﻿4 SMITHSONIAN MISCELLANEOUS COLLECTIONS [.VOL. 47 



atmospheric transmission should first be considered. Bouguer's 

 formula in a somewhat modified form may be written — 



e = kd = c,a 



O 



where e is the amount of energy of a particular wavelength as re- 

 ceived after its passage through the atmosphere. It is equal to the 

 deflection, d, of the galvanometer, multiplied by a constant, k, a 

 function of the receiving apparatus. 



e is the amount of this energy which would have been received 

 had there been no terrestrial absorbing medium in the path of the 

 rays. 



a, the coefficient of atmospheric transmission, is equivalent to the 

 fractional amount of energy transmitted by a unit layer. The unit 

 layer of water vapor is taken to be such that, if condensed into a 

 stratum of liquid water of the same cross-section, it would be one 

 centimeter thick. 



m, the air mass, is the length of the path of the ray through the 

 earth's atmosphere. The unit " an atmosphere " is the length of the 

 path with the sun in the zenith. 1 



'. is the ratio of the observed barometer ft to the standard height 



Pa 

 ft 760 mm. As the variations in ft have not amounted to more 

 than one percent, this factor has been neglected. 2 



1 m = „ . . ; James D. Forbes, Philosophical Trans- 



5 ,s .36 X sin. zenith distance 



actions of the Royal Society of London, 1842, part 1, p. 225. 



2 This barometric term is, however, very misleading. The terrestrial 

 atmosphere produces at least three kinds of absorption, due respectively to 

 dust, to the permanent gases, and to the fluctuating vapors. It seems im- 

 probable that absorption due to the dust above a station varies with the 

 barometer as indicated by the formula, for taking in illustration an extreme 

 case of a cyclonic low barometer, the correction would imply less absorption, 

 whereas the ascending currents may carry the dust into the upper and 

 pure) 1 the air, while the incoming currents at the bottom bring in 



additional dust, so that the absorption is actually increased. In ascending 

 from one station to another at a higher altitude, the factor again affords no 

 correct indication of the decrease of the absorption due to dust, unless indeed 

 rease of barometric pressure follows the same law as the decrease in 

 In the second case of the absorption of the permanent gases of 

 the use of the barometric pressure seems legitimate for obser- 

 vatioi lion; for stations at different altitudes the partial 



ing the absorption should be used. Water vapor 

 the third kind of absorbent, and a special factor for its 

 transmission is introduced in the formula. 



