the contracting mass, as small bodies radiate
heat much faster than large ones. The moon seems to be already
thoroughly refrigerated, while Jupiter and Saturn are very much hotter
than the earth, as is shown by the tremendous atmospheric phenomena
which occur on their surfaces. The sun, again, generates heat so
rapidly, owing to his great energy of contraction, and loses it so
slowly, owing to his great size, that his surface is always kept in a
state of incandescence. His surface-temperature is estimated at some
three million degrees of Fahrenheit, and a diminution of his diameter
far too small to be detected by the finest existing instruments would
suffice to maintain the present supply of heat for more than fifty
centuries. These facts point to a very long future during which the sun
will continue to warm the earth and its companion planets, but at the
same time they carry on their face the story of inevitable ultimate
doom. If things continue to go on as they have all along gone on, the
sun must by and by grow black and cold, and all life whatever throughout
the solar system must come to an end. Long before this consummation,
however, life will probably have become extinct through the
refrigeration of each of the planets into a state like the present state
of the moon, in which the atmosphere and oceans have disappeared from
the surface. No doubt the sun will continue to give out heat a long time
after heat has ceased to be needed for the support of living organisms.
For the final refrigeration of the sun will long be postponed by the
fate of the planets themselves. The separation of the planets from their
parent solar mass seems to be after all but a temporary separation. So
nicely balanced are they now in their orbits that they may well seem
capable of rolling on in their present courses forever. But this is not
the case. Two sets of circumstances are all the while striving, the one
to drive the planets farther away from the sun, the other to draw them
all into it. On the one hand, every body in our system which contains
fluid matter has tides raised upon its surface by the attraction of
neighbouring bodies. All the planets raise tides upon the surface of the
sun and the periodicity of sun-spots (or solar cyclones) depends upon
this fact. These tidal waves act as a drag or brake upon the rotation
of the sun, somewhat diminishing its rapidity. But, in conformity with a
principle of mechanics well known to astronomers