of the sun, yet
she is dead and lifeless and still. We can examine her surface with the
telescope, and see it all very plainly. Even with a large opera-glass
those markings which, to the naked eye, seem to be like a queer
distorted face are changed, and show up as the shadows of great
mountains. We can only see one side of the moon, because as I have said,
she keeps always the same face turned to the earth; but as she sways
slightly in her orbit, we catch a glimpse of sometimes a little more on
one side and sometimes a little more on the other, and so we can judge
that the unseen part is very much the same as that turned toward us.

At first it is difficult to realize what it means to have no air.
Besides supporting life in every breath that is drawn by living
creatures, the air does numerous other kind offices for us--for
instance, it carries sound. Supposing the most terrific volcano exploded
in an airless world, it could not be heard. The air serves as a screen
by day to keep off the burning heat of the sun's rays, and as a blanket
by night to keep in the heat and not let it escape too quickly. If there
were no air there could be no water, for all water would evaporate and
vanish at once. Imagine the world deprived of air; then the sun's rays
would fall with such fierceness that even the strongest tropical sun we
know would be as nothing in comparison with it, and every green thing
would shrivel up and die; this scorching sun would shine out of a black
sky in which the stars would all be visible in the daytime, not hidden
by the soft blue veil of air, as they are now. At night the instant the
sun disappeared below the horizon black darkness would set in, for our
lingering twilight is due to the reflection of the sun in the upper
layers of air, and a bitterness of deathly cold would fall upon the
earth--cold fiercer than that of the Arctic regions--and everything
would be frozen solid. It would need but a short time to reduce the
earth to the condition of the moon, where there is nothing to shrivel
up, nothing to freeze. Her surface is made up of barren, arid rocks, and
her scenery consists of icy black shadows and scorching white plains.

[Illustration: _Paris Observatory._

THE MOON.]

The black shadows define the mountains, and tremendous mountains they
are. Most of them have craters. A crater is like a cup, and generally
has a little peak in the middle of it. This is the summit of a volcano,
and when the volc