of meteors were caught every year, there must be an
incredible mass of meteoric matter roaming at large through the system.
There must be so many meteors that the earth would be incessantly pelted
with them, and heated to such a degree as to be rendered uninhabitable.
There are also other reasons which preclude the supposition that a
stupendous quantity of meteoric matter exists in the vicinity of the
sun. Such matter would produce an appreciable effect on the movement of
the planet Mercury. There are, no doubt, some irregularities in the
movements of Mercury not yet fully explained, but these irregularities
are very much less than would be the case if meteoric matter existed in
quantity adequate to the sustentation of the sun. Astronomers, then,
believe that though meteors may provide a rate in aid of the sun's
current expenditure, yet that the greater portion of that expenditure
must be defrayed from other resources.

It is one of the achievements of modern science to have effected the
solution of the problem--to have shown how it is that, notwithstanding
the stupendous radiation, the sun still maintains its temperature. The
question is not free from difficulty in its exposition, but the matter
is one of such very great importance that we are compelled to make the
attempt.

Let us imagine a vast globe of heated gas in space. This is not an
entirely gratuitous supposition, inasmuch as there are globes apparently
of this character; they have been already alluded to as planetary
nebulae. This globe will radiate heat, and we shall suppose that it emits
more heat than it receives from the radiation of other bodies. The globe
will accordingly lose heat, or what is equivalent thereto, but it will
be incorrect to assume that the globe will necessarily fall in
temperature. That the contrary is, indeed, the case is a result almost
paradoxical at the first glance; but yet it can be shown to be a
necessary consequence of the laws of heat and of gases.

Let us fix our attention on a portion of the gas lying on the surface
of the globe. This is, of course, attracted by all the rest of the
globe, and thus tends in towards the centre of the globe. If equilibrium
subsists, this tendency must be neutralised by the pressure of the gas
beneath; so that the greater the gravitation, the greater is the
pressure. When the globe of gas loses heat by radiation, let us suppose
that it grows colder--that its temperature accordingly falls; the