[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 rough body of the moon, shown in section (Fig. 59), illuminates the whole cavity at _a_, part of the one at _b_, casts a long shadow from the mountain at _c_, and touches the tip of the one at _d_, which appears to a distant observer as a point of light beyond the terminator, As the moon revolves the conical cavity, _a_ is illuminated on the forward side only; the light creeps down the backward side of cavity _b_ to the bottom; mountain _c_. comes directly under the sun and casts no shadow, and mountain _d_ casts its long shadow over the plain. Knowing the time of revolution, and observing the change of [Page 156] illumination, we can easily measure the height of mountain and depth of crater. An apple, with excavations and added prominences, revolved on its axis toward the light of a candle, admirably illustrates the crescent light that fills either side of the cavities and the shadows of the mountains on the plain. Notice in Fig. 58 the crescent forms to the right, showing cavities in abundance. [Illustration: Fig. 60.--Lunar Crater "Copernicus," after Secchi.] The selenography of one side of the moon is much better known to us than the geography of the earth. Our maps of the moon are far more perfect than those of the earth; and the photographs of lunar objects by Messrs. Draper and De la Rue are wonderfully perfect, [Page 157] and the drawings of Padre Secchi equally so (Fig. 60). The least change recognizable from the earth must be speedily detected. There are frequently reports of discoveries of volcanoes on the moon, but they prove to be illusions. The moon will probably look the same to observers a thousand years hence as it does to-day.