S. P. Langley—Selective Absorption of Solar Energy. 175 
have been little absorbed (at noon), the other when they have 
=n — absorbed (in the morning or afternoon). The 
ass of air through which the rays pass is taken proportional 
“ secant, oH for zenith distances less than 65°. and for those 
0174 tabular refraction 
greater to —— apparent altitude , and in oe cases to the 
the proportion of the radiation transmitted by a sun in the 
zenith to an observer at the sea-level, and this is here shown to 
vary greatly for each ray. Thus by reference to table III, we 
find of three solar rays whose wave- -lengths are ‘375, ‘600, “1 ,000, 
that of the ray whose wave-length is 0-375 (in the ultra violet 
61 per cent of the original energy would be absorbed and 3 
transmitted, of wave-length 0-600 (in the orange) 36 per cent 
would be absorbed and 64 transmitted, of wave-length 1-000 
(in the infra-red) 20 per cent is absorbed and 80 transmitted, 
etc. 
Allegheny pebinagonct Observations on the Solar gi a Spec- 
m previous to Mt. Whitney Hxpeditio 
- The poole list shows the dates at which bolometer ob- 
Servations were made at Allegheny up to June, 1881, for the 
measurement of heat in the spectrom and the determination of 
atmospheric transmission, by the comparison of noon and after- 
noon measures. Those days on which noon measurements 
causes are indicated by an asterisk. It will be seen 
twenty-nine days of observation only ten could be fully sabia 
Dates: 1880, Nov. 12,* Dec. 11,* Dec. 18.* 1881, Jan. 12,* 
Jan, 18,* Jan, 28, Feb. 9, Feb. 3, * Feb. 5,* Feb. 17, Feb. 19,* 
Feb. 22,* Feb. 26,* Mar. 2,* Mar. 10.* Mar. 11,* Mar. 25,* Mar. 
28," Apr. 7,* Apr. 16,* Apr. 22, Apr. oo Apr. 28,* Apr. 29, Apr. 
30, May 4,* May 26,” May 27,* May 2 
The following table gives the nies galvanometer deflec- 
tions reduced to a scale on which the readings are proportional 
to the current passing through the galvanometer. 
a = Pa ys enti rane’ aeetias es high sun 
- low sun. 
ii 
