184 S. F. Baird on the Distribution and 
are simply aqueous lines, but it is very difficult to distinguish at- 
mospheric lines from the true solar lines, and our knowledge of 
the first is as yet very incomplete. It still remains to make 
careful comparisons throughout the whole extent of the spec- 
trum before we can determine absolutely the relative absorbing 
power of the different constituents of our atmosphere 
rum. Here they are found in vast numbers, and I am not 
shall take pleasure in communicating through the pages of thi 
Journal any future results. 
‘Cambridge, Jan. 9, 1866. 
PEATE 
Art. XXV.—The Distribution and Migrations of North American 
Birds ; by SPENCE F. Barrp, Asst. Sec. Smithsonian Institu- 
tion. (Abstract of a memoir presented to the National Acad- 
emy of Sciences, Jan., 1865.) ; 
[Continued from’ p. 90.] 
ATTENTION has already been called to the fact that certain 
species, characterizing the eastern province, make their appeal 
ance in the Rocky mountains. The following is a list of those 
collected by Mr. Drexler at Fort Bridger (about in lat, 41°, long: 
110°) in the center of the Rocky mountain range, nearly of 
which have been found still farther to the northwest, toward 
Puget Sound. The birds found at Fort Bridger probably arrived 
