rate furnaces, but it does not attain the temperature of
the sun.

It must, however, be admitted that there is an apparent discrepancy
between a fact of common experience and the statement that the sun
possesses the extremely high temperature that we have just tried to
illustrate. "If the sun were hot," it has been said, "then the nearer we
approach to him the hotter we should feel; yet this does not seem to be
the case. On the top of a high mountain we are nearer to the sun, and
yet everybody knows that it is much colder up there than in the valley
beneath. If the mountain be as high as Mont Blanc, then we are certainly
two or three miles nearer the glowing globe than we were at the
sea-level; yet, instead of additional warmth, we find eternal snow." A
simple illustration may help to lessen this difficulty. In a greenhouse
on a sunshiny day the temperature is much hotter than it is outside. The
glass will permit the hot sunbeams to enter, but it refuses to allow
them out again with equal freedom, and consequently the temperature
rises. The earth may, from this point of view, be likened to a
greenhouse, only, instead of the panes of glass, our globe is enveloped
by an enormous coating of air. On the earth's surface, we stand, as it
were, inside the greenhouse, and we benefit by the interposition of the
atmosphere; but when we climb very high mountains, we gradually pass
through some of the protecting medium, and then we suffer from the cold.
If the earth were deprived of its coat of air, it seems certain that
eternal frost would reign over whole continents as well as on the tops
of the mountains.

The actual distance of the sun from the earth is about 92,900,000 miles;
but by merely reciting the figures we do not receive a vivid impression
of the real magnitude. It would be necessary to count as quickly as
possible for three days and three nights before one million was
completed; yet this would have to be repeated nearly ninety-three times
before we had counted all the miles between the earth and the sun.

Every clear night we see a vast host of stars scattered over the sky.
Some are bright, some are faint, some are grouped into remarkable forms.
With regard to this multitude of brilliant points we have now to ask an
important question. Are they bodies which shine by their own light like
the sun, or do they only shine with borrowed light like the moon? The
answer is easily stated. Most of those bodies shine by their o