S. P. Langley—Atmospheric Absorption. 179 
considerable color is present) remain nearly the same with the 
false hypothesis as with the true one, and it is with their rela- 
tive magnitudes that the student of stellar photometry is chiefly 
concerfed, for he desires to know their relative brightness at 
the zenith rather than their absolute brightness outside the 
atmosphere. ° 
ith the sun, however, it is otherwise, for here it is the 
absolute heat or light which is in question. 
Accordingly, when we apply our above conclusions either to 
problems of solar physics or of meteorology, the result is of an 
altogether different importance. Almost all the phenomena of 
meteorology would become predictable if we knew how much 
heat reaches the soil, and how much, and in what altered kind, 
is returned to outer space. To solve these problems we must 
know how much is absorbed by our atmosphere; and there are 
further reasons, independent of those cited, for believing that 
this may be more than double what is commonly supposed. 
It may be observed that the comparison of observations at 
‘the base and summit of a very high mountain will enable us to 
obtain much better determinations than the method of high 
such observations I have been led independently of theory to 
‘conclude that the absorption is greater than is commonly sup- 
posed. But beside this method, which is in reach of but few, 
there is another at the command of all, the significance of 
whose results seems to have been hitherto overlooked. 
If we are willing to agree that most of the solar heat is not 
absorbed by our air in the sense of being accumulated there 
{since this would heat the atmosphere to the condition of a 
glowing gas) we must, it’seems to me, admit that it is mainly 
diffused toward and away from us by particles, so that nearly 
Say that they seem to show not only that the average amount 
of blue light (to speak for the moment of blue light only) which 
* See the investigation of Tyndall on the cause of the blue color of the sky, 
Proc, Roy. Soc., vol. xvii, p. 223. See also the theoretical investigations of 
Clausius. Poggendorf Annalen, vol. exxix, p. 330, et seg., and o Rayleigh, 
; £0 p. 
London, Edinburgh and Dublin Phil. Mag., Feb., 1871, e¢ 
seq. 
Am. Jour. Sci.—Tuirp Series, Vou. XXVIII, No. 165.—Supt., 1884. 
12 
