the cooling chamber at the top and
bottom. The warm air from above is cooled by melting the ice. It then
becomes denser and settles down into the main chamber. The articles in
the cooling room warm the air as they cool, so it rises to the top and
again comes in contact with the ice, thus completing the cycle. The
rate of this natural circulation is automatically regulated by the
temperature of the articles in the cooling chamber and by the amount
of ice in the icing compartment; hence the efficiency of such a box is
high.

Now let us apply this principle to the drying of lumber. First we must
understand that as long as the lumber is moist and drying, it will
always be cooler than the surrounding air, the amount of this
difference being determined by the rate of drying and the moisture in
the wood. As the lumber dries, its temperature gradually rises until
it is equal to that of the air, when perfect dryness results. With
this fact in mind it is clear that the function of the lumber in a
kiln is exactly analogous to that of the ice in an ice box; that is,
it is the cooling agent. Similarly, the heating pipes in a dry kiln
bring about the same effect as the articles of food in the ice box in
that they serve to heat the air. Therefore, the air will be cooled by
the lumber, causing it to pass downward through the piles. If the
heating units are placed at the sides of the kiln, the action of the
air in a good ice box is duplicated in the kiln. The significant point
in this connection is that, the greener and colder the lumber, the
faster is the circulation. This is a highly desirable feature.

A second vital point is that as the wood becomes gradually drier the
circulation automatically decreases, thus resulting in increased
efficiency, because there is no need for circulation greater than
enough to maintain the humidity of the air as it leaves the lumber
about the same as it enters. Therefore, we advocate either the
longitudinal side-wise inclined pile or edge stacking, the latter
being much preferable when possible. Of course the piles in our kiln
were small and could not be weighted properly, so the best results as
to reducing warping were not obtained.

=Preliminary Steaming.=--Because the fibres of the gums become plastic
while moist and hot without causing defects, it is desirable to heat
the air-dried lumber to about 200 degrees Fahrenheit in saturated
steam at atmospheric pressure in order to reduce the warping.