ft-wood charcoal is preferred. It is found that it is
not usually advantageous to build kilns of over 160 to 180 cubic meters
in capacity. Larger furnaces have been used, and give as good a yield,
but they are much more cumbersome to manage. The largest yield got from
kilns is from 50 to 60 bushels for hard wood to 50 for soft wood. The
average yield, however, is about 45 bushels. In meilers, two and a half
to three cords of wood are required for 100 bushels, or 30 to 40 bushels
to the cord. The kiln charcoal is very large, so that the loss in fine
coal is very much diminished. The pieces usually come out the whole
size, and sometimes the whole length of the wood.

The rectangular kilns were those which were formerly exclusively in use.
They are generally built to contain from 30 to 90 cords of wood. The
usual sizes are given in the table below:

1 2 3 4
Length 50 40 40 48
Width 12 15 14 17
Height 12 15 18 18
Capacity, in cords 55 70 75 90

1 and 2. Used in New England. 3. Type of those used in Mexico. 4. Kiln
at Lauton, Mich.

The arch is usually an arc of a circle. A kiln of the size of No. 4, as
constructed at the Michigan Central Iron Works, with a good burn, will
yield 4,000 bushels of charcoal.

The vertical walls in the best constructions are 12 to 13 feet high, and
1-1/2 brick thick, containing from 20 to 52 bricks to the cubic foot of
wall. To insure sufficient strength to resist the expansion and
contraction due to the heating and cooling, they should be provided with
buttresses which are 1 brick thick and 2 wide, as at Wassaic, New York;
but many of them are built without them, as at Lauton, Michigan, as
shown in the engraving. In both cases they are supported with strong
braces, from 3 to 4 feet apart, made of round or hewn wood, or of cast
iron, which are buried in the ground below, and are tied above and below
with iron rods, as in the engraving, and the lower end passing beneath
the floor of the kiln. When made of wood they are usually 8 inches
square or round, or sometimes by 8 inches placed edgewise. They are
sometimes tied at the top with wooden braces of the same size, which are
securely fastened by iron rods running through the corners, as shown.
When a number of kilns are built together, as at the Michigan Central
Iron Works, at Lauton, Michigan, shown in the plan view,