ave their winter and summer
alternately near and far from the sun. It is easily seen that when the
summer season comes to a hemisphere in the part of the orbit which is
then nearest the sun the period will be very hot. When the summer
came farthest from the sun that part of the year would have the
temperature mitigated by its removal to a greater distance from the
source of heat. A corresponding effect would be produced in the winter
season. As long as the orbit remained eccentric the tendency would be
to give alternately intense seasons to each hemisphere through periods
of about twelve thousand years, the other hemisphere having at the
same time a relatively slight variation in the summer and winter.

At first sight it may seem to the reader that these studies we have
just been making in matters concerning the shape of the orbit and the
attendant circumstances which regulate the seasons were of no very
great consequence; but, in the opinion of some students of climate, we
are to look to these processes for an explanation of certain climatal
changes on the earth, including the Glacial periods, accidents which
have had the utmost importance in the history of man, as well as of
all the other life of the planet.

It is now time to give some account as to what is known concerning the
general conditions of the solar bodies--the planets and satellites of
our own celestial group. For our purpose we need attend only to the
general physical state of these orbs so far as it is known to us by
the studies of astronomers. The nearest planet to the sun is Mercury.
This little sphere, less than half the diameter of our earth, is so
close to the sun that even when most favourably placed for observation
it is visible for but a few minutes before sunrise and after sunset.
Although it may without much difficulty be found by the ordinary eye,
very few people have ever seen it. To the telescope when it is in the
_full moon_ state it appears as a brilliant disk; it is held by most
astronomers that the surface which we see is made up altogether of
clouds, but this, as most else that has been stated concerning this
planet, is doubtful. The sphere is so near to the sun that if it were
possessed of water it would inevitably bear an atmosphere full of
vapour. Under any conceivable conditions of a planet placed as
Mercury is, provided it had an atmosphere to retain the heat, its
temperature would necessarily be very high. Life as we know it coul