expenditure of force, limited in supply. The sun and stars are
continually sending out a flood of heat. They are exhausting the
internal supply of something which must be limited in extent.
Whence comes the supply? How is the heat of the sun kept up? If it
were a hot body cooling off, a very few years would suffice for it
to cool off so far that its surface would become solid and very
soon cold. In recent years, the theory universally accepted has
been that the supply of heat is kept up by the continual
contraction of the sun, by mutual gravitation of its parts as it
cools off. This theory has the advantage of enabling us to
calculate, with some approximation to exactness, at what rate the
sun must be contracting in order to keep up the supply of heat
which it radiates. On this theory, it must, ten millions of years
ago, have had twice its present diameter, while less than twenty
millions of years ago it could not have existed except as an
immense nebula filling the whole solar system. We must bear in
mind that this theory is the only one which accounts for the
supply of heat, even through human history. If it be true, then
the sun, earth, and solar system must be less than twenty million
years old.

Here the geologists step in and tell us that this conclusion is
wholly inadmissible. The study of the strata of the earth and of
many other geological phenomena, they assure us, makes it certain
that the earth must have existed much in its present condition for
hundreds of millions of years. During all that time there can have
been no great diminution in the supply of heat radiated by the
sun.

The astronomer, in considering this argument, has to admit that he