PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 761st meeting was held on October 30, 1915, at the Cosmos 

 Club. President Eichelberger in the chair, 33 persons present. 



Regular Program 



Mr. F. E. Fowle presented a paper on The transparency of air and 

 water vapor. Radiant energy (e \) coming through the air from a 

 heavenly body suffers five losses: 1, non-selective, as to wave-length, 

 in the permanent gases of the air; 2, non-selective associated with water 

 vapor; 3, selective in the permanent gases; 4, selective in the water 

 vapor; 5, losses due to the dust. The losses (1) have been shown to be 

 due to the scattering by the molecules of the air and may be com- 

 puted accurately from the number of molecules in the path; it is ex- 

 pressed by coefficients, a a \ (Astrophysical Journal, 40:435. 1914; 

 38:392. 1913). The losses (2) are greater than would be expected 

 from the number of molecules of water vapor and are treated in the 

 same paper; they are expressed by coefficients, a w \; coefficients for (1) 

 and (2) vary slowly with the wave-length. The losses (3) and (4) 

 are treated in detail in the present communication. The first 

 are practically only in bands of limited wave-lengths and are expressed 

 empirically as a function of the length of path, m, through the 

 air. The losses (4) are similar to (3) but are expressed as a function 

 of the amount of water vapor, w, in the path. The losses (5) due 

 to dust vary at sea level from day to day but are nearly invariable 

 with the wave-length in the region considered here. At Washington 

 they may vary on good days from 3 to 11 per cent or more; above an 

 altitude of about 1000 meters they are generally negligible. The 



resultant energy may be expressed by e = e \ \a a \<£w) ■ Tables were 

 given showing the losses from the incomng solar energy in calories and 

 per cents due to scattering and absorption for sea level and other alti- 

 tudes, various amounts of water vapor and zenith distances of the sun; 

 also percentage absorptions due to water vapor at various wave- 

 lengths and formulae for computing the various losses for particular 

 cases. 



Discussion: Mr. C. A. Briggs cited a practical application of absorb- 

 ing power of water vapor by the use of a steam curtain before doors of 

 steel-heating furnaces. Messrs. Swann, Wells, and Bauer asked 

 regarding presence of dust and possible effects at higher altitudes. Mr. 



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