108 GENERAL DESCRIPTION OF THE MOON. 



them exceedingly conspicuous on the dark background 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. More- 

 over, even were water present, without an atmosphere there could be 

 no such circulation as takes place on the earth, upward to clouds and 

 thence downward by the rain and streams to the ocean. Clouds can 

 not exist unless there be an atmosphere in which they can float, and 

 even if there be an air of exceeding tenuity on the moon, it is surely 

 insuflicient to support a trace of clouds. Some distinguished astrono- 

 mers have thought to discern something floating of a cloud-like 

 nature, but these ol)servations, 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 l)oth air and water in any such 

 quantities as is necessarj^ to maintain organic life appears to exclude 

 the possibility of there being an}' such life as that of plants and ani- 

 mals on the lunar surface. It may be stated that very few astronomers 

 are now inclined to believe that the moon can possibly be the abode of 

 living forms. 



Being without an efl'ective atmosphere, for the possible but unproved 

 remnant that may exist there would be quite inefl'ective, 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, is occupied l\v wide 

 plains, which in their general shape are more nearl}' level than any 

 equally extensive areas of the land, or, so far as we know, of the ocean 

 floor of the earth, though they are beset with very many slight irregu- 

 larities. These areas of rough, dark-hued plains are the seas or maria 

 of selenographers, so termed because of old the}^ were, from their rela- 

 tivel}" level nature, supposed to be areas of water. These maria occupy 

 about one-third of the visible surface. Their height is somewhat less 

 than that of the crust outside of their area. The remaining portion of 



