34*0 Mr. J. P. Cooke on the Aqueous Lines 



and a scries of comparisons made under the same conditions, at 

 different heights, would give data for determining the law ac- 

 cording to which the amount of vapour decreases with the eleva- 

 tion above the sea-level. 



All the aqueous lines change in intensity like the line ft. 

 They first appear very faintly when the amounj: of vapour in the 

 air reaches a definite point, varying for the different lines, and 

 gradually gain in intensity as the amount of vapour increases. 

 Thus the group of three lines, 8, e, f, do not appear in fig. 2, are 

 barely visible in fig. 3, but become very marked in fig. 4*. 

 The lines rj and S and the nebulous band k do not appear until 

 the air is very moist, and even when it contains 6*57 grains of 

 vapour per cubic foot they are still very faint. Under still more 

 unusual atmospheric conditions they will undoubtedly become 

 more intense, and we shall then probably be able to completely 

 resolve the nebulous band and count the lines of which it 

 consists. 



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 /3 only occasionally appear- 

 ing when the atmosphere becomes more moist. During the 

 warm weather of summer, when the absolute amount of mois- 

 ture in the air is in almost all cases greater than in winter, the 

 appearance of the 1) 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 

 must evidently concur in order that the aqueous lines may be 

 developed in their greatest intensity. In the first place, the air 

 must be charged with vapour not only near the surface of the 

 earth, but also through a great height of the atmosphere. Local 

 causes might greatly increase the amount of moisture in the 

 lower strata of the atmosphere and affect powerfully the hygro- 

 meter, which would not, to the same extent at least, influence 

 the indications of the spectroscope. In the second place, other 

 things being equal, the intensity of the aqueous lines must be 

 strengthened by increasing the length of the path of the sun's 

 rays through the atmosphere; and this is the longer the lower 

 the altitude of the sun. But then, again, the intensity of the 

 light has such an important influence on the definition of the 

 lines, and the slightest haze in the atmosphere so greatly 



* With nn increasing quantity of vapour in the atmosphere the line y of 

 fig. 3 is seen before the group of lines 6\ e, £; and an intermediate figure be- 

 tween figs. 2 and 3 might be given, showing only the lines D lt x,y,(3, D2. 



