8 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
move it along until the drying process is completed, when it is taken 
out at the opposite end of the kiln. An attempt is usually made in 
these kilns to maintain one end moist and the other end dry. This is 
known as the " progressive " type of kiln, and is the one most com- 
monly used in large operations. It is the least satisfactory of the two, 
however, where careful drying is required, since the conditions can 
not be so well regulated and the temperatures and humidities are apt 
to change with change of wind. The compartment method can be 
arranged so that it will not require any more kiln space or any more 
handling of lumber than the progressive type. It does, however, 
require more intelligent operation, since the conditions in the kiln 
must be changed as the drying progresses. With the progressive 
type the conditions, once established, remain the same. 
To obtain draft or circulation three methods are in use — by forced 
draft or a blower usually placed outside 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 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 differ- 
ences among these, however, have more to do with 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 kiln. 
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. 
In the new humidity-regulated kiln the humidity is controlled di- 
rectly. The majority of kilns make no attempt whatever to regulate 
this all-important factor beyond retaining an indeterminate amount 
at the beginning of the operation and drying the air, either by con- 
densers or by ventilation at the end. 
Other methods of drying in vacuum and in various gases have been 
tried from time to time. 
DRYING BY SUPERHEATED STEAM. 
There is still another type of kiln which is not included in the 
former classification, viz, that using superheated steam. What this 
term really signifies is simply water vapor in the absence of air in a 
condition of less than saturation. Such kilns are, properly speak- 
ing, vapor kilns, and usually operate at atmospheric pressure, but 
