166 S. P. Langley—Atmospherie Absorption. 
which is here supposed to be of uniform density and constitu- 
tion. (The effects of the actually unequal density of successive 
strata, can, it is assumed, be calculated and allowed for.) Let 
S be the observer’s station, then ES would be the direction of 
a ray where the sun is in the zenith, and, to fix our ideas, let 
FS=2 ES; GS=3 ES; KS=4 KS, etc. The original heat A 
would become Ap after passing through one stratum (ES); and, 
according to what has been assumed, it would become (when 
the sun’s zenith distance became ESF) Ap’ after absorption by. 
the two strata between F and S, Ap’ after absorption by the 
three strata between G and §, etc. A, the original heat,andp 
the coefficient of transmission, are unknown; but if we make an 
observation of the heat actually reaching S along FS (let us _ 
call this heat m) and again later in the day along GS (calling _ 
this second observed quantity n) we have in the particular case 
suppose : 
earth’s surface, and HK the upper surface of the atmosphere, — 
, 
Ap’=m Ap’=n 
that the coefficient of transmission (7) is a constant. 
It is no doubt true that a very sensible portion of the solar — 
h 
is of different kinds. To use Melloni’s illustration, radiant heat 
would to any eye that could ‘see’ it, appear to be of totally — 
for it has long been in one sense well known. But in another 
hence treat all wave-lengths nearly alike, or diminish the dire! 
radiation by a nearly general absorption: minuter ones begin 
