369 | B. HE. Iavingston 171 
condition into its two components, the moisture condition of 
the air and the velocity of air movement or circulation. For 
such an analysis atmometric observations are of course inade- 
quate. Furthermore, it is often requisite to compare different 
evaporating powers of the air when the air movement is 
known to be constant, in which case the moisture condition 
is the only variable to be taken into account. Finally, in the 
artificial control of the air conditions of culture chambers, 
the rooms of dwellings, ete., it is frequently possible to main- 
tain air circulation without much fluctuation and then to con- 
trol the evaporating power by controlling the moisture con- 
dition. In such cases it becomes important that serious at- 
tention be given to the moisture condition of the air and its 
adequate measurement. 
By moisture condition is here meant that factor in atmos- 
pheric evaporating power that is independent of the rate of 
air movement. It is thus an index of a condition determined 
by the state of saturation of the air (with aqueous vapor) and 
by the air temperature. Humidity, as commonly measured, 
does not involve temperature. To make these relations clearer, 
it may be added that the index of atmospheric evaporating 
power should be equal to the product of the index of the mois- 
ture condition and the index of circulation: J —1, x I, 
Of course it is here assumed that all measurements of con- 
ditions have been properly weighted and brought into corre- 
spondence, in deriving the indices. Otherwise a coefficient of 
proportionality needs to be applied to each of the quantities. 
In this equation, the value Jf, is the one with which this 
discussion deals. 
The tendency of water to evaporate into air lying next to 
the water surface is measured by the maximum vapor pressure 
possible with the prevailing conditions of the surface. If pure 
water is considered the maximum value may be obtained for 
any given temperature, from published physical tables. It 
will be lower than these published values if the water is im- 
pure, or if it is held by imbibing solids, etc. It is a gas pres- 
