S. P. Langley on the Selective Absorption of Solar Energy. 167 



indicates that the true solar constant is larger than that com- 

 monly given. The ratio of the dark to luminous heat has 

 been so wholly changed by selective absorption that we must 

 greatly modify our usual estimates, not only of the sun's heat- 

 radiation, but of his effective temperature. 



The sun to an eye without our atmosphere would appear of 

 a bluish tint. 



In spite of the care with which the experiments on which 

 the above conclusions rest have been conducted, owing to the 

 importance of the subject, and to their departure in" some 

 respects from received opinion, it seemed desirable to repeat 

 them under conditions differing as much as possible from those 

 in which they were made. If the preceding conclusions are 

 sound, we ought to find like results by actually ascending to 

 a great height and directly measuring there, as well as below, 

 the absorptions which each ray has actually undergone. 



Expedition to Mount Whitney. 



In July 1881 an expedition, fitted out and instrumentally 

 equipped at the Allegheny Observatory, proceeded in the 

 writer's charge, but with the aid of transportation furnished 

 by the War Department and under the official direction of 

 General W. B. Hazen, U. S. A., Chief Signal Officer of the 

 Army, to Mount Whitney in Southern California, where these 

 observations and others were repeated at two contiguous 

 stations at very different absolute altitudes. The results will 

 shortly appear in an official publication (some of the drawings 

 prepared for which have been used for the present memoir by 

 the kind permission of General Hazen). At present it is suf- 

 ficient to say of them, that the conclusions just rehearsed were 

 confirmed aud extended. 



While on the mountain, at an elevation of 13,000 feet, a 

 hitherto unrecognized extension was observed by the bolo- 

 meter of the infra-red prismatic spectrum in the vicinity of 

 the great absorption-band marked on our prismatic chart as 

 H, and beyond it; and on the return to Allegheny it was found 

 that this last observation could still be continued in our lower 

 atmosphere. The generosity of a citizen of Pittsburg had 

 enabled the observatory to provide for the expedition several 

 pieces of special apparatus. Among these was a Foucault 

 siderostat, of the dimensions of that at the Paris Observatory, 

 but of a much improved design, and a special apparatus (the 

 spectro-bolometer) to enable the deviations of the invisible 

 rays to be measured with an error of less than a minute of arc 

 &c. ; and this apparatus has been used in the new research we 

 now describe. (The siderostat was made by A. Hilger, of 



