GENERAL DESCRlPTlOlSf OF THE MOON. 107 



it were not more than about -iO miles from the observer. The condi- 

 tions of this seeing are much more favorable than tliose under which 

 we behold a range of terrestrial mountains at tliat distance, for the 

 reason that the air, and especialh" the moisture, in our atmosphere 

 hinders and confuses the light, and there is several times as much of 

 this obstruction encountered in a distance of iO miles along the earth's 

 surface as there is in looking vertically upward. 



Seen with the greater telescopes, the surface of the moon may reveal 

 to able observers, in the rare moments of the best seeing, circular 

 objects, such as pits, which are perhaps not more than 500 feet in 

 diameter. Elevations of much less height ma}^ be detected by their 

 shadows, which, because there is no trace of an atmosphere on the 

 moon, are extraordinarily sharp, the line between the dark and light 

 being as distinct as though drawn by a ruler. Elongate objects, such 

 as rifts or crevices in the surface, because of their length, may be vis- 

 ible even when they are onl}" a few score feet in width, for the same 

 reason that while a black dot on a wall may not make any impression 

 on the e3^e, a line no wider than the dot can be readily perceived. 

 Owing to these conditions, the surface of the moon has revealed many 

 of its features to us, perhaps about as well as we could discern them 

 by the naked eye if the sphere were no more than 20 miles awa}^ 



Separated from all theories and prepossessions, the most important 

 points which have been ascertained as to the condition of the moon's 

 surface are as follows: 



The surface di tiers from that of the earth in the fact that it lacks 

 the envelopes of air and water. That there is no air is indicated by 

 the feature above noted — that there is no diffusion of the sunlight, the 

 shadows being absolutely black and with perfectly clean-cut edges. 

 It is also shown l)y the fact that when a star is occulted or shut out by 

 the disc of the moon it disappears suddenly without its light being 

 displaced, as it would ])e by refraction if there were any sensi])le 

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

 if there is an}" air at all on the moon's surface it is probabl}" less in 

 amount than remains in the nearest approach to a vacuiau 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 b}' 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 whicli 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 fea- 

 tures existed, the reflection of the sun from their surfaces would make 



