This is the
contraction which is produced in the great incandescent bodies of
the universe by the loss of the heat which they radiate. As
remarked in the preceding essay, the energy generated by the sun's
contraction could not have kept up its present supply of heat for
much more than twenty or thirty millions of years, while the study
of earth and ocean shows evidence of the action of a series of
causes which must have been going on for hundreds of millions of
years.
The antagonism between the two conclusions is even more marked
than would appear from this statement. The period of the sun's
heat set by the astronomical physicist is that during which our
luminary could possibly have existed in its present form. The
period set by the geologist is not merely that of the sun's
existence, but that during which the causes effecting geological
changes have not undergone any complete revolution. If, at any
time, the sun radiated much less than its present amount of heat,
no water could have existed on the earth's surface except in the
form of ice; there would have been scarcely any evaporation, and
the geological changes due to erosion could not have taken place.
Moreover, the commencement of the geological operations of which
we speak is by no means the commencement of the earth's existence.
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