By the observed temperature of mines, and that at the bottom of
artesian wells, it has been established that the rate at which such
temperature increases as we descend varies considerably in different
localities, where the depths are comparatively small; but where the
depths are great, we find a much nearer approximation to a common
rate of increase, which, as determined by the best observation in the
deepest mines, shafts, and artesian wells in Western Europe, is very
nearly 1° F. _for an increase in depth of fifty feet_.--_W. Hopkins,
M.A., F.R.S._
Humboldt states that, according to tolerably coincident experiments in
artesian wells, it has been shown that the heat increases on an average
about 1° for every 54·5 feet. If this increase can be reduced to
arithmetical relations, it will follow 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 Himalaya.
The following is the opinion of Professor Silliman:
That the whole interior portion of the earth, or at least a great
part of it, is an ocean of melted rock, agitated by violent winds,
though I dare not affirm it, is still rendered highly probable by
the phenomena of volcanoes. The facts connected with their eruption
have been ascertained and placed beyond a doubt. How, then, are
they to be accounted for? The theory prevalent some years since,
that they are caused by the combustion of immense coal-beds, is
puerile and now entirely abandoned. All the coal in the world could
not afford fuel enough for one of the tremendous eruptions of
Vesuvius.
This observed increase of temperature in descending beneath the earth’s
surface suggested the notion of a central incandescent nucleus still
remaining in a state of fluidity from its elevated temperature. Hence
the theory that the whole mass of the earth was formerly a molten
fluid mass, the exterior portion of which, to some unknown depth, has
assumed its present solidity by the radiation of heat into surrounding
space, and its consequent refrigeration.
The mathematical solution of this problem of Central Heat, assuming
such heat to exist, tells us that though the central portion of the
earth may consist of a mass of molten matter, the temperature of its
surface is not thereby increased by more than the small fraction of
a degree. Poisson has calculated that it would require _a thousand
millions of centuries_ to reduce this fraction to a degree by half its
present amount, supposing always the external conditions to remain
unaltered. In such cases, the superficial temperature of the earth may,
in fact, be considered to have approximated so near to its ultimate
limit that it can be subject to no further sensible change.