being brought together at different temperatures, and being drawn into
the mixing chamber through the same chimney, it will be seen that the
dimensions of the flues that conduct them should vary with the kind of
coal used; and the manner in which the gases are brought together is not
a matter of indifference.

[Illustration: THE BICHEROUX SYSTEM OF FURNACE.

Vertical Section, and Horizontal Section through MNOPQR]

The gas generator consists of a hopper, A, into which drops, through
small apertures a, the coal piled up on the platform, D. These apertures
are closed with coal or bricks. The bottom of the generator is formed of
a small standing grate. The coal, on falling upon a mass in a state of
ignition, distills and becomes transformed into coke, which gradually
slides down over a grate to produce afterward, through its own
combustion, a distillation of the coal following it. But as these are
features found in all generators we will not dwell upon them.

The gases that are produced flow through a long horizontal flue, B, into
a vertical conduit, E, into which there debouches at the upper part a
series of small orifices, F, that conduct the air that has been heated.
The gases are inflamed, and traverse the furnace c (not shown in the
cut), from whence they go to the chimney. Before the air is allowed to
reach the intervening chamber it is made to pass into the sole of the
furnace and into the walls of the chamber, so that to the advantage of
having the air heated there is joined the additional one of having those
portions of the furnace cooled that cannot be heated with impunity.

The incompletely burned gases that escape from the furnace are utilized