NO. 8 SAMUEL PIERPONT LANGLEY ABBOT I5 



surface temperature of this planet ; and how, in diminished quantity 

 and altered kind, it is finally returned to outer space. 



". . . . We are trying to estimate the amount of solar heat before 

 absorption (the solar constant). 



" Could we ascend above the atmosphere, this heat might be directly 

 measured. Evidently, since this is impossible, and since we can only 

 observe the portion which filters down to us after absorption, we must 

 add to this observed remnant a quantity equal to that which the 

 atmosphere has taken out, in order to reproduce the original amount. 



" Toi find what it has taken out, we must study the action in detail, 

 and, from the knowledge thus gained frame a rule or formula which 

 shall enable us to infer the loss since we cannot directly determine it. 



" It is because the exact determination of the solar constant thus 

 presupposes a minute knowledge of the way in which the sun's heat 

 is afifected by the earth's atmosphere ; and because every change in 

 our atmosphere comes from this same heat, that the solution of the 

 problem interests meteorology as well as astronomical physics. 



". . . . Let us consider what the problem appears to be at a first 

 glance, and what the first suggestion is for solving it. If a beam of 

 sunlight enters through a crevice in a dark room, the light is partly 

 interrupted by the dust particles in the air, the apartment is visibly 

 illuminated by the light reflected from them, and the direct beam 

 having lost something by this process, is not so bright after it has 

 crossed the room as before it entered it. If a quarter of the light 

 was thus scattered, and the beam after it crossed the room would be 

 but three- fourths as bright as when it entered it, and if we were to 

 trace the now diminished beam through a second apartment alto- 

 gether like the other, it seems at first reasonable to suppose that the 

 same proportion, or three-fourths of the remainder, would be trans- 

 mitted, and so on, and that the light would be the same kind of light 

 as before, and only diminished in amount. The assumption originally 

 made by Bouguer ' and followed by Herschel and Pouillet was that it 

 was in this manner that the solar heat was interrupted by our atmos- 

 phere, and that by using such a simple progression the original heat 

 could be calculated. 



" Now, it is no doubt true that a very sensible portion of light and 

 heat are scattered by an analogous process in our atmosphere ; but we 

 have in our present knowledge to consider that heat is not a simple 

 emanation, but a compound of an infinite number of radiations, and 

 that these are afifected in an infinite diversity of ways by the different 



* Bouguer, Traite de la luniiere. Paris, 1760. 



