CarrutHErs.—Retardation of Earth’s Rotation by Voleanie Action. 861 
The theory that the earth increases in temperature about one degree for 
every fifty feet as we descend is founded on observations made principally 
in mines in western Europe, that is, along a line of volcanic action, the 
present activity of which in England was shown by the earthquake of 
Lisbon affecting the waters of Loch Ness. 
Superficial action must here still be taking place and maintaining the 
warmth of the upper strata. 
This theory cannot be at all considered as proved by observation. It 
requires to be tested by sinking mines in various parts of the world, espec- 
cially in areas where subsidence has been long continued. 
We have spoken of the solid crust and liquid interior of the earth more 
for convenience of illustration than anything else. There is no sufficient 
reason, however, to suppose that any part of the earth is, or ever has been, 
fluid, but it is as well established as any other scientific knowledge founded 
only on inference, that as we descend, the elasticity increases, and for the 
purposes of our hypothesis, nothing more is required. 
On the assumption we have made above, the temperature increases only 
about 2,000 degrees in 800 miles. If this is near the truth the earth must 
be solid to the very centre, and some explanation is required of the slight 
power of resistance to tensile strain which we know it to possess. The 
explanation lies in the increasing elasticity as we approach the centre. 
Where stone is under compression, and is only kept by force from ex- 
panding, it has no tensile strength whatever to resist a strain tending 
slightly to lengthen it, or rather its strength is a negative quantity. The 
inner part of the earth is in this position, so that the whole strain is thrown 
on a thin outer layer which has tobear not only its own tendency to change 
its shape, but also that of the much larger inner part. It is, therefore, not 
surprising that it yields to very little more strain than that caused by the 
tides. That it does not yield to the tides alone is probably largely due to 
the rapidity of the earth’s rotation, which carries away the part under the 
greatest strain so quickly from under the moon that the molecular motion 
has not full time to bring up all its forces against it. 
We must now show that the elasticity we have assumed for the interior 
of the earth is sufficient to prevent further condensation except as the 
interior heat is radiated away. 
The weight of a pillar of stone one square inch in area and one mile 
long, would be about 5000 Ibs. At a depth of 800 miles the specific 
gravity is doubled, but the earth has there lost half of its attracting power, 
so the effective compressing force of equal bulks of stone would be the 
same in both positions. The total weight therefore which each square inch 
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