The Sun and Moon, which cause
the sea to ebb and flow, most probably also affect these subterranean
depths. We may suppose that the periodic elevations and depressions of the
molten mass under the already solidified strata must have caused
inequalities in the vaulted surface from the force of pressure. The amount
and action of such oscillations must, however, be small; and if the relative
position of the attracting cosmical bodies may here also excite "spring
tides," it is certainly not to these, but to more powerful internal forces,
that we must ascribe the movements that shake the Earth's surface. There
are groups of phenomena to whose existence it is necessary to draw
attention, in order to indicate the universality of the influence of the
attraction of the Sun and Moon on the external and internal conditions of
the Earth, however little we may be able to determine the quantity of this
influence.
According to tolerably accordant experiments in Artesian wells, it has been
shown that the heat increases on an average about 1 degree for every 54.5
feet. If this increase can be reduced
p 174
to arithmetical relations, it will follow, as I have already observed,* that
a stratum of granite would be in a state of fusion at a depth of nearly
twenty-one geographical miles, or between four and five times the elevation
of the highest summit of the Hinalaya.
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