118 U. S. BUREAU OF FISHERIES 
ture with itself, due to its change in temperature. If the air after 
heating were cooled to its original temperature (62°), it would again 
become saturated; and if cooled further, water would immediately 
begin to condense from it. The temperature at which water vapor 
begins to condense from air that is being cooled is known as the dew 
point—in this case 62°. Ifin this example, however, after heating the 
air it had been allowed to take up some water from wet fish before 
being cooled, the dew point would have become higher; and, with cer- 
tain limitations, the more water it had taken up the higher the dew 
point would have been. In fact, it is actually possible for so much 
water to be taken up in the first part of a tunnel full of fish that when 
the air later strikes cold fish just entering the far end of the tunnel it 
will condense on them. This must be avoided, of course. 
Fractions of the total amount of water vapor that can be present 
in air at a given temperature are expressed as ‘‘per cents relative 
humidity.” For example, saturated air has a “relative humidity” 
of 100 per cent and half-saturated air 50 per cent. 
Air is heated to increase its moisture-absorbing capacity. Another 
reason for warming the air is to furnish the heat needed to vaporize 
the water that is to be removed. It takes much heat to- change 
water to vapor (steam) at the boiling point; yet it takes virtually 
the same quantity to change water to vapor at lower temperatures. 
In drying, the warm air that strikes an object furnishes this heat. 
Were it not furnished, a body from which water can evaporate 
freely would soon become so cool that the rate of evaporation would 
become negligible. 
Air in contact with a wet object soon becomes saturated, and if 
evaporation is to proceed this air must be replaced. This is done 
by continuously blowing air through the drier. This air conveys 
heat to the object and carries away the water that has vaporized. 
Increasing the temperature of the drying air causes a marked increase 
in the drying rate, and this is also true for increased air velocity. 
An idea of the effect of changing the intensity of these factors can be 
gained from the following comparisons, which are based on relations 
given in Tables 6 and 7. If, from a definite water surface and in a 
given time, air blowing at the rate of 250 feet per minute will evaporate 
1 pound of water with the temperature of the air and water at 72°, 
then at a temperature of 132° 6.75 pounds will be removed; and if 
at this higher temperature the air velocity is increased to 1,000 feet 
per minute the amount of water evaporated will be 9.25 pounds. 
Due to limitations set by the substance being dried, such increases 
in drying seldom are realized by bettering drying conditions. The 
temperature of the drying air may have to be kept low, so that the 
temperature of the substance itself will not rise to a point where 
undesirable changes take place or the substance may give up its 
moisture so slowly that the cost of maintaining a high air velocity 
will not be justified. 
The amount of moisture in the drying air also has its effect on the 
drying rate. Except where modified by other influences, the higher 
the humidity the lower the drying rate. 
