LOWELL. — TEMPERATURE OF MARS. 653 



Violle, Crova, Hansky, and others to the determination of this quan- 

 tity 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 modification, and 

 that to an exact determination of the Solar Constant it is necessary to 

 consider the action of each separately, and then to sum them together. 

 To this end he invented his spectro-bolometer. 



By means of this instrument Langley mapped the solar radiation to 

 an extension of the heat spectrum unsuspected before. He then car- 

 ried 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 gi'eater 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 toward the red, the effect 

 of the solar radiation proving not negligible as far as A = 15 fx. 



In 1901 Professor Very, who had been his assistant earlier, published 

 an important memoir on the Solar Constant, based upon these bolometric 

 observations, but with a value for it got from spectral curves derived 

 from simultaneous actinometric and bolometric determinations at Camp 

 Whitney and Lone Pine, and extended from them outside the atmosphere 

 by taking both air and dust effects into account in selectively reflecting 

 and diffracting the energy waves. The air effect is proportionate to the 

 air mass, but the dust effect increases in greater ratio as one nears 

 the surface of the ground. The formula he used were adaptations of 

 those by Rayleigh for accounting for the selective reflection and dif- 

 fraction of small particles. 2 



Energy of Visible and Invisible Spectrum. 



Planimetrical measurement of the area enclosed by the curve deduced 

 for outside our atmosphere gives the following results : 



Distribution of Heat in the Spectruji. 



Wave-lengths. Percentage. 



Invisible, X = 0.2 /ia-0.393 /*...«. 2.5 



Visible, A = 0..393 fx-OJG fx. 32. 



Invisible, X = 0.76 /a-15 /x 65.5 



100. 



2 U. S. Department of Agriculture, Weather Bureau, No. 254. 



