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



all act to send divided matter from higher to lower positions. Except the first 

 and the last, they incidentally provide means of carriage by which the fragments 

 may be conveyed to indefinite distances ; chemical decay and the increase and 

 decrease in bulk due to variable heat acting by themselves do no more than give 

 the separated bits a chance to move down declivities of considerable slope. 



It is evident that all the chemical change which occurs on the earth depends 

 on the presence of an atmosphere containing water. This condition apparently, 

 I think surely, does not now exist on the moon and probably, as I shall hereafter 

 o-ive reasons for believincr, has never existed there ; for this reason we may set 

 aside this agent as a possible source of changes of lunar topography. From the 

 same facts we are led to dismiss the possibility of wind action. The only sug- 

 gestion of such work has been to explain the radial light bands on the supposi- 

 tion that the vapors emanating from the craters by their rapid diffusion caused 

 winds that blew the material which forms the rays to the places it occupies. We 

 have seen that this hypothesis does not account for the facts, and that they are 

 apparently explained by a much simpler view of the matter. 



The idea of water having been at some time in the past an agent of erosion 

 on the moon is so persistently recurring that it is worth while to set forth, in 

 some detail, the results of my studies of the matter. I gave over fifty nights of 

 observing with the Harvard College Mertz refractor, which has an excellent 

 glass, to the question of a possible aqueous history of the several divisions of the 

 lunar field. The result Was to convince me that no part of that surface, new or 

 old, has ever been shaped by aqueous erosion, and this for the following reasons : 

 Aqueous erosion by river action has one characteristic effect : it, in all cases, 

 except where pot holes are formed by waterfalls, brings about a system of 

 continuous down-grades from the heights to the lower ground. My inspection 

 of the moon's surface, which, from this point of view, was carefully made, 

 satisfied me that the streams had never done their inevitable work on that 

 sphere ; for I was unable to find a single case of a depression of considerable 

 length having a continuous down-grade, or an instance where it might be sup- 

 posed that a valley, so shaped, had been subsequently deformed. None of the 

 rills which have been supposed to be stream-like in shape are in the least so to 

 an eye trained in terrestrial topography. They have no gathering grounds, no 

 trace of that digitated system of valleys which must have been formed if they 

 had been water channels ; moreover, they have a perverse habit of branching the 

 wrong way, when they branch at all. Most selenographers have quite aban- 

 doned the idea that any of the features of the moon are due to water action, 

 though some of them adhere to the notion that there may be some slight trace 

 of water vapor in a supposed remnant of an atmosphere lying very near the 

 surface. 



The same arguments that exclude river action on the moon will a fortiori 

 exclude glaciers. Both these forms of water require extensive evaporation areas 

 and the machinery of an atmosphere for their maintenance. Now that it is gen- 

 erally accepted that the maria are not and never could have been seas, but are 



