520 F. W. Very — Transmission of Terrestrial Radiation. 



of Homen were made under conditions that must be regarded 

 as exceptional " is not necessarily justified. 



The mean value of 14 percent which Angstrom derives for 

 the transmission of terrestrial radiation by the atmosphere from 

 considerations which I have shown above to be based on falla- 

 cies, is further advocated by him because it " is not far from 

 that derived by Abbot and Fowle from the measurements on 

 the absorptive power of the water-vapor" ; but the apparent 

 support of these measurements is also fallacious, since, as I 

 proved conclusively in " Atmospheric Radiation " (p. 99), 

 diluted water-vapor, that is, a given depth of water spread in 

 the form of vapor through a very large depth of air, transmits 

 much more freely than the same amount of aqueous vapor in 

 concentrated form and equal equivalent depth. " The same 

 quantity of water in the form of steam takes out about 15 per 

 cent, and distributed as atmospheric vapor only one-half of 1 

 per cent." I have already pointed out the inadequacy of the 

 theory of aqueous absorption held by these observers.* 



Much vagueness and misconception would be removed if it 

 were realized that there is a fundamental distinction between 

 the transmission of a large part of the solar radiation by the 

 atmosphere, and that of terrestrial radiation by the same 

 atmosphere. Diffuse selective reflection enters very largely 

 into the skylight problem and the process is conducted with 

 the speed of light, which for atmospheric distances is prac- 

 tically instantaneous ; but the terrestrial radiation which does 

 not immediately escape to space, becomes air radiation and 

 passes from molecule to molecule by a very large number of 

 alternations and thus remains a long time in the medium. 

 This has been shown in my " Note on Atmospheric Radia- 

 tion," f and also more elaborately in a computation of the flux 

 of volume energy, which appears to constitute air radiation, 

 through the height of the atmosphere, in my paper : " On the 

 Solar Constant," % where it is shown that " the thermal energy 

 which gives air pressure is maintained by the internal radiation 

 between the air molecules." 



Certain theorists have attached a great importance to changes 

 in atmospheric carbon dioxide as a climatic factor. It is true 

 that this gas has a strong absorption band at 14 - 7/-i, which 

 looks very imposing when plotted on a percentage scale of 

 absorption ; but when it is recognized that there is only a small 

 part of the total terrestrial radiation included within the limits 

 of this band, it may be seen that its effect is relatively insig- 

 nificant. Ozone also, in spite of some large claims by Mr. 



* See Astrophysical Journal, vol. xxxiv, p. 373. 

 f This Journal, vol. xxxiv, p. 533, December, 1912. 

 jlbid., vol. xxxix, p. 204, et seq., February, 1915. 



