the kiln, by ventilation, and
by internal circulation and condensation. A great many patents have
been taken out on different methods of ventilation, but in actual
operation few kilns work exactly as intended. Frequently the air moves
in the reverse direction for which the ventilators were planned.
Sometimes a condenser is used in connection with the blower and the
air is recirculated. It is also--and more satisfactorily--used with
the gentle internal-gravity currents of air.

Many patents have been taken out for heating systems. The differences
among these, however, have more to do the mechanical construction than
with the process of drying. In general, the heating is either direct
or indirect. In the former steam coils are placed in the chamber with
the lumber, and in the latter the air is heated by either steam coils
or a furnace before it is introduced into the drying chamber.

Moisture is sometimes supplied by means of free steam jets in the kiln
or in the entering air; but more often the moisture evaporated from
the lumber is relied upon to maintain the humidity necessary.

A substance becomes dry by the evaporation of its inherent moisture
into the surrounding space. If this space be confined it soon becomes
saturated and the process stops. Hence, constant change is necessary
in order that the moisture given off may be continually carried away.

In practice, air movement, is therefore absolutely essential to the
process of drying. Heat is merely a useful accessory which serves to
decrease the time of drying by increasing both the rate of evaporation
and the absorbing power of the surrounding space.

It makes no difference whether this space is a vacuum or filled with
air; under either condition it will take up a stated weight of vapor.
From this it appears that the vapor molecules find sufficient space
between the molecules of air. But the converse is not true, for
somewhat less air will be contained in a given space saturated with
vapor than in one devoid of moisture. In other words the air does not
seem to find sufficient space between the molecules of vapor.

If the temperature of the confined space be increased, opportunity
will thereby be provided for the vaporization of more water, but if it
be decreased, its capacity for moisture will be reduced and visible
water will be deposited. The temperature at which this takes place is
known as the "dew-point" and depends upon the initial degree of
saturation