, at which distance
the side walls of the furnace must be erected, the back one similarly,
and the front one round the four projecting doors, which are,
respectively, the ash-pit door, the fire door, and two doors for
cleansing the horizontal smoke-box and interior of convolutions. The
furnace walls must be continued up to a few inches above the bend of
iron smoke flue, and then--if, as shown, the furnace be small--covered
with a 4-in. York slab in one piece. If the furnace be large, a flat
brick arch must form the covering, as at Fig. 8, where this arch
supports the flooring of the laconicum. The openings for the admission
of the heated air into the conduit leading into the hot rooms may be
either directly above, as shown in the last-named illustration, or in
the side, as in Fig. 6, with inclined flues. As a rule, it is more
economical, in heating on the principle now under consideration, to
place the furnace below the level of the hot rooms; but if desirable to
place both on one level, the back wall of the furnace chamber becomes
the party wall of the laconicum, and it must be stopped short of the
ceiling, and the air debouched over it.

In cheap baths the interior face of furnace chamber may be of stock
brickwork; but best glazed work should be adopted in good ones. All hot
and cold-air ducts should be similarly lined with glazed ware. In
first-class work the floors of horizontal and inclined flues should be
of white glazed tiles set in cement. Manholes must be provided for
cleaning when necessary. Every portion of furnace chamber, flues,
shafts, and conduits for hot and cold air must be "get-at-able" either
by means of manholes or by long brushes. Air-tight doors must be
indicated on the plans wherever this necessity demands them.

The iron smoke-pipe from furnace must be conducted to the smoke flue,
and the connection between furnace chamber and flue hermetically sealed.
The walls for a small furnace chamber need not be more than 4-1/2 in.
thick. Large furnaces require walls one-brick thick.

[Illustration: FIG. 7.

An Air Filter.]

The cold-air flues leading from either side of the furnace must be
conducted to their respective inlets. If possible, at least two inlets
should be provided, facing different ways: this with regard to the
possibility of certain winds drawing the air out where it is wanted to
enter. The openings should be vertical, like windows, and, in cities,
furnished with a solid frame and case