hat to which his
researches have finally conducted him, is thus expressed: "Upon the
whole, then, we may venture to assert that the minimum thickness of the
crust of the globe, which can be deemed consistent with the observed
amount of precession, cannot be less than one-fourth or one-fifth of the
earth's radius." That is from 800 to 1000 miles.[746]

It will be remarked, that this is a _minimum_, and any still _greater_
amount would be quite consistent with the actual phenomena; the
calculations not being opposed to the supposition of the general
solidity of the entire globe. Nor do they preclude us from imagining
that great lakes or seas of melted matter may be distributed through a
shell 400 or 800 miles thick, provided they be so inclosed as to move
with it, whatever motion of rotation may be communicated by the
disturbing forces of the sun and moon.

_Central heat._--The hypothesis of internal fluidity calls for the more
attentive consideration, as it has been found that the heat in mines
augments in proportion as we descend. Observations have been made, not
only on the temperature of the air in mines, but on that of the rocks,
and on the water issuing from them. The mean rate of increase,
calculated from results obtained in six of the deepest coal mines in
Durham and Northumberland, is 1 degrees Fahr. for a descent of forty-four
English feet.[747] A series of observations, made in several of the
principal lead and silver mines in Saxony, gave 1 degrees Fahr. for every
sixty-five feet. In this case, the bulb of the thermometer was
introduced into cavities purposely cut in the solid rock at depths
varying from 200 to above 900 feet. But in other mines of the same
country, it was necessary to descend thrice as far for each degree of
temperature.[748]

A thermometer was fixed in the rock of the Dolcoath mine, in Cornwall,
by Mr. Fox, at the great depth of 1380 feet, and frequently observed
during eighteen months; the mean temperature was 68 degrees Fahr., that
of the surface being 50 degrees, which gives 1 degrees for every
seventy-five feet.

Kupffer, after an extensive comparison of the results in different
countries, makes the increase 1 degrees F. for about every thirty-seven
English feet.[749] M. Cordier announces, as the result of his
experiments and observations on the temperature of the interior of the
earth, that the heat increases rapidly with the depth; but the increase
does not follow the same law ove