have the most water in the sapwood; thus sycamore
has more water than hickory, etc.

The efficiency of the drying operations depends a great deal upon the
way in which, the lumber is piled, especially when the humidity is not
regulated. From the theory of drying it is evident that the rate of
evaporation in dry kilns where the humidity is not regulated depends
entirely upon the rate of circulation, other things being equal.
Consequently, those portions of the wood which receive the greatest
amount of air dry the most rapidly, and vice versa. The only way,
therefore, in which anything like uniform drying can take place is
where the lumber is so piled that each portion of it comes in contact
with the same amount of air.

In the Forestry Service kiln (Fig. 30), where the degree of relative
humidity is used to control the rate of drying, the amount of
circulation makes little difference, provided it exceeds a certain
amount. It is desirable to pile the lumber so as to offer as little
frictional resistance as possible and at the same time secure uniform
circulation. If circulation is excessive in any place it simply means
waste of energy but no other injury to the lumber.

The best method of piling is one which permits the heated air to pass
through the pile in a somewhat downward direction. The natural
tendency of the cooled air to descend is thus taken advantage of in
assisting the circulation in the kiln. This is especially important
when cold or green lumber is first introduced into the kiln. But even
when the lumber has become warmed the cooling due to the evaporation
increases the density of the mixture of the air and vapor.

Kiln-drying Gum

The following article was published by the United States Forestry
Service as to the best method of kiln-drying gum:

=Piling.=--Perhaps the most important factor in good kiln-drying,
especially in the case of the gums, is the method of piling. It is our
opinion that proper and very careful piling will greatly reduce the
loss due to warping. A good method of piling is to place the lumber
lengthwise of the kiln and on an incline cross-wise. The warm air
should rise at the higher side of the pile and descend between the
courses of lumber. The reason for this is very simple and the
principle has been applied in the manufacture of the best ice boxes
for some time. The most efficient refrigerators are iced at the side,
the ice compartment opening to