26 PROCEEDINGS OP THE AMERICAN ACADEMY 



Investigations on Light and Heat, made a>d pnunsnED wholly ob in part with 



APPaOPRIAIION FROM THE IluMfoRD YCSD. 



IV. 



ATMOSPHERIC ECONOMY OF SOLAR RADIATION. 



By Arthur Searle. 



Presented October 10, 1888. 



The terrestrial atmosphere acquires energy from the solar radiation 

 by direct absorption, by the absorption of terrestrial radiation, and by 

 conduction from terrestrial solids and liquids. It loses energy in three 

 corresponding ways ; by radiation into space, by downward radiation, 

 and by conduction. 



As some time must elapse between the acquisition and the loss of 

 any given amount of energy, the air always contains a certain accumu- 

 lated store of activity resulting from the solar radiation, and manifested 

 in warmth, expansion, and movement. It is a general and apparently 

 well founded belief, the reasons for which need not here be rejDeated, 

 that terrestrial temperatures are maintained to a great extent by the 

 aid of this atmospheric accumulation of energy ; so that a far lower 

 temperature would prevail in the absence of the air. The hypothesis 

 which has been current until recently with regard to this protective 

 action of the atmosphere depended upon a supposed effect of. selective 

 absorption, which has now been largely, if not entirely, disproved by 

 Langley's experiments. The supposition, indeed, was always some- 

 what difficult to reconcile with the familiar fact that celestial bodies 

 appear redder at a small than at a great altitude ; since, so far as the 

 visible spectrum is concerned, this proved that among the constituents 

 of the atmosphere there were some, abounding in its lower strata, 

 which absorbed radiations of small wave-length more readily than the 

 others. Hence it did not seem probable that the radiation from ter- 

 restrial substances the temperature of which was far below red heat 

 would be absorbed by the air with peculiar readiness, and thus pre- 

 vented from escaping into space. This reasoning, however, could not 

 be conclusive, and actual experiment was required to overthrow the 

 assumption that the air was much more transparent to solar than to 

 terrestrial radiation. 



As we are now obliged to abandon this assumption, it is natural to 



