371] B. E. Livingston 173 



if the evaporating power of the air for any particular surface 

 is to be studied. The condensation pressure should be deter- 

 mined for the general atmosphere of the space under con- 

 sideration. If air circulation were infinitely rapid, which 

 means, practically, if there is a high wind, this deficit value 

 should be a measure of the evaporating power for the particu- 

 lar location considered. Also, if two sets of conditions are to 

 be compared, in which the air circulation is the same, then 

 the two atmospheric evaporating powers 'should be propor- 

 tional to the corresponding vapor pressure deficits; for the 

 other factor is then common to both sets. 



To illustrate the use of the vapor pressure deficit, let it be 

 supposed that there are two rooms in which the air circulation 

 is alike, and let it be required to estimate the relative values 

 of the evaporating powers corresponding to the two rooms. 

 The data involved and the results obtained are shown below, 

 together with the two relative humidity values, as usually 

 given in such comparisons. 



Air tern- VAPOR PRESSURE Vapor pres- Relative 

 perature Maximum Actual sure deficit humidity 



deg. c. mm. of Hg. mm. of Hg. mm. of Hg. per cent. 

 Room 1. 20 17.41 14.50 2.91 83 



Room 2. 25 23.55 6.14 17.41 26 



The values used in this example have been so chosen that the 

 deficit for Eoom 2 is 17.41 mm., just what it would be for 

 Room 1 if the actual vapor pressure were taken as zero. This 

 is the maximum deficit for a temperature of 20. Neverthe- 

 less, it is seen that the actual vapor pressure for Room 2 is far 

 from zero. This emphasizes the point that the maximum 

 evaporating power of the air increases with the temperature, 

 air pressure, and circulation remaining the same. 



Such comparisons have usually been made in terms of rela- 

 tive humidity, the values for which are presented in the last 

 column of the tabular arrangement just given. This mathe- 

 matical abstraction is the ratio of the actual to the maximum 



