g52 proceedings of the american academy. 



Distance but One of Several Factors. 



In these and similar determinations, the only thing considered is 

 the distance from the sun, as if the surfaces of the planets were im- 

 material points in space, and the whole heat arriving there went to 

 warm the bodies. But such is far from being the fact. Not only are 

 the surfaces material, but any air the bodies possess as a covering is 

 material too. Now, postponing for the moment consideration of the 

 blanketing effect of air, the actual amount of heat received at the sur- 

 faces in consequence of their constitution or of their air-envelope, is not 

 at all what mere distance would lead one to infer. 



Division of Radiant Energy. 



So soon as a radiant ray strikes matter it suffers division of its energy. 

 Part of it is reflected, part absorbed, and part transmitted. What is re- 

 flected is sentoff again intospace, performing noworkintheway of heating 

 the body. Now the amount reflected is not the same in all cases, depend- 

 ing for its proportion upon the character of the matter the ray strikes. 



If the surface of a planet be itself exposed unblanketed by air, the 

 absorbed and transmitted portions go to heat the planet, directly or 

 indirectly. 



If the planet be surrounded by air, the portion transmitted by this 

 air, plus what is radiated or reflected from it to the solid surface, must 

 first be considered. Then, upon this quota as a basis, must secondly 

 be determined how much the surface in its turn reflects. The balance 

 alone goes to warm the ground or ocean. 



Light and Heat. 



Radiant energy is light, heat, or actinism, merely according to the 

 effect we take note of. If our eyes were sensitive equally to all wave- 

 lengths, we could gauge the amount of heat received by a body by the 

 amount of light it reflected, — that is, by its intrinsic brightness, or 

 albedo. For this percentage deducted from unity would leave the 

 percentage of heat received. This procedure may still be applied, 

 provided account be also taken of the heat depletion suffered by 

 the invisible rays. Two problems, then, confront us. 



We must find the albedoes of the several planets in order to compare 

 one with another in its reception of heat, and we must find the relation 

 borne by the visible and invisible rays to the subject. The latter 

 problem may best be attacked first. 



Actinometers and pyrheliometers are instruments for measuring in 

 toto the heat received from the sun ; and they have been used by 



