S. P. Langley on the Selective Absorption of Solar Energy, 163 

 Table VI.* 



\= -375 -400 -450 -500 -600 -700 -800 -900 1-000 



d, (cor.) winter, 1881. 1926 3634 579-3 767'9 724-9 527'9 338-3 215-4 173-6 

 d„ „ spring, 1881. 111*9 235-4 4237 5696 621-0 552-5 372-3 238-0 234-6 



The mean air-mass for winter = 13°88 

 „ „ „ springs 9-33 



We now proceed to the calculation of the energy outside 

 the atmosphere for homogeneous rays with the data which 

 have been given. For this purpose we have used the formula 



log E = log <*,-]*[,£, log*, 



where E is the energy in any ray outside the atmosphere 

 (i. e. before telluric absorption), d / the average galvanometer- 

 deflection at noon for the same ray, /3, the barometer-pressure 

 in units of one decimetre, or the mass of air in the vertical 

 column, M / (3 / the corresponding air-mass for the sun's zenith- 

 distance at noon, and t the adopted coefficient of transmission. 

 The following table has been prepared with the values 

 observed in the spring of 1881, using mean coefficients of 

 transmission, to show the relation between energy outside the 

 atmosphere and that for high and low sun at Allegheny, the 

 various actual absorbing air-masses at the low-sun observations 

 being reduced to a uniform value, double that at high sun. 



Table VII. 











X= -375 -400 -450 -500 



•600 



•700 



•800 



•900 1-000 



E = energy before absorption 353 683 1031 1203 



1083 



849 



519 



316 309 



d, = energy after absorption 1 1 12 235 424 570 

 (corrected high sun) J 



621 



553 



372 



238 235 



d tl = energy after absorption 1 07 fi3 140 2°5 

 (corrected low sun) f 



311 



324 



246 



167 167 



E can be computed from d y and d n by the formulas already 

 given ; and with these values the curves in fig. 2 have been 

 plotted. 



* It will be seen that, although the winter absorbing air-mass was 

 nearly half as large again as in the spring, the heat received from the 

 shorter wave-lengths was actually greater in the winter. It appears 

 probable, then, that the transmissibility of the atmosphere for the light- 

 producing radiations is relatively greater in winter than in spring. As 

 this effect may be connected in some way with the unequal prevalence of 

 atmospheric moisture at the two seasons, it may be well to state that the 

 tension of aqueous vapour during the winter observations was in the 

 neighbourhood of 2 millimetres, in the spring of 8 millimetres. 



