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



Area outside curve above = 1^*000 . . 47*26 

 Area outside curve below = 1^*000 . . 26*40 



Total 73*66 



Area high-sun curve above = 1 M *000 . . 26*96 



Area high-sun curve below = 1 M, 000 . . 20*00 



Total 46*96 



mi „., . area outside curve 73*66 ., rnr . 



The ratio of these areas is ^— = = — — — = 1*569. 



area nign-sun curve 4b*9o 



We have, then, adopting 1*81 cal. as the solar radiation at 

 Allegheny with clear sky, 1*81 cal. x 1*57 = 2*84 calories as 

 an approximate value of the solar constant. 



In all these observations the object has been to avoid the 

 registering of small variations analogous to the Fraunhofer 

 lines, and to give only the general distribution of the energy. 

 The mapping of the interruptions of the energy caused by 

 visible or invisible lines or bands forms a distinct research; 

 and the results are given later in the present article. 



We find from these preliminary observations that the maxi- 

 mum energy in the normal spectrum of a high sun at the 

 earth's surface is near the yellow, and that the position of the 

 maximum of heat does not in fact differ widely from that of 

 the maximum of light. It has been long known that certain 

 ultra-violet and violet rays were much absorbed ; but it has 

 been supposed that the absorption increased also in the infra- 

 red, so that the luminous part of the spectrum was on the 

 whole the most transmissible. 



But we see here, not only how enormous the absorption at 

 the violet end really is, but that the light-rays have suffered 

 a larger absorption before they reach us than the " heat "-rays 

 (i. e. than the extreme red and infra-red rays) — a conclusion 

 opposed to the present ordinary opinion, and, if true, of far- 

 reaching importance. For if this "dark" heat escapes by 

 radiation through our atmosphere more easily than the lumi- 

 nous heat enters, our view of the heat-storing action of this 

 atmosphere and of the conditions of life on our planet must be 

 changed. Within the limits of the present charts the "dark" 

 heat apparently does so escape. 



We can, from the data now gathered as to the rate of ab- 

 sorption for each ray, compute the value of the heat or energy 

 before absorption (the solar constant) by a new process which 

 is in strict accordance with theory. This preliminary value 



