KILN DRYING HANDBOOK. 23 
and provided with a revolution counter. This counter is ordinarily 
in the form of a dial and pointer, one revolution of the pointer 
usually representing an air movement of 100 feet. It is necessary 
to use a watch with the anemometer, to determine the time corre- 
sponding to a certain air movement. It is customary to let the 
anemometer run a definite number of minutes, and then to divide 
the number of feet recorded by. the number of minutes, the quotient 
being the velocity expressed in feet per minute. It must be re- 
membered that the velocity in any duct varies throughout the cross- 
section, being greatest at the center and least along the sides, and 
that a single reading will probably not represent a true average. 
For accurate results the cross-section of the duct should be divided 
into squares about equal to the diameter of the anemometer and a 
reading taken on each square. This will seldom be necessary, how- 
ever, in ordinary work. In using anemometers in open places care 
must be exercised to set the anemometer with its axis truly parallel 
with the air movement. Otherwise it will register less than it should. 
Smoke may be used to indicate the direction of the air movement. 
Anemometers are imperfect in that the speed of the fan is not 
truly proportional to the air velocity over the entire range of use- 
fulness of the instrument, and it becomes necessary to apply a cor- 
rection factor. This correction factor is determined by actual trial 
or calibration at the factory, and a curve showing the amount of 
correction to be applied at different velocities should accompany the 
instrument. 
DRYING AND DRYING STRESSES. 
MOISTURE GRADIENT. 
The moisture in wood tends to equalize itself by flowing to areas 
of least moisture. If we desire to produce a flow of moisture in a 
piece of wood of uniform moisture content, we must first upset this 
uniform condition. This is done by removing some of the moisture 
from the surface by circulating air of proper temperature and 
humidity around the piece. As soon as evaporation from the sur- 
face commences, a " moisture gradient " has been established ; that 
is, we have made the wood drier on the surface than in the interior, 
and have thereby started the movement of the moisture from the 
interior toward the surface. If we continue to remove the moisture 
from the surface through evaporation a moisture gradient will con- 
tinue to exist. If the moisture be removed from the surface faster 
than it can transfuse from the interior, the moisture gradient will 
increase or become steeper, whereas if it be removed more slowly, the 
gradient will become less. 
SHRINKAGE. 
As the drying of green wood progresses the amount of free water 
in the cells gradually diminishes, and soon the cells near the sur- 
face have lost all their free water, i. e., they have reached the fiber- 
saturation point. It is at this point, which is a very definite one for 
most species, usually between 25 and 30 per cent moisture, that the 
changes in the properties of the wood begin to take place. As wood 
