826 TrANSACTIOSS of the A?IER1CAX lySTITUTE. 



a strip wliich so perfectly resembles a long river flowing from a 

 crater like reservoir to a lake that to believe it anything else than a 

 river is difficult." 



To account for the absence of large bodies of water upon the 

 moon's surface, we have only to reflect that the facts demonstrate the 

 quantity of water pertaining to tlie moon to be relatively but a mere 

 percentage of that belonging to the earth ; and the earthy matter of 

 the moon being lighter than that of our world, its tendency to absorb 

 moisture would be greater in proportion. 



The atmosphere of the moon being lighter than that of the earth, 

 in the relative proportion of their solids, could not, of course, sustain 

 the vapors of water, in such a form as the cloud system of our world ; 

 but even the earth's atmosphere cannot sustain the vapors of water in a 

 visible form, at a greater elevation than about three miles above the sea 

 level, and at this elevation the cirrus cloud becomes so thin as to be 

 almost invisible. Nor is it improbable that the lower strata of the lunar 

 atmosphere really sustain a cloud system, even more dense than our 

 cirri. Indeed, some observers do assert that they have seen light 

 clouds floating over the moon's surface, and the lunar clouds to 

 become visible at all must.be denser than our cirri, for every 

 observer knows that a cloud of considerable density ma}^ pass 

 betweon the telescope and the moon, without perceptibly affecting the 

 definition. And our cloud is so near to the telescope that it covers 

 the whole disc of the moon, but if it were removed to the lunar sur- 

 face it would cover only a mere speck, which would make it exces- 

 sively difficult to find at all, especially as there is nothing to relieve 

 it from the luminous disc of the orb. And from this cause alone the 

 moon's of Jupiter become entirely invisible throughout three quar- 

 ters of every transit of tlie planet, and when an opaque body becomes 

 invisible in its passage across a luminous disc, you will see how small 

 our chances have been of determining pro or con the existence of 

 lunar clouds. 



The question of the extent, nature and power of refraction of the 

 assumed lunar atmosphere necessarily connects itself w^ith this part of 

 the subject, but the time required to present those considerations 

 makes it necessary to pass them over for the present. 



It will be evident from the nature of our own earth, that -whatever 

 fluid was active in producing the lunar formations must be there 

 still ; because it could not escape from that body any more than the 

 fluids of our own earth can escape into space. The heat of the sun 



