IN PASSING THROUGH THE ATMOSPHERE. 
271 
Section IX. — Conclusions. 
112. On the whole, it appears from the facts and reasonings of this paper, — 
1. That the absorption of the solar rays by the strata of air to which we have im- 
mediate access, is considerable in amount for even moderate thicknesses. 
2. That the diurnal curve of solar intensity has, even in its most normal state, 
several inflections, and that its character depends materially upon the elevation of 
the point of observation. 
3. That the approximations to the value of extra atmospheric solar radiation, on 
the hypothesis of a geometrical diminution of intensity, are inaccurate. 
113. The coincidence found by M. Pouillet (Art. 22.) between the logarithms of 
the intensities and the thicknesses, may be ascribed to his having used a formula 
which gives the greater thicknesses sensibly too small, and thus makes an accidental 
compensation. Perhaps another accidental compensation may be found in the con- 
tinuity of the Curve XV. I have mentioned (Art. 82.) that I expected to find a 
different law for extinction in the higher and lower regions of the atmosphere. It 
may be that the greater purity of the air in the higher regions, and its great dryness, 
counterbalance the greater absorptive power which we have attributed (Art. 85.) to 
the first portions of an absorbing medium traversed by light or heat. 
114. We further conclude, — 
4. That the tendency to absorption through increasing thicknesses of air is a di- 
minishing one. That in fact the absorption almost certainly reaches a limit, beyond 
which no further loss will take place by an increased thickness of similar atmospheric 
ingredients. That the residual heat (tested by the absorption into a blue liquor) may 
amount to from a half to a third of that which reaches the surface of the globe after 
a vertical transmission through a clear atmosphere. 
5. That the law of absorption in a clear and dry atmosphere, equivalent to between 
one and four times the mass of air traversed vertically, may be represented (within 
those limits) by an intensity diminishing in a geometrical progression, plus a constant 
quantity, which is the limiting value already mentioned. Hence the amount of 
vertical transmission has always hitherto been greatly overrated, or the value of extra 
atmospheric solar radiation greatly underrated. 
6. The value of extra atmospheric solar radiation upon the hypothesis of the above 
law being generally true, is 73° of the actinometer marked B. 2. The limiting value 
of the solar radiation, after passing through an indefinite atmospheric thickness, is 15 0, 2. 
7. The absorption in passing through a vertical atmosphere of 760 millimetres of 
mercury, is such as to reduce the incident heat from 1 to - 534. 
8. The physical cause of this law of absorption appears to be the non- homogeneity 
of the incident rays of heat ; which by parting with their more absorbable elements 
become continually more persistent in their character, as Lambert and others have 
shown to take place where plates of glass are interposed between a source of heat and 
a thermometer. 
