of material,
becomes denser, and consequently tends to sink. Unless the material to
be dried is so arranged that the air can pass gradually downward
through the pile as it cools, poor circulation is apt to result.

In edge-piled lumber, with the heating system beneath the piles, the
natural tendency of the cooled air to descend is opposed by the hot
air beneath which tends to rise. An indeterminate condition is thus
brought about, resulting in non-uniform drying. It has been found that
air will rise through some layers and descend through others.

Humidity

Humidity is of prime importance because the rate of drying and
prevention of checking and case-hardening are largely dependent
thereon. It is generally true that the surface of the wood should not
dry more rapidly than the moisture transfuses from the center of the
piece to its surface, otherwise disaster will result. As a sufficient
amount of moisture is removed from the wood to maintain the desired
humidity, it is not good economy to generate moisture in an outside
apparatus and force it into a kiln, unless the moisture in the wood is
not sufficient for this purpose; in that case provision should be made
for adding any additional moisture that may be required.

The rate of evaporation may best be controlled by controlling the
amount of vapor present in the air (relative humidity); it should not
be controlled by reducing the air circulation, since a large
circulation is needed at all times to supply the necessary heat.

The humidity should be graded from 100 per cent at the receiving end
of the kiln, to whatever humidity corresponds with the desired degree
of dryness at the delivery end.

The kiln should be so designed that the proper degree may be
maintained at its every section.

A fresh piece of sapwood will lose weight in boiling water and can
also be dried to quite an extent in steam. This proves conclusively
that a high degree of humidity does not have the detrimental effect on
drying that is commonly attributed to it. In fact, a proper degree of
humidity, especially in the loading or receiving end of a kiln, is
just as necessary to good results in drying as getting the proper
temperature.

Experiments have demonstrated also that injury to stock in the way of
checking, warping, and hollow-horning always develops immediately
after the stock is taken into the kiln, and is due to the degree of
humidity being too low. Th