340 



SCIENCE 



[N. S. Vol. XXXIX. No. 1001 



prepared speetro-bolometric apparatus for 

 this purpose, and made many solar constant 

 measurements therewith at Upsala. These 

 measurements are still being continued 

 there by his successors. It is hoped that 

 this long and interesting series will soon 

 be published. 



Passing from this work of Angstrom, 

 which belongs in a later period, and omit- 

 ting mention of valuable pyrheliometric 

 observations by numerous observers in 

 Italy, Switzerland and Russia, which I 

 regret that space forbids me here to dis- 

 cuss, attention must now be directed to the 

 work of Langley, which marked an epoch 

 in this kind of investigation. 



LANGLEY 'S OBSERVATIONS 



Prior to Langley 's observations there 

 had been numerous attempts to determine 

 the solar constant, which are well summed 

 up in the excellent little book of Radau, 

 entitled " Actinometrie. " It is shown that 

 nearly aU observers were in comparative 

 agreement, so far as their actual observa- 

 tions go, and if the transmission of radia- 

 tion by the atmosphere be estimated by the 

 simple formula 



which was employed by Pouillet and many 

 others, the value of the solar constant 

 would be found in the neighborhood of 

 1.75 calories. 



But Forbes, Desains, VioUe, Crova and 

 others showed convincingly that this equa- 

 tion does not accurately express the dimi- 

 nution of radiation attending the decline 

 of the sun from zenith to horizon, or the 

 descent of the observer from a high alti- 

 tude to a lower one. Accordingly several 

 empirical formulte of more complexity were 

 proposed, which, owing to their more nu- 

 merous constants, could be made to fit the 

 observed variation of the total intensity of 

 radiation under different conditions more 



closely. By the aid of such empirical 

 formula higher values of the solar constant 

 have been obtained. Some of these in our 

 own time have gone as high as 4 calories. 

 Radau however says: 



It is clear tbat the intensity of tlie solar radia- 

 tion outside the atmosphere can not be certainly 

 obtained from experiments which have been made 

 [prior to 1871], for the result depends essentially 

 on the manner of calculation. 



This conclusion is still applicable to 

 pyrheliometer measurements not supported 

 by spectrum observations. 



The tendency toward high values of the 

 solar constant was powerfully stimulated 

 by the publication of the report of the 

 Mount Whitney expedition by Langley in 

 1884. As Forbes and Radau had stated, so 

 Langley emphasized and acted upon the 

 fact that the formula 



A = AopBI'^iO sec 3_ 



applies only to a homogeneous bundle of 

 rays in a pure atmosphere; and the inten- 

 sity of solar radiation outside the atmos- 

 phere can only be exactly determined when 

 the atmospheric transmission coefficients of 

 the rays of all wave-lengths, which go to 

 make up the complex beam of the sun, are 

 separately determined and allowed for. 

 Langley was the first to determine and 

 apply atmospheric transmission coefficients 

 for numerous rays of different wave-lengths 

 in the solar spectrum. For this purpose 

 he invented the bolometer, a delicate elec- 

 trical thermometer, and observed with it 

 the variation of the intensity of each ray 

 of the spectrum from low sun to high. He 

 found it impracticable to determine the 

 transmission coefficients in the water vapor 

 bands of the infra-red, but assuming that 

 there were no water vapor bands in the 

 solar spectrum outside our atmosphere, he 

 avoided this difficulty by smoothing the 

 spectriun energy curve, which he computed 

 from his bolometric observations to repre- 



