that a
mighty sphere of the prodigious dimensions of our sun would, if once
heated, cool gradually, but the duration of the cooling would be so long
that for thousands and for millions of years it could continue to be a
source of light and heat to the revolving system of planets. This
suggestion will not bear the test of arithmetic. If the sun had no
source of heat beyond that indicated by its high temperature, we can
show that radiation would cool the sun a few degrees every year. Two
thousand years would then witness a very great decrease in the sun's
heat. We are certain that no such decrease can have taken place. The
source of the sun's radiation cannot be found in the mere cooling of an
incandescent mass.

Can the fires in the sun be maintained by combustion, analogous to that
which goes on in our furnaces? Here we would seem to have a source of
gigantic heat; but arithmetic also disposes of this supposition. We know
that if the sun were made of even solid coal itself, and if that coal
were burning in pure oxygen, the heat that could be produced would only
suffice for 6,000 years. If the sun which shone upon the builders of the
great Pyramid had been solid coal from surface to centre, it must by
this time have been in great part burned away in the attempt to maintain
its present rate of expenditure. We are thus forced to look to other
sources for the supply of the sun's heat, since neither the heat of
incandescence nor the heat of combustion will suffice.

There is probably--indeed, we may say certainly--one external source
from which the heat of the sun is recruited. It will be necessary for us
to consider this source with some care, though I think we shall find it
to be merely an auxiliary of comparatively trifling moment. According to
this view, the solar heat receives occasional accessions from the fall
upon the sun's surface of masses of meteoric matter. There can be hardly
a doubt that such masses do fall upon the sun; there is certainly no
doubt that if they do, the sun must gain some heat thereby. We have
experience on the earth of a very interesting kind, which illustrates
the development of heat by meteoric matter. There lies a world of
philosophy in a shooting star. Some of these myriad objects rush into
our atmosphere and are lost; others, no doubt, rush into the sun with
the same result. We also admit that the descent of a shooting star into
the atmosphere of the sun must be attended with a flash of