s at present. In fact, we can prove that the form of
the moon is protuberant toward the earth. Its centre of gravity is
thirty-three miles beyond its centre of magnitude, which is the same
in effect as if a mountain of that enormous height rose on the earth
side. Hence any fluid, as water or air, would flow round to the
other side.

The moon's day, caused by the sun's light, is 29-1/2 times as long
as ours. The sun shines unintermittingly for fifteen days, raising a
temperature as fervid as boiling water. Then darkness and frightful
cold for the same time succeed, except on that half where the earth
acts as a moon. The earth presents the same phases--crescent, full,
and gibbous--to the moon as the moon does to us, and for the same
causes. Lord Rosse has been enabled, by his six-foot reflector, to
measure the difference of heat on the moon under the full blaze
of its noonday and midnight. He finds it to be no less than five
hundred degrees. People not enjoying extremes of temperature should
shun a lunar residence. The moon gives us only 1/6180000 as much
light as the sun. A sky full of moons would scarcely make daylight.

[Page 154]
[Illustration: Fig. 58.--View of the Moon near the Third Quarter.
From a Photograph by Professor Henry Draper.]

There are no indications of air or water on the moon. When it occults
a star it instantly shuts off the light and as instantly reveals
it again. An atmosphere would gradually diminish and reveal the
light, and by refraction [Page 155] cause the star to be hidden in
much less time than the solid body of the moon would need to pass
over it. If the moon ever had air and water, as it probably did,
they are now absorbed in the porous lava of its substance.

_Telescopic Appearance._

[Illustration: Fig. 59.--Illumination of Craters and Peaks.]

Probably no one ever saw the moon by means of a good telescope
without a feeling of admiration and awe. Except at full-moon, we
can see where the daylight struggles with the dark along the line
of the moon's sunrise or sunset. This line is called the terminator.
It is broken in the extreme, because the surface is as rough as
possible. In consequence of the small gravitation of the moon, utter
absence of the expansive power of ice shivering the cliffs, or the
levelling power of rains, precipices can stand in perpendicularity,
mountains shoot up like needles, and cavities three miles deep
remain unfilled. The light of the sun falling on the