44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



influences that are likely to produce a deviation of the same kind. 

 Among these we will consider : 



(1) The influence of the temperature gradient. It is evident 

 that for a radiating atmosphere of low density, a larger part of the 

 radiation reaching the surface of the earth must come from farther 

 and therefore colder layers than for a dense atmosphere. From this 

 it follows that a decrease in the density of the atmosphere must 

 produce a decrease in its radiation in a twofold way: (A) in con- 

 sequence of the diminished radiating power of the unit volume ; and, 

 (B) because of the simultaneous shifting of the effective radiating 

 layer to higher altitudes. 



(2) We must consider that the radiation is determined by the 

 integral humidity, and that the water-vapor pressure comes into play 

 only in so far as it gives a measure of this quantity. At a certain 

 place we may obtain the integral humidity by multiplying the pressure 

 by a certain constant ; but this constant varies with the altitude. At 

 sea level this constant has a value equal to 2.3 against 1.8 at the alti- 

 tude of the summit of Mount Whitney ; these values can be obtained 

 from the formula of Suring, which has been discussed in a previous 

 chapter. 



This means that, in order to compare the integral humidities of 



two different localities as indicated by their absolute humidities, we 



should apply a reduction factor to the latter values. Thus, if the 



absolute humidity on the top of Mount Whitney is the same as at 



sea level (which naturally is unlikely to be the case at the same time), 



1 S 

 the integral humidity at the former place will be only -^ of that at 



the latter. 



(3) The coefficient of absorption, and consequently also that of 

 the emission for a unit mass of water vapor, is a function of the total 

 pressure to which it is subjected. This important fact has been 

 revealed by the investigations of Eva von Bahr 1 who found that water 

 vapor at a pressure of 450 mm. absorbs only about yy per cent of 

 what an identical quantity absorbs at 755 mm. pressure. The ab- 

 sorption coefficient will change in about the same proportion, and 

 consequently the effective amount of water vapor .(if we may use 

 that term for the amount of water vapor that gives a constant radia- 

 tion) will not be proportional to its mass but will be a function of 

 the pressure, i. e., a function also of the altitude. Miss v. Bahr's 



1 Eva v. Bahr, Uber die Einwirkung des Druckes auf die Absorption 

 Ultraroter Strahlung durch Gase. Inaug. Diss., Upsala, 1908, p. 65. 



