky material which we know of, viz. 
granite or porphyry, crushing at 14 tons per square inch, or a little above 2000 tons per 
square foot, and of specific gravity = 2 ‘85 8, or weighing from 175 to 180 lbs. per cubic 
foot. Our results, which only require, as for illustration here, to be approximate, tvill 
not be materially affected by our assuming the diameter of the globe 8000 miles each of 
G000 feet or 1000 fathoms, which will save a mass of figures. The result will be quite 
comparable with those we should obtain by taking the sphere at 7920 British statute 
miles. N ow in equation III. 
£=K, the radius above taken for our globe, and therefore 
T=2000P; 
but in the case of the shell of 1 mile thickness, P=the weight in tons of 1 cubic mile 
of its material, i. e. of granite or porphyry, which, as a cubic fathom of water weighs 
very nearly G tons, will be 6 X 2*858 tons per cubic fathom, or 1000 3 X 6 X 2*858 tons per 
cubic mile. Then T= 2000 X 1000 :3 X 6 X 2*858 tons, the horizontal thrust per square 
mile, which, divided by the number of square feet in a square mile, viz. 1000 2 X 36, gives 
as the value of T for the square foot 
T= 952666 tons, 
which is more than 472 times greater than the crushing load given for granite or porphyry. 
This simple investigation will sufficiently apply to any cubic mile in a like covering 
shell, even if 100 miles (or even more) in thickness, the variation in the force of gravity 
at that depth, ^ of the assumed radius of the globe, being so small that we may here 
neglect it. 
The subject may be viewed more intelligibly perhaps in the following way. As T is 
