abu 
If in these experiments the pan contained salt water, which is 
hygroscopic, a lower value of vapor pressure would be reached for 
the same temperature. For average sea water the vapor pressure is 
about 2 per cent less than for pure water. Vapor pressure over sea 
water is given in Table 2, 
Evavoration involves a loss of heat from the water surface. At 
50°F the loss is 590 calories for cach gram of water evanorated. 
This quantity, known as latent heat, varies with temperature only 
slizhtly. 
One manifestation of this cooling effect is in the so-called 
wet-bulb temperature, which may be defined as the temperature the 
air will reach when its vapor becomes saturated without any other 
fain or loss of heat than the latent heat utilized during the sat- 
uration. Air may be brought approximately to its wet-bulb tempera- 
ture by passing it through wet gauze, as in this case the latent 
heat required for saturation is supplied by the cooling of the air. 
The wet-bulb temperature forms the basis cf a common instrument 
for measuring humidity, the psychrometer. When a small wet object 
is placed in a stream of unsaturated air it assumes a temperature 
lower than the air temperature. It loses heat by evaporation and 
gains heat by conduction from the warmer air, Initially the objecs 
cools, but when the loss and gain of heat reach a balance it reaches 
an equilibrium temperature. In general this temperature depends on 
the air temperature, humidity and pressure, and also on the shape 
and size of the object and on the air speed. The equilibrium ten- 
perature is however independent of these last factors if the Pd °Stout 
the size of an ordinary thermometer bulb and if the air speed is 7 
miles per hour or more. The object then comes very close to the wet~ 
bulb temperature of the air. The psychrometer consists of a wet-bulb 
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