the Surface- Temperature of the Planets: 163 



its turn reflects. The balance alone goes to warm the ground 

 or ocean. 



Light and Heat, Albedo. — Now 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 

 one hundred would leave the percentage of heat received. 

 Though our eyes are not so sensitive this procedure may 

 still be applied, provided account be taken of the heat- 

 depletion suffered by the invisible rays, as well as of that- 

 undergone by the visible ones. 



To solve the problem, therefore, we must find, first the 

 ratio of the visible to the invisible rays, second the albedoes 

 of the several planets, and thirdly the loss suffered by the 

 rays we cannot see. 



Actlnometer. — Actinometers and pyrheliometers are instru- 

 ments for measuring in toto the heat received from the sun ; 

 and they have been applied by Violle, Crova, Hansky, and 

 others to the determination of this quantity at given places, 

 and so to a conclusion as to the amount of heat outside our air, 

 or the Solar Constant. Langley's great contribution to the 

 subject was the pointing out that the several wave-lengths of 

 the different rays were not of homogeneous action or modifi- 

 cation, and that for an exact determination of the Solar Con- 

 stant it is necessary to consider the action of each separately, 

 and then to sum them together. To this end he invented 

 his spectro-bolometer. 



Bolometer. — By means of this instrument Langley mapped 

 the solar radiation to an extension of the heat spectrum un- 

 suspected before. He then carried it up Mt. Whitney, in 

 California, and discovered two important facts : one that the 

 loss in the visible part of the spectrum was much greater, not 

 only actually but relatively to the rest, than had been supposed, 

 and the other that the greater the altitude at which the 

 observations were made the larger the value obtained for the 

 solar constant. Both of these are pertinent to our present 

 inquiry. 



With a rock-salt prism instead of a glass one he next 

 extended still farther the limits of the heat spectrum towards 

 the red, the effect of the solar radiation proving not negligible 

 as far as X=15yL6. 



Very's Work. — In 1901 Prof. Very, who had been his 

 assistant earlier, published an important memoir on the Solar 

 Constant based upon these bolometric observations, but with 



M2 



