FOR DEDUCING THE RELATIVE HUMIDITY OF THE AIR. 



Example. 

 Suppose the 



Temperature of the Air, or t, to be 43 F. 



Temperature of the Dew-Point, or f, to be = 35 F. 



Difference between the two, or t t', to be = 8 F. 

 Taking in Table VI. the Elastic Forces due to t and f, we have 



= -I* = '3-4, Relative Humidity in Hundredth. 



The following Table VIII. gives, in hundredths, the fraction of saturation, or Rel- 

 ative Humidity, corresponding to each degree of t', or of the temperature of the air, 

 from to 104 ; and for every half degree of t t', or of the difference between 

 the temperature of the air and of the dew-point, from 0.5 to 24.5. Regnault's Table 

 of Elastic Forces of Vapor, reduced to English measures, has been used in the 

 computation. 



Though the fraction of saturation expressed in hundredths indicates the Relative 

 Humidity with sufficient accuracy, the thousandths have been added to facilitate, as 

 remarked above in the preface to the Psychrometrical Tables, the interpolations for 

 any number falling between those given in the table. 



USE OF THE TABLE. 

 Example. 



Temperature of Air, or t, being = 62 F. 



Temperature of the Dew-Point, or t', = 53 F. 



Difference, or t t', = 9 F. 

 Find out the Relative Humidity. 



In the column of temperatures, the first on the left, find 62 ; on the same horizon- 

 tal line, in the column headed 9, is found 72.4, which is the Relative Humidity 

 required. 



Should it seem desirable to compute the Relative Humidity for values of t t' 

 not contained in the table, the factors given below in Table IX. may be used. It 

 may be seen, however, that an interpolation at sight will always suffice for meteo- 

 rological purposes. 



B 74 



