in going up it is turned a quarter over, by the hands holding it steady, the same side is visible.  Three causes enable us to see a little more than half the moon’s surface:  1.  The speed with which it traverses the ellipse of its orbit is variable.  It sometimes gets ahead of us, sometimes behind, and we see farther around the front or back part. 2.  The axis is a little inclined to the plane of its orbit, and its orbit a little inclined to ours; hence we see a little over its north pole, and then again over the south pole. 3.  The earth being larger, its inhabitants see a little more than half-way around a smaller body.  These causes combined enable us to see 576/1000 of the moon’s surface.  Our eyes will never see the other side of the moon.  If, now, being solid, her axial revolution could [Page 153] be increased enough to make one more revolution in two or three years, that difference between her axial and orbital revolution would give the future inhabitants of the earth a view of the entire circumference of the moon.  Yet if the moon were once in a fluid state, or had oceans on the surface, the enormous tide caused by the earth would produce friction enough, as they moved over the surface, to gradually retard the axial revolution till the two tidal elevations remained fixed toward and opposite the earth, and then the axial and orbital revolutions would correspond, as 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.