Both types of kilns are equally well adapted to the drying of 1-inch green Norway and white pine, elm, hemlock, and such woods as are used in the manufacture of flooring, ceiling, siding, shingles, hoops, tub and pail stock, etc. The selection of one or the other for such work is largely matter of personal opinion. Kilns of Different Types All dry kilns as in use to-day are divided as to method of drying into two classes: The "Pipe" or "Moist-air" kiln; The "Blower" or "Hot Blast" kiln; both of which have been fully explained in a previous article. The above two classes are again subdivided into five different types of dry kilns as follows: The "Progressive" kiln; The "Apartment" kiln; The "Pocket" kiln; The "Tower" kiln; The "Box" kiln. The "Progressive" Dry Kiln Dry kilns constructed so that the material goes in at one end and is taken out at the opposite end are called Progressive dry kilns, from the fact that the material gradually progresses through the kiln from one stage to another while drying (see Fig. 31). In the operation of the Progressive kiln, the material is first subjected to a sweating or steaming process at the receiving or loading end of the kiln with a low temperature and a relative high humidity. It then gradually progresses through the kiln into higher temperatures and lower humidities, as well as changes of air circulation, until it reaches the final stage at the discharge end of the kiln. Progressive kilns, in order to produce the most satisfactory results, especially in the drying of hardwoods or heavy softwood timbers, should be not less than 100 feet in length (see Fig. 35). In placing this type of kiln in operation, the following instructions should be carefully followed: When steam has been turned into the heating coils, and the kiln is fairly warm, place the first car of material to be dried in the drying room--preferably in the morning--about 25 feet from the kiln door on the receiving or loading end of the kiln, blocking the wheels so that it will remain stationary. [Illustration: Fig. 35. Exterior View of Four Progressive Dry Kilns, each 140 Feet long by 18 Feet wide. Cross-wise piling, fire-proof construction.] Five hours later, or about noon, run in the second car and stop it about five feet from the first one placed in the drying room. Five hours later, or in the evening push car number two up against the first car; then run in car number three, stopping it about five feet from car number two.