98 abbot: solak constant of radiation 



radiation outside the atmosphere, so as to leave no water vapor 

 bands in it at all. Had Langley stopped with these steps ac- 

 comphshed, he would have left us as the result of the Mount 

 Whitney expedition, 2.060 calories, the mean value as deter- 

 mined by high and low sun observations at Lone Pine, or 2.220 

 calories, the mean value similarly determined from observations 

 at Mountain Camp. But, by the train of reasoning given on 

 pages 142-144 of his report, he convinced himself that the expo- 

 nential formula does not hold for the earth's atmosphere, even for 

 a strictly homogeneous ray. He therefore altered his results by 

 two different procedures, one of which he states was of a kind to 

 give too low a value of the solar constant, and the other too 

 high. By this means he obtained the values 2.630 and 3.505. 

 The mean of these, 3.068, or in round numbers 3.0 calories per 

 square centimeter per minute he adopted as the solar constant. 

 But in fact, both procedures were calculated to give too high 

 results, and the most probable results of Langley's observations 

 lies below either of them, and is in fact 2.22, or 2.06 calories, ac- 

 cording as the work at Lone or Mountain Camp is regarded as 

 the better. Li order to recognize this, it is necessary to examine 

 the argument which led him to doubt the accuracy of the expo- 

 nential formula, as applied to the transmission of homogeneous 

 rays through the earth's atmosphere, but first let us consider 

 the basis of the formula. 



We have seen that Bouguer's formula rests on the fundamental 

 assumption that the light is not changed in its nature in passing 

 from one layer to another, so that equal layers take out equal 

 fractions. This is not the case except for homogeneous rays. 

 It is therefore necessary to divide the beam up into parts, each 

 containing rays of approximately homogeneous transmissibility. 

 For this purpose it is necessary to observe the spectrum of the 

 sunlight by the aid of the bolometer or other satisfactory delicate 

 heat-measuring instrument. Even so, it is not possible to ob- 

 serve the transmission of the atmosphere at every wave length, 

 so as to determine the coefficients of transmission in the fine 

 lines of absorption by water vapor and oxygen which are in- 

 troduced by the earth's atmosphere. These lines are mainly 



