d by
corresponding adaptations in the pliable organisms of cultivated plants.
All we can certainly conclude is that no marked change has taken place
in the heat of the sun during historical time. But when we come to look
back into much earlier ages, we find copious evidence that the earth has
undergone great changes in climate. Geological records can on this
question hardly be misinterpreted. Yet it is curious to note that these
changes are hardly such as could arise from the gradual exhaustion of
the sun's radiation. No doubt, in very early times we have evidence
that the earth's climate must have been much warmer than at present. We
had the great carboniferous period, when the temperature must almost
have been tropical in Arctic latitudes. Yet it is hardly possible to
cite this as evidence that the sun was then much more powerful; for we
are immediately reminded of the glacial period, when our temperate zones
were overlaid by sheets of solid ice, as Northern Greenland is at
present. If we suppose the sun to have been hotter than it is at present
to account for the vegetation which produced coal, then we ought to
assume the sun to be colder than it is now to account for the glacial
period. It is not reasonable to attribute such phenomena to fluctuations
in the radiation from the sun. The glacial periods prove that we cannot
appeal to geology in aid of the doctrine that a secular cooling of the
sun is now in progress. The geological variations of climate may have
been caused by changes in the earth itself, or by changes in its actual
orbit; but however they have been caused, they hardly tell us much with
regard to the past history of our sun.

The heat of the sun has lasted countless ages; yet we cannot credit the
sun with the power of actually creating heat. We must apply to the
tremendous mass of the sun the same laws which we have found by our
experiments on the earth. We must ask, whence comes the heat sufficient
to supply this lavish outgoing? Let us briefly recount the various
suppositions that have been made.

Place two red-hot spheres of iron side by side, a large one and a small
one. They have been taken from the same fire; they were both equally
hot; they are both cooling, but the small sphere cools more rapidly. It
speedily becomes dark, while the large sphere is still glowing, and
would continue to do so for some minutes. The larger the sphere, the
longer it will take to cool; and hence it has been supposed