330 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



ft _ ft 



1 + a t 2 1 + a t t 



and then for x 2 = x t and / 2 > t v we liave 



/ 2 = /, but e 2 > £j and i? 2 < 7?i 



that is to say, the warming of a quantity of air having a given 

 invariable composition and inclosed within a non-expansible vessel, 

 causes a rise of vapor pressure and a diminution of relative humidity 

 while the absolute humidity remains unchanged. 



This example is very instructive because it allows us to recognize 

 very clearly the difference between vapor pressure and absolute 

 humidity, whereas otherwise not infrequently one tends to consider 

 these two ideas as equivalent. 



The reason for this latter error lies in the fact that the quotient 



1.060 



1 + at 



occurring in equation (i) is equal to unity when t = 16.38 C. 

 and varies less than 2 per cent from unity between the values t = io° 

 and t = 22 . Since in the metric system the numbers for vapor 

 pressure and for absolute humidity are nearly the same for the 

 temperatures that most frequently occur, therefore in ordinary 

 language the difference between these two ideas is frequently 

 entirely overlooked. 



These analyses may suffice to help us clearly recognize the mean- 

 ing of the specific moisture and the mixing ratio. 



I will now pursue the example further and show how the average 

 distribution of aqueous vapor in a vertical column of air is expressed 

 by the use of this idea. 2 



I assume the atmospheric pressure at sea-level to be 76o mm , the 

 temperature 9.0 C. and the vapor pressure 6.5 mm , which corre- 

 spond to annual average conditions in the neighborhood of Berlin. 



As to the diminution of temperature with altitude I have used the 

 numbers deduced by A. Berson. 3 The value of the vapor pressure 

 for different altitudes is computed by the formula given by R. Suring 



e = £o 10 6 ^ + 20^ 



2 The following portion of this memoir has been revised, taking into con- 

 sideration the results of recent balloon voyages. [1905. W. v. B.] 



3 Wissenschaftlichen Luftfahrten., Vol. Ill, p. 63. 



