HUMIDITY-REGULATED AND RECIRCULATING DRY KILN"- 7 
face of cold water. In the new kiln a fine spray of water is used in- 
stead of a condenser. This has the additional advantage that when 
the water is heated above a certain temperature (the temperature of 
the wet bulb in a wet-and-dry bulb hygrometer) it will humidify the 
air. By simply changing the temperature of the spray the air may 
be supplied at any desired humidity. 
INSTRUMENTS FOR MEASURING HUMIDITY. 
A common instrument used for measuring humidity is the wet- 
and-dry bulb hygrometer or " psychrometer." This consists of two 
thermometers mounted side by side, the bulb of one of which is 
covered by a silk cloth or wick which dips into a vessel of water. 
This should be placed in a fairly strong draft of air. The evapora- 
tion from the " wet bulb " reduces its temperature below that of the 
dry bulb, and the rate of this evaporation, and consequently the 
temperature of the wet bulb, depends upon the relative humidity in 
the air. By noting the two temperatures of the dry and the wet 
bulb thermometers the relative humidity can be determined by tables 
which have been carefully worked out by the Weather Bureau. 1 
In the humidity diagram in Forest Service Bulletin 104 the values 
are expressed in curves (the convex series of curves on the diagram) , 
by means of which the relative humidity may be read off directly 
without numerical calculations. This instrument is probably the 
simplest reliable means for determining humidity. There are instru- 
ments which read directly from a hand on a dial, the motion of the 
hand being produced by the swelling of vegetable or animal tissues. 
These are very convenient but fragile and not to be depended upon. 
The most direct way of determining humidity is, of course, to de- 
termine the dew point. This may be accomplished by gradually 
cooling a bright surface, as polished metal, in contact with the mov- 
ing air, until a mist is precipitated thereon. Special interest attaches 
to the wet-and-dry bulb hygrometer for the reason that the wet wood 
in the dry kiln is actually in the same condition as the wet bulb. It 
is affected in the same way. The actual temperature of the wood, 
while it is moist, is therefore that of the wet bulb, provided there is 
sufficient circulation. 
TYPES OF KILNS. 
There are two distinct ways of handling lumber in kilns. One' 
way is to place the load of lumber in a chamber where it remains in 
the same place throughout the operation, while the conditions of the 
drying medium are varied as the drying progresses. This is the 
compartment kiln or stationary method. The other is to run the 
lumber in one end of the chamber on a wheeled truck and gradually 
1 See Psychrometer Tables by Marvin, Bulletin 235 of United States Weather Bureau. 
