f

8 APPLICATION OF THE LAWS OF HYDRAULICS

is also introduced as before, through the fireboxer, It follows that
when the damper or stopper at the top of the chimney is opened
slightly, the model will represent a downdraft brick kiln in
operation.

It is seen that in the downdraft kiln the hottest gases rise to
the highest point under the roof, where they accumulate, forcing
the cold gases to the chimney through the ports in the hearth of
the kiln. Descending little by little toward the sole of the kiln,
the flames or hot gases finally fill the entire kiln chamber and
maintain themselves throughout it, only passing to the chimney
as they are displaced by hotter gases. In this manner the free
lower surface of the hot layer of gases is very nearly stationary,
which insures a practically uniform burn to the brick. In this
atmosphere, which varies very little, the reactions of combustion
are readily effected until only very slight traces of the combustible
elements and free oxygen can be found in the gases. That is,
combustion takes place with very nearly the theoretical supply of
oxygen. The flames of this combustion traverse the entire mass
of the gases and there are no definite points at which high tem-
peratures may be found. For this reason the downdraft kiln
is successfully employed when it is desired to obtain slow and
uniform heating.

These experiments with a model of a furnace immersed in
water confirm, with sufficient clearness, the fundamental principle
that the circulation of the hot gases within a furnace is similar
to the circulation of a light liquid within an enclosure filled with a
heavy liquid.

III. THE CURRENT OF THE HOT GASES MAY BE COMPARED TO
A STREAM OF WATER TURNED UPSIDE DOWN OR
INVERTED
Streams composed of a heavy fluid in motion within a lighter
fluid are seen everywhere. Do not all rivers represent the dis-
placement of a light fluid—the air—by a heavy fluid—the water?
In this case, it is very well known that the stream is confined
on the bottom and the sides.
If the flame and the hot gases within the furnace were fluids
heavier than the air, it would be found that they flowed in the
same manner as the stream of water. But as they are much lighter