306 Mr. S. P. Langley on the Amount 



nearly the same with the false hypothesis as with the true 

 One, and it is with their relative magnitudes that the student 

 of stellar photometry is chiefly concerned, for he desires to 

 know their relative brightness at the zenith rather than their 



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absolute brightness outside the atmosphere. 



With 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 to 

 problems either of solar physics or of meteorology, the result 

 is of an altogether different importance. Almost all the phe- 

 nomena 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 atmo- 

 sphere; and there are further reasons, independent of those 

 cited, for believing that this is 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- 

 and low-sun observations, not only on account of the diminu- 

 tion of the absorbing air-mass, but because we thus avoid 

 certain systematic differences between the atmospheric con- 

 ditions at noon and evening (or morning) which introduce 

 constant errors into the results, in addition to those already 

 considered. From such observations I have been led, inde- 

 pendently of theory, to conclude that the absorption is greater 

 than is commonly supposed. 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 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 all we 

 lose by selective absorption from the direct light and heat of 

 the sun we ought to find reflected or selectively diffused to 

 us from the sky, or turned away from us to outer space *.-, I 

 have been engaged in measuring the heat and light from the 



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sky around the sun, and that from the whole sky apart from 

 the sun. These measures are still incomplete, and I will only 



* See the investigation of Tyndall on the cause of the hlue colour of 

 the sky (Proc. Roy. Soc. vol. xvii. p. 223). See also the theoretical in- 

 vestigations of Clausius (Poggendortf's Annalen, vol. cxxix. p. 330 et seq.), 

 and of Lord Rayleigh (Phil. Mag. Feb. 1871 et seq.). 



