REDUCTION OF HUMIDITY DATA VON BEZOLD 327 



We were therefore forced long since, in our theoretical investiga- 

 tions to^introduce two other quantities by the use of which not 

 only did such investigations first become possible, but which are 

 also adopted to give a deeper insight into the condition of the atmos- 

 phere as to moisture. These quantities are on the one hand the 

 quantity of vapor contained in the unit mass of moist air, which can 

 be conveniently called the "specific humidity;" 1 and, on the other 

 hand, the quantity of moisture mixed with the unit mass of dry air 

 or briefly "the mixing ratio." 



How important the knowledge of these quantities may be in the 

 discussion of the observed numerical data gained by balloon ascen- 

 sions is shown by the simple consideration that these must remain 

 constant so long as the balloon preserves a course in companion- 

 ship with the air that surrounds it, no matter how complex the 

 changes may be that this air experiences as to its pressure and tem- 

 perature and consequently also as to its absolute and relative 

 humidity. 



So also do these quantities experience no change so long as the 

 balloon remains within an ascending or descending current provided 

 that no other air, with a different vapor constant, becomes mixed 

 with it. 



Hence also, conversely, any change in these quantities becomes in 

 one sense a measure of the admixture of foreign masses of air, a 

 process whose study is of the highest importance. 



This much by way of introduction. We will now first consider 

 the relations that exist between these two quantities and those 

 others that are ordinarily used as characteristic of the moisture in 

 the atmosphere. 



To this end I shall use the following notation: 



e the vapor pressure in millimeters of mercury. 



e' the maximum possible vapor pressure for the temperature t. 



f the absolute humidity or the number of grams of water vapor in 

 a cubic meter. 



R the relative humidity. 



x the mixing ratio or the mass of vapor mixed with a unit mass of 

 dry air expressed as a fraction of this latter unit. 



1 In the mechanical theory of heat, as is well known, the fraction of water, 

 that is in the form of vapor, in a mixture of saturated vapor and water is 

 called the "specific quantity of vapor. " By using the name above proposed 

 we give expression to the analogy of the two quantities (vapor and humidity), 

 whereas on the other hand we prevent any confusion in the two different ideas. 



