6 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



amount of air in the line of its rays. This evidence affords proof that if there is 

 any air at all on the moon's surface it is probably less in amount than remains in 

 the nearest approach to a vacuum we can produce by means of an air-pump. 

 Like proof of the airless nature of the moon is afforded by the spectroscope 

 applied to the study of the light of an occulting star or that of the sun as it is 

 becoming eclipsed by the moon. In fact a great body of evidence goes to show 

 that there is no air whatever on the lunar surface. 



The evidence of lack of water at the present time on the surface of the 

 moon appears to be as complete as that which shows the lack of an atmosphere. 

 In the first place, there are evidently no seas or even lakes of discernible size. 

 There are clearly no rivers. If such features existed, the reflection of the sun 

 from their surfaces would make them exceedingly conspicuous on the dark back- 

 ground of the moon, which for all its apparent brightness is really as dark as the 

 more somber-hued rocks of the earth's surface when lit by the sun. Moreover, 

 even were water present, without an atmosphere there could be no such circula- 

 tion as takes place on the earth, upward to clouds and thence downward by the 

 rain and streams to the ocean. Clouds cannot exist unless there be an atmos- 

 phere in which they can float, and even if there be an air of exceeding tenuity on 

 the moon, it is surely insufiicient to support a trace of clouds. Some distin- 

 guished astronomers have thought to discern something floating of a cloud-like 

 nature, but these observations, though exceedingly interesting, are not sufficiently 

 verified to have much weight against the body of well-observed facts that shows 

 the moon to be essentially waterless. 



The well-established absence of both air and water in any such quantities as 

 are necessary to maintain organic life appears to exclude the possibility of there 

 being any such life as that of plants and animals on the lunar surface. The reader 

 will find below a further discussion of this question, and it may therefore here be 

 passed with the statement that very few astronomers are now inclined to believe 

 that the moon can possibly be the abode of living forms. 



Being without an eff^ective atmosphere, for the possible but unproved rem- 

 nant that may exist there would be quite ineffective, the moon lacks the defense 

 against radiation of heat which the air affords the earth. Therefore in the long 

 lunar night the outflow of heat must bring the temperature of the darkened part 

 to near that of the celestial spaces, certainly to some hundred degrees below 

 Fahrenheit zero. Even in the long day this lack of air and consequent easy 

 radiation must prevent any considerable warming of the surface. The temper- 

 ature of the moon has been made the matter of numerous experiments. These, 

 for various reasons, have not proved very effective. The most trustworthy, the 

 series undertaken by S. P. Langley, indicate that at no time does the heat attain 

 to that of melting ice. 



Turning now to the shape and structure of the moon's crust, we observe that » 

 it differs much from that of the earth. Considering first the more general features, 

 we note that there are none of those broad ridges and furrows, — the continents 

 and the sea basins. A portion of the surface, mainly in the northern hemisphere, 



