172 Vapor Tension Deficit [370 
sure, and is expressed in pressure units, as the height of a 
mercury column, fractions of an atmosphere, ete. It may be 
considered as equal to the pressure that drives the water vapor 
out of the liquid surface, which may be termed the vaporiza- 
tion pressure. The temperature of the liquid lying close to 
the surface exerts a marked influence upon the magnitude of 
this pressure. 
This tendency for water to evaporate is opposed by another 
tendency, that of the air to deposit liquid water on the evapo- 
rating surface; it is the tendency of water vapor to condense. 
This is measured by the partial pressure of water vapor in the 
air lying next to the evaporating surface, and it may have any 
value between zero and the maximum vapor pressure of water 
vapor for the given air temperature. It also is a gas pressure 
and is measured in pressure units. The most satisfactory 
method of measuring it is by means of the Regnault dew-point 
apparatus, through determining the temperature of the dew- 
point, the partial pressure of water vapor in the air being equi- 
valent to the maximum vapor pressure of liquid water at the 
temperature of the dew-point of the air. Another less satis- 
factory method of determining this partial pressure is by 
means of the sling psychrometer, the readings being inter- 
preted by physical tables published for this purpose. This 
actual partial pressure of water vapor in the air may be termed 
the condensation pressure. 
From this it follows that the difference between the vapori- 
zation pressure and the condensation pressure must deter- 
mine the value of that factor of atmospheric evaporating 
power that is not determined by air circulation. This differ- 
ence is the vapor pressure deficit, measured as a pressure; it 
is the excess of vaporization pressure over condensation pres- 
sure. For most purposes of approximation it may be sup- 
posed that the temperature of the. liquid surface and that of 
the general air are the same. but this is not strictly true, and 
the temperature value employed in deriving the vaporization 
pressure ought really to be measured just within the liquid, 
