tains a
certain store of energy, part of which is continually disappearing in
the form of radiant heat. The energy remaining in the sun is partly
transformed in character; some of it is transformed into heat, which
goes wholly or partly to supply the loss by radiation. The total energy
of the sun must, however, be decreasing; and hence it would seem the sun
must at some time or other have its energy exhausted, and cease to be a
source of light and of heat. It is true that the rate at which the sun
contracts is very slow. We are, indeed, not able to measure with
certainty the decrease in the sun's bulk. It is a quantity so minute,
that the contraction since the birth of accurate astronomy is not large
enough to be perceptible in our telescopes. It is, however, possible to
compute what the contraction of the sun's bulk must be, on the
supposition that the energy lost by that contraction just suffices to
supply the daily radiation of heat. The change is very small when we
consider the present size of the sun. At the present time the sun's
diameter is about 860,000 miles. If each year this diameter decreases by
about 300 feet, sufficient energy will be yielded to account for the
entire radiation. This gradual decrease is always in progress.

These considerations are of considerable interest when we apply them
retrospectively. If it be true that the sun is at this moment shrinking,
then in past times his globe must have been greater than it is at
present. Assuming the figures already given, it follows that one hundred
years ago the diameter of the sun must have been nearly six miles
greater than it is now; one thousand years ago the diameter was
fifty-seven miles greater; ten thousand years ago the diameter of the
sun was five hundred and seventy miles greater than it is to-day. When
man first trod this earth it would seem that the sun must have been many
hundreds, perhaps many thousands, of miles greater than it is at this
time.

We must not, however, over-estimate the significance of this statement.
The diameter of the sun is so great, that a diminution of 10,000 miles
would be but little more than the hundredth part of its diameter. If it
were suddenly to shrink to the extent of 10,000 miles, the change would
not be appreciable to ordinary observation, though a much smaller change
would not elude delicate astronomical measurement. It does not
necessarily follow that the climates on our earth in these early times
must ha