18 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
can be used equally well as a wet -bulb recorder, though sometimes 
it is necessary to change the shape or design of the bulb. 
The wet -bulb temperature alone is of no value in determining- 
humidity without the corresponding dry-bulb temperature. Hence, 
wet and dry recorders are set up side by side, or, as is much more 
convenient, combined in one instrument called a two-pen recorder, 
which records both temperatures on one chart. The space between 
the two represents the wet-bulb depression. For general use, two-pen 
recorders have the tubes and bulbs separate from each other, but 
when designed exclusively for use as wet and dry bulb recorders the 
extension tubes are frequently encased in a single protecting armor 
extending from the case of the instrument almost to the bulbs. The 
wick trough is made with two sets of brackets, one set for the wet 
bulb and one for the dry bulb. Instruments of this type are usually 
called recording psychrometers. The facts concerning recording 
thermometers apply to wet and dry bulb recorders. 
CONTROL OF KILN HUMIDITY. 
It is simpler to increase the humidity in a kiln than to decrease it. 
The universal method of increasing humidity is to inject steam into 
the kiln chamber. 
Atmospheric air is usually drier than that in the wood-drying 
kiln and can be used only for dehumidification, a practice common 
with ventilated kilns. The moist air is drawn off through ventilating 
flues and the fresh air enters through intake flues or ducts. As the 
fresh air is heated its relative humidity falls while the dew point 
remains the same. 
Moisture may also be removed from the air by condensation. The 
water vapor in the air condenses as it passes over a substance colder 
than the dew point of the air. Condenser pipes with cold water 
flowing through them are commonly used for this purpose. When 
cold water is not available, *i refrigerator plant may be installed 
and brine circulated through the condenser pipes. 
Cold-water sprays are also used to dehumidify air. The spray 
temperature must be below the dew point of the air passing through. 
If the sprays are powerful enough the air will be cooled to about 
the temperature of the water and will come out saturated at a tem- 
perature below its original dew point. In other words, the dew point 
will have been lowered. If the air be heated to its original tempera- 
ture it will be drier than it originally was. 
In the chemical laboratory air is dried by passing it through 
chemicals which have affinity for moisture. Principal among these 
are calcium chloride and sulphuric acid. Their use has not been 
developed for commercial wood drying. 
The control of humidity is more difficult than temperature control, 
and greater attention must be given to the apparatus to secure sat- 
isfactory results. One principal reason is that a small difference in 
the wet-bulb temperature produces a comparatively large differ- 
ence in humidity, and to secure good control requires an accurate 
instrument. 
The controllers of greatest importance are those which depend 
partly or wholly upon a wet-bulb of one type or another. Temper- 
ature controllers of various types can be made into wet-bulb con- 
