due to this character of our
atmosphere. But this mean temperature could not be maintained, were not
that of the earth itself in harmony with it. The surface might, no
doubt, be cooled or heated by the adjacent air, but the heat, if given
out from an earth warmer than the atmosphere, would be rapidly replaced
from within, and a constant accumulation ensue in the air, while, if the
earth were cooler, a diminution, equally constant, of the temperature of
the atmosphere, must take place. The earth is, however, itself subject
to the same law. All the materials of which it is composed, are capable
of compression, in a greater or less degree, and of being heated by
compression. The tendency of all material substances to the centre of
attraction, loads the parts nearest to that centre with the whole weight
of the superincumbent mass. And in the depth of four thousand miles,
which intervenes between the centre and the surface, the heat must be
far more than equal to that obtained by the compound blow-pipe or
galvanic deflagrator, under whose intense energies the most refractory
substances liquefy. Hence it may be inferred as a fact, as certain as
any in physical science, that the interior of the earth is at present in
a state resembling igneous fusion, not produced, however, by any of the
more familiar sources of heat, but by the intense pressure the upper
masses exert upon those nearer to the centre.

Here, then, we find the reason of the earth's having assumed a figure
consistent with the equilibrium of a fluid mass, whose particles are
endued with a mutual attraction, and which has a motion around an axis.

Let us suppose all the particles which now constitute the earth, to have
been originally disseminated throughout a vast space, and to have
approached their common centre of gravity by the force of mutual
attraction; the consideration thus caused would have produced the state
of intense heat that is now kept up within by pressure; and the
conducting power of the bodies would have propagated the heat nearly
equal throughout the mass. The surface would then have existed in a
liquid state as well as that beneath. But as the radiation from the
surface of a heated body is in exact proportion to its temperature, this
cause of cooling would have been intense, and a crust must soon have
formed upon the outer surface; this crust would have increased in
thickness so long as the heat thrown off by radiation exceeded that
received fr