THE MOON 335 



"canals", etc., of indicating what is really meant, that is plains or 

 slopes covered with some form of vegetation. * * * 



The barren and unchanging region to the south of the crater 

 will be referred to as the "mare". The brightness of two other 

 regions has been (estimated), the "floor" and the "inner field". 

 The latter refers to that small portion of the inner slope of the 

 crater lying directly between the "source", or point from which 

 the canals originate, and the crater floor. This region changes 

 very notably in shape as the lunation progresses. Between the 

 "field" and "the crater lies a bright triangular area which contains 

 several more or less variable canals. Indeed, nearly all of the 

 small dark areas on the moon are variable under high illumina- 

 tion, when well seen, either in shape, or density, or both. Except 

 in the case of a few small spots very near the equator, never has 

 any variation been observed on the moon near the time of sunrise 

 or sunset, that is to say near the terminator. * * * 



Nearly everyone who looks at the moon through a telescope 

 confines his attention exclusively to these regions, because they 

 furnish striking views, are easily identified, and because what 

 is seen is easy to understand. It is largely due to this fact that 

 the impression that the moon is lifeless has so long maintained its 

 existence, in spite of the facts to the contrary. The only time 

 when the moon is interesting to one who is really used to its 

 surface, is when changes are taking place upon it, and these only 

 occur far from the terminator, that is during the lunar summer 

 time. Then they are conspicuous all over the surface to any 

 who will take the trouble to watch and to study them carefully. * * 



At colongitude 30 each of the outer canals fork, but the 

 four branches are very faint. The next day they reach their 

 maximum length of 24 miles from source to terminus. Besides 

 the two main canals others are seen in the immediate vicinity. 

 In the dark region just outside the crater they are seen branching 

 in all directions giving somewhat the effect of a country road 

 map. In places one could scarcely travel three miles without 

 crossing one. They are of all degrees of faintness from that 

 of the two main canals already described, down to the extreme 

 limit of visibility. The narrowest visible, however, can hardly be 

 less than 200 feet in width. 



The lunar atmosphere is very possibly considerably more 

 dense than is generally supposed. We know but little about the 

 refractive powers of very rarefied atmospheres when the mean 

 free path of the molecule is of appreciable length. It is not likely 

 however that its density exceeds a few milliimeters, and in that 

 case ice when warmed would pass directly into vapor without 

 passing through the liquid form. However tempting the idea 

 might be, and their appearance certainly suggests it, these canals 



