those, m, which allow the hot air to pass into the chambers. The hot air
then descends from the top of the chamber into the wool or rags, and,
becoming saturated and heavier, descends and makes its exit from the
chamber through an aperture, n, near the floor, whence it flows to the
central chimney. This latter, which is built of brick or stone, contains
in its center a second chimney (formed of cast or forged iron pipes)
that serves to carry off into the atmosphere the products of combustion
from the heating apparatus. The heat that radiates from these pipes
serves at the same time to heat the annular space through which the
vapors derived from the wool are disengaged.

The air, heated to 40° or 50°, is made to pass thus for several hours,
until the greater part of the humidity has been removed. The temperature
is then raised to 80° or 90° by gradually closing the apertures that
give access to the ventilating chimney. In order that it may be possible
to further increase the temperature during the last hour, and raise it
to 90° or 120°, an arrangement is provided that prevents all entrance of
the external air into the heating apparatus, and that replaces such air
with the hot air of the chamber; so that this hot air circulates in the
pipes of the stove and thus becomes gradually hotter and hotter. The hot
vapors that issue from the lower chamber rise into the upper one, where
they are used for the preliminary drying of another part of the
materials.

The hot air stove should be well lined with refractory clay, in order to
prevent the iron from getting red hot, and the grate should be of
relatively wide surface. All the pipes should be of cast iron, and all
the joints be well turned. Every neglect to see to such matters, with a
view to saving money, will surely lead in the long run to bad results.