20 BULLETIN 1136, U. 8S. DEPARTMENT OF AGRICULTURE 
heat required for evaporation is absorbed from the air that carries 
away the water vapor, with resultant cooling of the air; the net 
effect in consequence is to make the air heavier, since the increase 
in weight brought about by the cooling outweighs the decrease 
caused by the increase in humidity. 
The humidity of the surrounding air not only determines largely 
the rate at which materials will dry, but it also determines the extent 
to which they can be dried; there is a definite balance between the 
humidity of the air and the moisture content of wood. With minor 
variations, all kinds of wood held long enough in an atmosphere of 
constant temperature and constant humidity will come to the same 
moisture content, which is called the “ equilibrium moisture content.” 
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FELATIVE HUMIDITY IN ATMOSPHERE — PLRCENT. 
Figure 4.—Relation of the equilibrium moisture content of wood to the relative humid- 
ity of the surrounding atmosphere, at three temperatures 
The time required for this adjustment varies with different species, 
other factors being the same. The relation between humidity in the 
air and moisture in the wood is an important one, since it is closely 
related to all drying schedules and, further, determines the extent 
to which wood for use under specified conditions of temperature and 
humidity should be dried. Figure 4 presents curves showing the 
humidity-moisture relation at three temperatures. 
HUMIDITY-MEASURING INSTRUMENTS 
Since humidity determines the drying characteristics of air at any 
given temperature, the control of humidity in the kiln is of prime 
importance. It is essential that the moisture be removed from the 
wood surface at the maximum safe drying rate. If the humidity is 
too low the wood will dry too fast and will ‘be injured ; if the humidity 
is too high the drying will be slow and expensive. 
