170 HOWARD E. PULLING 



increases with the amount of the effective substance between 

 the observer and the sun, that is to say, the effect increases not 

 only with the increase in concentration of effective material 

 per unit distance along the path of the light beam, but also with 

 the total distance traversed by the light beam through the 

 atmosphere with any given concentration of effective material. 



The general scattering and selective absorption due to the 

 gas molecules will increase as the density of the atmosphere 

 increases and with a longer path of the light rays through the 

 atmosphere. It will thus vary with the altitude, with baro- 

 metric pressure at the same altitude, and with the time of day 

 and the time of year at the same altitude and with the same 

 barometric pressure. 



The general scattering and selective absorption produced 

 by water vapor will vary with the changes in the total amount 

 of water vapor in the atmosphere between the earth and the 

 sun, with the time of day and the time of year. 



The dust effects will vary with the composition, shape, size, 

 etc., of the dust particles and the position of the sun with 

 reference to them and to the observer. 



So little is known of the effects of clouds that a summary 

 of their possible effects may be omitted. 



The graphs in figure 4 illustrating the total energy reaching 

 the earth in the sun's radiation are self-explanatory and may 

 serve as an integrated summary of that portion of the fore- 

 going discussion that considers the effects of the altitude of 

 the observer, the sun's altitude above the horizon, the general 

 scattering by the permanent gases and by water vapor, the 

 irregular contour of the sun's energy curve outside the atmos- 

 phere and the constant shifting of the maximum intensity to- 

 ward the longer waves as the sun's altitude decreases. Table 

 5 indicates the manner in which these losses are distributed 

 between the permanent gases of the atmosphere and the water 

 vapor for different amounts of the latter and for different alti- 

 tudes of the sun above the horizon. It may be noted that the 

 graphs are averages of a number of individual determinations 

 of the sun's energy and that variations from this distribution 

 are to be expected. 



