10 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
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. 
Table 3 shows analytically that the spontaneous cooling of the mix- 
ture produced by the evaporation alone increases its density. This 
fact is of great significance, and the method of piling lumber in the 
Forest Service kiln takes advantage of this principle. 
THEORY AND DESCRIPTION OF THE FOREST SERVICE KILN. 
The humidities and temperatures in the piles of lumber are largely 
dependent upon the circulation of air within the kiln. The tempera- 
ture and humidity within the kiln, taken alone, are no criterion of 
the conditions of drying within the pile of lumber if the circulation 
in any portion is deficient. It is possible to have an extremely rapid 
circulation of the air within the dry kiln itself and yet have stag- 
nation within the pile, the air passing chiefly through open spaces 
and channels. Wherever stagnation exists or the movement of air 
is too sluggish the temperature will drop and humidity increase, 
perhaps to the point of saturation. 
When in large kilns the forced circulation is in the opposite di- 
rection from that induced by the cooling of the air by the lumber 
there is always more or less uncertainty as to the movement of the 
air through the piles. Even with the boards placed edgewise, with 
stickers running vertically, and with the heating pipes beneath the 
lumber, it was found that although the air passed upward through 
most of the spaces it was actually descending through others, so that 
very unequal drying resulted. While edge piling would at first 
thought seem ideal for the freest circulation in an ordinary kiln with 
steam pipes below, it in fact produces an indeterminate condition; 
air columns may pass downward through some channels as well as 
upward through others, and probably stagnate in others. Neverthe- 
less, edge piling is greatly superior to flat piling where the heating 
system is below the lumber. 
From experiments and from a study of conditions in commercial 
kilns the idea was developed of so arranging the parts of the kiln 
and the pile of lumber that advantage might be taken of this cooling 
of the air to assist the circulation. That this can be readily accom- 
plished without doing away with the present features of regulation 
of humidity by means of a spray of water is clear from figure 1, 
which shows a cross section of the improved humidity-regulated 
dry kiln. 
In the form shown in the sketch a chamber or flue B runs through 
the center near the bottom. This flue is only about 6 or 7 feet in 
height and, together with the water spray F and the baffle plates D D, 
constitutes the humidity -control feature of the kiln. This control 
of humidity is effected by the temperature of the water used in the 
