J. P. Cooke on Aqueous Lines of the Solar Spectrum. 181 — 
With a very dry atmosphere the line « is the only one which 
appears within the D lines, as shown in fig. 1. As the amount 
of vapor increases the line ? makes its appearance. At first it 
is barely visible, but as the amount of vapor increases still 
further it becomes more and more prominent, until at last, as 
shown in fig. 4, it is even more intense than the line « A 
eareful comparison of these two lines might indeed serve as an 
approximate measure of the amount of vapor in the atmosphere, 
a series of comparison, made under the same conditions at 
different heights would give data for determining the law ac- 
cording to which the amount of vapor decreases with the eleva- 
tion above the sea level. 
Thus the group of three lines 9¢¢ do not appear in fig. 2, are 
barely visible in fig. 8, but 
nsists, 
It is hardly necessary to repeat that the examples here given 
are selected from a large number of observations. During the 
cold dry weather of winter the appearance of the D line is uni- 
formly as shown in fig. 1, the line @ only occasionally appear- 
ing when the atmosphere becomes more moist. During the 
warm weather of summer, when the absolute amount of moist- 
ure in the air is in almost all cases greater than in winter, the 
appearance of the D line is as uniformly that shown in fig. 3. 
It is only very rarely in the dry climate of New England, even 
during the summer, that all the lines shown in fig. 4 are visible, 
and, as already stated, I never saw them before so sharply de- 
fined as on the 17th of November last. Several conditions 
* With an increasing quantity of vapor in the atmosphere the line y of fig. 3 is 
before the group of Beas ae tal an intermediate figure between 2 ands 
might be given showing only the lines D,278D,. “i 
