2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 68 



chiefly of the transparency of water vapor at wave-lengths greater 

 than 2 fi. 



Figure i shows graphically the relative positions and intensities 

 in the normal spectrum of the energy radiating from two bodies of 



!/<. 2345678 9 10 II 12 13 14 15 16 17 Ig 19 tO/j. 



Fig. I. — Computed black-body energy curves for two bodies of equal size, one 

 at 6,ooo° K. (sun), the other at 287° K. (earth). 



equal size, one at the temperature of the sun, the other at that of the 

 earth. 



HISTORY OF THE PRESENT RESEARCH 



At first thought it might seem easy to obtain the main object of 

 this research by observing in the laboratory the total radiation of 

 a body at the temperature of the earth, after that radiation had passed 

 through various amounts of atmospheric aqueous vapor. However, 

 the complication then introduced by the radiation from the vapor 

 itself renders the analysis and interpretation of the results difficult 

 if not ambiguous. So recourse is necessary to the use of the radiation 

 from a source at a high enough temperature to make negligible the 

 radiation from the vapor itself and its low temperature surroundings. 

 But then a new trouble arises for the total radiations of bodies at 

 different temperatures are of different qualities and differently 

 affected by the absorption powers of the aqueous vapor. In order 

 to apply the results to the case of the earth's radiation, it is there- 

 fore necessary to know the absorptions from wave-length to wave- 

 length and the comparative distribution of energy in the spectrum 

 of the laboratory source of radiation and of the earth. This requires 

 the introduction of the spectroscope with its attending difficulties. 



Preparations for observing quantitively the transmissibility of 

 radiation by aqueous vapor in the spectrum region of the earth's 

 emission were begun in 1908. The radiation which it was proposed 



