7. In drying refractory hardwoods, such as oak, best results are obtained at a comparatively low temperature. In more easily dried hardwoods, such as maple, and some of the more difficult softwoods, as cypress, the process may be hastened by a higher temperature but not above the boiling point. In many of the softwoods, the rate of drying may be very greatly increased by heating above the boiling point with a large circulation of vapor at atmospheric pressure. 8. Unequal shrinkage between the exterior and interior portions of the wood and also unequal chemical changes must be guarded against by temperatures and humidities suited to the species in question to prevent subsequent cupping and warping. 9. The degree of dryness attained should conform to the use to which the wood is put. 10. Proper piling of the material and weighting to prevent warping are of great importance. Requirements in a Satisfactory Dry Kiln The requirements in a satisfactory dry kiln are: 1. Control of humidity at all times. 2. Ample air circulation at all points. 3. Uniform and proper temperatures. In order to meet these requirements the United States Forestry Service has designed a kiln in which the humidity, temperature, and circulation can be controlled at all times. Briefly, it consists of a drying chamber with a partition on either side, making two narrow side chambers open top and bottom. The steam pipes are in the usual position underneath the material to be dried. At the top of the side chambers is a spray; at the bottom are gutters and an eliminator or set of baffle plates to separate the fine mist from the air. The spray accomplishes two things: It induces an increased circulation and it regulates the humidity. This is done by regulating the temperature of the spray water. The air under the heating coil is saturated at whatever temperature is required. This temperature is the dew point of the air after it passes up into the drying chamber above the coils. Knowing the temperature in the drying room and the dew point, the relative humidity is thus determined. The relative humidity is simply the ratio of the vapor pressure at the dew point to the pressure of saturated vapor (see Fig. 30). [Illustration: Fig. 30. Section through United States Forestry Service Humidity-controlled Dry Kiln.] Theory and Description of the Forestry Service Kiln