20 BULLETIN 1136, IT. S. DEPARTMENT OE AGRICULTURE. 
Time is an element in certain classes of control in which it is de- 
sirable to change the setting of the control instrument at definite 
intervals. To meet this need both single-duty and double-duty 
instruments are made with time attachments. These are usually in 
the form of clock-driven cams, upon which ride levers controlling 
the adjustment of the air valves. By providing a suitable assort- 
ment of cams and a sufficiently flexible system of gearing between 
the clock and the cam any desired drying schedule can be repro- 
duced automatically. 
AIR CIRCULATION IN THE KILN. 
It is absolutely necessary to have a certain amount of circulation 
of air in a kiln to convey the heat from the steam coils or other 
source to the lumber and to carry away the evaporated moisture. 
PRODUCTION OF CIRCULATION. 
The simplest way to produce circulation is by means of chimneys 
or fines. This natural draft is caused by the difference in tempera- 
ture of the outside air and the air in the kiln. The warm air in the 
kiln is lighter than the air outside and is continually escaping 
through the top. The cold outside air is drawn in at the bottom. 
There is always inleakage at the bottom and outleakage at the top 
of the kiln, no matter how well it may be built ; and when the path 
of the air is made easy by providing chimneys and fresh-air in- 
takes the circulation becomes quite brisk. The velocity in the chim- 
neys may be 600 feet per minute or more, depending upon cir- 
cumstances. A reasonable amount of draft may be secured through 
the chimneys, even though no air intake openings are provided. 
There may also be considerable draft through the intake when 
there are no chimneys, or when the chimney dampers are closed. 
Under such conditions the whole kiln acts as a chimney, and the 
leakage is sufficient to permit the escape or entrance of appreciable 
amounts of air. 
Air intakes are usually placed at the bottom of the kiln and the 
outlets from the kiln to the chimneys at varying heights along the 
sides and in the ceiling. The chimneys usually, but not always, 
project above the roof. The higher the chimneys the more rapid 
will be the circulation. 
It is sometimes considered advantageous to draw the air over a 
circuitous route through which the circulation will ordinarily not 
start of its own accord. This may be done by some special means 
to stimulate the circulation, such as the use of radiators, aspira- 
tors, or inspirators. The simplest form of radiator for this pur- 
pose is a single length of pipe running the full length of the chim- 
ney and fed with steam from the bottom. These radiators produce 
an upward draft of air in the chimneys by heating the air. They 
may be left on throughout the entire drying period if the added 
circulation is desirable. The heat given off by these radiators is 
lost, except in so far as it does useful work in producing circulation. 
Condensers if properly located will assist materially in produc- 
ing circulation, and will also reduce the humidity. If air is being 
