S. P. Langley on the Selective Absorption of Soiar Energy. 161 

 M is determined from the formula 



•0174 x tabular refraction 



M= 



cosapp. altitude 

 Table II. 



Date of obser- 

 vation. 



High Sun. 



Low Sun. 



Sun's 

 hour- 



Sun's 

 zenith 



dis- 

 tance. 



Baro- 

 meter. 



Air- 

 mass. 



I 



Sun's 

 hour- 



Sun's 

 zenith 

 dis- 

 tance. 



Baro- 

 meter. 



Air- 

 mass. 





angle. 



(ft). 



(MA)- 



angle. 



(ft). 



(M,ft)- 





h m 





d m 





h m 





d m 





Jan. 28, 1881 



00 



58 29 



745 



1425 



2 57 



71 28 



745 



23-24 



Feb. 2 



09 

 38 



57 09 



52 57 



7-39 

 7-43 



13-63 

 12-33 



3 00 



2 56 



70 45 

 66 09 



7-39 

 7-42 



22-24 



18-25 



Feb. 17 



April 22 



12 



28 13 



7-36 



8-35 



4 36 



66 22 



7-36 



18-32 



April 23, a.m. 



11 



27 49 



7-40 



8-37 



2 45 



45 30 



7-40 



10-56 



April 23, p.m. 



11 



27 49 



7-40 



8-37 



4 26 



63 57 



7-40 



16-85 



April 29, a.m. 



06 



25 50 



735 



8-17 



3 11 



48 46 



7-35 



11-15 



April 29, p.m. 



06 



25 50 



7-35 



8-17 



5 23 



73 36 



7-35 



35-73 



April 30 



04 



25 31 



741 



8-21 



3 54 



56 31 



7-41 



13-43 



May 28 



11 



19 03 



7'32 



7-75 



5 33 



71 14 



7-32 



22-33 



By combining the high- and low-sun observations of each 

 day separately, coefficients of atmospheric transmission are 

 obtained by using the formula 



Iogf _ log<*//-log<*/ J 



Jo ^-MA-MA 



where t is the coefficient of vertical transmission by air at a 

 barometric pressure of one decimetre. A tabular statement of 

 these coefficients has been prepared; but the average or adopted 

 value only is here given. 



Table III. 



X= -375 -400 -450 -500 '600 



Adopted £ ... -884 -892 -909 -923 -942 



J-Q 



•392 -420 -485 -544 -636 



•700 

 •955 

 •705 



•800 -900 1-000 

 •965 -970 -971 

 •763 -794 -799 



It is important to notice that (contrary to a generally 

 received opinion) the tr admissibility of the atmosphere is here 

 found to be greatest for the infra-red rays. 



All good noon observations have been reduced to a uniform 

 battery-current of 0'25 weber, and the results arranged in two 

 sets — the first for winter and the second for spring measures. 



These tables are not here given ; but the average results are 

 as follows : — 



