J. Croll— Aqueous Vapor in Relation to Perpetual Snow. 108 
motion, we can hardly avoid the conclusion that they are not 
due to the terms of a higher order of magnitude which have 
been neglected in the development of the theory. They must 
therefore result from some systematic error among terms of 
more importance in the lunar theory. 
In bringing to a close this account of my researches, I would 
repeat that my only object has been to discover if possible, by 
means of a new method of investigation, any false steps which 
may have been committed by previous investigators in the 
mathematical development of the lunar theory. The history 
of philosophy affords numerous examples of the advantages of 
independent methods of investigation over independent calcu- 
lations by the same method. It often happens that for particu- 
lar values of the known quantities of a problem some terms of 
the solution become infinite or indeterminate, when certain 
attention afford the means of a much needed improvement in 
the lunar theory. 
Cleveland, May 25, 1880. 
Arr. XTIL—Agqueous Vapor in Relation to Perpetual Snow; by 
JAMES CROLL, LL.D., F.R.S.* 
SOME twelve years ago I gave (Phil. Mag., March, 1867, 
Climate and Time, p. 548) what appears to be the true explana- 
tion of that apparently paradoxical fact observed by Mr. 
Glaisher that the difference of reading, between a thermometer 
exposed to direct sunshine and one shaded, diminishes instea 
of increases, as we ascend in the atmosphere. This le to 
_ an important conclusion in regard to the influence of aqueous 
vapor on the melting of snow, but recent objections to some of 
my views convince me that I have not given to that conclusion 
the prominence it deserves. I shall now state in a few words 
the conclusion to which I refer. 
€ reason why snow at great elevations does not melt but 
remains permanent, is owing to the fact that the heat received 
from the sun is thrown off into stellar space so rapidly by radi- 
* Communicated to this Journal by the author. 
