t-RyS'] Condition of the Moon's Surface. 41 



The great objection to this view of the moon's past 

 history consists in the difficulty of accounting for the lunar 

 atmosphere. It must be remembered that owing to the small- 

 ness of the moon's mass, an atmosphere composed in the 

 same way as ours would have a much greater depth com- 

 pared with its density at the mean level of the moon's 

 surface than our atmosphere possesses compared with its 

 pressure at the sea-level. If there were exactly the same 

 quantity of air above each square mile of the moon's 

 surface as there is above each square mile of the earth's 

 surface, the lunar air would not only extend to a much 

 greater height than ours, but would be much less dense at 

 the moon's surface. The atmospheric pressure would in 

 that case be about i-6th that at our sea-level, and instead 

 of the lower half of such an atmosphere (that is, the lower 

 half in actual quantity of air) lying within a distance of about 

 3J miles from the mean surface, as in the case of our earth, 

 it would extend to a distance of about 22 miles from the 

 surface. Now this reasoning applies with increased force to 

 the case of an atmosphere contained within the cavernous 

 interior of the moon ; for there the pressure due to the at- 

 traction of the moon's mass would be reduced. It is very 

 difficult to conceive that under such circumstances room 

 would not only exist for lunar oceans, but for a lunar 

 atmosphere occupying, one must suppose, a far greater 

 amount of space even before their withdrawal into these 

 lunar caverns, and partially freed from pressure so soon as 

 such withdrawal had taken place. That the atmosphere 

 should be withdrawn so completely that no trace of its 

 existence could be recognised does certainly appear very 

 difficult to believe, to say the least. 



Nevertheless, it is not to be forgotten that so far as 

 terrestrial experience is concerned water is absolutely 

 essential to the occurrence of volcanic action. If we are 

 to extend terrestrial analogies to the case of our moon, 

 notwithstanding the signs that the conditions prevailing in 

 her case have been very different from those existing in the 

 case of our earth, we are bound to recognise at least the 

 possibility that water once existed on the moon. Moreover, 

 it must be admitted that Professor Frankland's theory seems 

 to accord far better with lunar facts than any of the others 

 which have been advanced to account for the disappearance 

 of all traces of water or air. The theory that oceans and an 

 atmosphere have been drawn to the farther side of the 

 moon cannot be entertained when due account is taken of 

 the range of the lunar librations. Sir J. Herschel, indeed, 



vol. in. (n.s.) g 



