THE LUNAR WORLD. 87 



that gaseous vestment which seems appropriate to every 

 planet. 



Here again our satellite the moon stands in striking 

 contrast with the other globes that are accessible to our 

 observation. If the moon ever possessed an atmosphere, 

 and this is a point on which we cannot feel any certainty, 

 it has at least now vanished. I should perhaps qualify 

 this statement with the remark that acute observers have 

 detected occasional indications of some gaseous material 

 on the moon in extremely limited quantity confined to 

 certain valleys or depressions. At first sight it may 

 perhaps seem difficult to imagine that the telescope could 

 be invoked to study the question as to whether there was 

 atmosphere on the moon ; for atmosphere seems so trans- 

 parent, and is indeed so invisible, that we might naturally 

 ask, how is it to be observed ? No doubt if there were 

 water on the moon, we might reasonably expect to see 

 clouds or vapours if air existed in which those vapours 

 could be suspended. But the air itself we could not ex- 

 pect to see ; how then can its absence be demonstrated by 

 the telescope? It may be true that we never could 

 observe material quite translucent, but what we might 

 expect to see are certain indications that would be per- 

 ceived if the atmosphere were present, and as we do not 

 find them we infer that the atmosphere does not exist. 



The simplest method of demonstrating the absence of 

 atmosphere of sensible amount surrounding the moon is 

 by observing the phenomenon presented in what is called 

 the occultation of a star. As the moon wends its way 

 over the starry heavens it sometimes passes between the 

 earth and a star, and the phenomenon is often one of 



