my. Sir William Herschel began by trying
to sound its depths; at one time he thought he had succeeded; but
before he died he saw that they were unfathomable with his most
powerful telescopes. Even today he would be a bold astronomer who would
profess to say with certainty whether the smallest stars we can
photograph are at the boundary of the system. Before we decide this
point we must have some idea of the form and distance of the cloudlike
masses of stars which form our great celestial girdle. A most curious
fact is that our solar system seems to be in the centre of this
galactic universe, because the Milky Way divides the heavens into two
equal parts, and seems equally broad at all points. Were we looking at
such a girdle as this from one side or the other, this appearance would
not be presented. But let us not be too bold. Perhaps we are the
victims of some fallacy, as Ptolemy was when he proved, by what looked
like sound reasoning, based on undeniable facts, that this earth of
ours stood at rest in the centre of the heavens!

A related problem, and one which may be of supreme importance to the
future of our race, is, What is the source of the heat radiated by the
sun and stars? We know that life on the earth is dependent on the heat
which the sun sends it. If we were deprived of this heat we should in a
few days be enveloped in a frost which would destroy nearly all
vegetation, and in a few months neither man nor animal would be alive,
unless crouching over fires soon to expire for want of fuel. We also
know that, at a time which is geologically recent, the whole of New
England was covered with a sheet of ice, hundreds or even thousands of
feet thick, above which no mountain but Washington raised its head. It
is quite possible that a small diminution in the supply of heat sent us
by the sun would gradually reproduce the great glacier, and once more
make the Eastern States like the pole. But the fact is that
observations of temperature in various countries for the last two or
three hundred years do not show any change in climate which can be
attributed to a variation in the amount of heat received from the sun.

The acceptance of this theory of the heat of those heavenly bodies
which shine by their own light--sun, stars, and nebulae--still leaves
open a problem that looks insoluble with our present knowledge. What
becomes of the great flood of heat and light which the sun and stars
radiate into empty space with a v