S. P. Langley—The Solar Atmosphere. 495 
tained since La Place wrote. These facts, as they seem to me, 
demanding a changed formula I expect to point out in a defi- 
nite form in a subsequent memoir. It seems fitter then to 
defer an exact statement of the limits of the amount by which 
solar radiation is absorbed by the atmosphere, until it is accom- 
panied by demonstration. I willonly here repeat that we may 
feel certain that the estimates cited from La Place and Secehi, 
and which make the sun’s atmosphere absorb from { to +4 of 
the radiation we should receive in its absence, are in excess of 
the truth. For the sake of indicating approximately the real 
value, I may also state that from my computation of the so- 
called “Juminous-heat” rays, not greatly less or more than one- 
half the whole are absorbed by, turned back by, or converted 
into work in, the sun’s atmosphere. The total thermal absorp- 
tion is somewhat less than that of the luminous heat. If, how- 
ever, we admit this value provisionally, certain results seem to 
follow which deserve mention. : 
The mean surface temperature of our globe is separated from 
that of absolute zero by about 500° of the Fahrenheit scale. — 
The internal heat supplied to the surface may be neglected. 
"he temperature of interplanetary space is, according to Fou- 
ner — 60°C., according to Pouillet —142, according to Liais — 97. 
Liais, in a memoir whence we cite these estimates, admits, as a 
not called upon here to adopt, further than to observe that 
with this, as with either of the other estimates, the obscure 
We shall perhaps be warranted in entertaining it then as a 
reasonable assumption, that, in the complete absence of the sun, 
e earth’s temperature would fall very nearly to —273 C. We 
prefer as the basis of an estimate, confessedly but approximate, 
to take the mean of this value and Pouillet’s, and, using the 
Fahrenheit scale, we may state that of the 500° F., which on 
the natural scale is the approximate mean temperature of our 
globe, as much as four-fifths is derived from the sun. 
