. The heated air rises above the
pipes HH and between the piles of lumber. As it comes in contact with
the piles, portions of it are cooled and pass downward and outward
through the layers of boards into the space between the condensers GG.
Here the column of cooled air descends into the spray flue B, where
its velocity is increased by the force of the water spray. It then
passes out from the baffle plates to the heaters and repeats the
cycle.

One of the greatest advantages of this natural circulation method is
that the colder the lumber when placed in the kiln the greater is the
movement produced, under the very conditions which call for the
greatest circulation--just the opposite of the direct-circulation
method. This is a feature of the greatest importance in winter, when
the lumber is put into the kiln in a frozen condition. One truckload
of lumber at 60 per cent moisture may easily contain over 7,000 pounds
of ice.

In the matter of circulation the kiln is, in fact, seldom
regulatory--the colder the lumber the greater the circulation
produced, with the effect increased toward the cooler and wetter
portions of the pile.

Preliminary steaming may be used in connection with this kiln, but
experiments indicate that ordinarily it is not desirable, since the
high humidity which can be secured gives as good results, and being at
as low a temperature as desired, much better results in the case of
certain difficult woods like oak, eucalyptus, etc., are obtained.

This kiln has another advantage in that its operation is entirely
independent of outdoor atmospheric conditions, except that barometric
pressure will effect it slightly.

KILN-DRYING

Remarks

Drying is an essential part of the preparation of wood for
manufacture. For a long time the only drying process used or known was
air-drying, or the exposure of wood to the gradual drying influences
of the open air, and is what has now been termed "preliminary
seasoning." This method is without doubt the most successful and
effective seasoning, because nature performs certain functions in
air-drying that cannot be duplicated by artificial means. Because of
this, hardwoods, as a rule, cannot be successfully kiln-dried green or
direct from the saw.

Within recent years, considerable interest is awakening among wood
users in the operation of kiln-drying. The losses occasioned in
air-drying and in improp