180 8S. P. Langley—Selective Absorption of Solar Energy. 
E can be computed from d, and d,, by the formule already 
given, and with these values the curves in fig. 2 have been 
plotte : 
The middle curve (1) is that at high sun. Except for the 
heat below wave-length (1%0), the area of the curve may be 
considered to represent the heat actually observed by the 
actinometers, at noon, as presently given. 
The lower curve (II) is that at low sun. Its area is propor- 
tional to the heat received when the sun shone through double 
e. 
The above values (in table VII) are relative only. To obtain 
absolute ones we have now to combine this result with the 
actual measurements of solar radiation in calories, or other 
days, we have 1°81 calories observed at Allegheny in March, 
1881. This is the absolute amount of heat represented by the 
area of a completed “high sun” curve. 
To this result, the energy distributed through the whole 
spectrum has contributed, while our bolometer measurements 
ance for the (here) uncharted area below A= 1”0: 
