S. P. Langley— Atmospheric Absarption. 165 
‘the light laterally reflected or diffused from them, and the direct 
beam, having lost something by this process, is not so bright 
after it has crossed the room, as before. In common language, 
‘the direct light, to an observer in the path of the beam, has 
‘been partly “absorbed,” and the problem is, to determine in 
what degree. If a certain portion of the light (suppose one- 
fifth) was thus scattered, the beam after it crossed the room 
would be but four-fifths as bright as when it entered it; and, if 
we were to trace the now diminished beam through a second 
apartment altogether like the other, it seems at first, reasonable 
to suppose that the same proportion (i. e. four-fifths of the re- 
mainder) would be transmitted there also, and that the light 
would be the same kind of light as before, and only diminished 
im amount (in the proportion 4x4.) The assumption originally 
made by Bouguer* and followed by Herschel and Pouillet, was 
that it was in this manner that the solar heat was absorbed b 
“our atmosphere, and that by assuming such a simple progression 
the original heat could be calculated. (The relatively minute 
expenditure of energy in the actual warming of the air is of 
course to be included.) 
Let us (to repeat Bouguer’s reasoning) divide in imagination 
any homogeneous absorbing medium, into successive strata of 
identical thickness and chemical constitution. 
E ok G K 
