intensely heated
state, and to have been parting ever since with portions of its heat,
and at the same time contracting its dimensions. There are, undoubtedly,
good grounds for inferring from recent observation and experiment, that
the temperature of the earth increases as we descend from the surface to
that slight depth to which man can penetrate: but there are no positive
proofs of a secular decrease of internal heat accompanied by
contraction. On the contrary, La Place has shown, by reference to
astronomical observations made in the time of Hipparchus, that in the
last two thousand years at least there has been no sensible contraction
of the globe by cooling; for had this been the case, even to an
extremely small amount, the day would have been shortened, whereas its
length has certainly not diminished during that period by 1/300th of a
second.

Baron Fourier, after making a curious series of experiments on the
cooling of incandescent bodies, considers it to be proved
mathematically, that the actual distribution of heat in the earth's
envelope is precisely that which would have taken place if the globe
had been formed in a medium of a very high temperature, and had
afterwards been constantly cooled.[210] He contends, that although no
contraction can be demonstrated to have taken place within the
historical period (the operation being slow and the time of observation
limited), yet it is no less certain that heat is annually passing out by
radiation from the interior of the globe into the planetary spaces. He
even undertook to demonstrate that the quantity of heat thus transmitted
into space in the course of every century, through every square metre of
the earth's surface, would suffice to melt a column of ice having a
square metre for its base, and being three metres (or 9 feet 10 inches)
high.

It is at the same time denied, that there is any assignable mode in
which the heat thus lost by radiation can be again restored to the
earth, and consequently the interior of our planet must, from the moment
of its creation, have been subject to refrigeration, and is destined
together with the sun and stars forever to grow colder. But I shall
point out in the sequel (chapter 31) many objections to these views, and
to the theory of the intense heat of the earth's central nucleus, and
shall then inquire how far the observed augmentation of temperature, as
we descend below the surface, may be referable to other causes
unconne