HUMIDITY-REGULATED AND RECIRCULATHSTG 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.^ 



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. 



