broad cross-hatching represents brickwork]

For the ordinary room, therefore, a fair average size for the fireplace
opening is three feet in width by two and a half feet high, with a depth
half the width. From such a fireplace it is possible to get a maximum of
heat with a minimum of draft.

There are two vital principles that should be observed in the design of
any fireplace. One of these is the relation between the size of the
opening into the room and the size of the flue itself. A cross-section
of the flue--which incidentally should be kept the same throughout its
extent--should be one-tenth of the area of the opening into the room.
The second vital consideration is the introduction of what are known as
a "smoke shelf" and a "smoke chamber." The reason for constructing a
fireplace with these two features will appear more readily by reference
to the diagram. This is drawn to show that when a fire is kindled on the
hearth the warm air current, which is generated immediately, begins to
rise through the throat (the opening between the fire chamber and the
smoke chamber) and at once induces a down-draft of cold air. If the back
of the fireplace were on the same continuous plane with the rear side of
the chimney flue, this downward current of cold air would strike
directly upon the fire itself and force smoke out into the room. The
smoke shelf is built just where it will prevent this action. The
sectional diagram does not perhaps make quite clear the shape of this
smoke chamber, but the accompanying perspective outline sketch will
indicate the fact that the throat and the smoke chamber at the bottom
must extend across the full width of the fire chamber. This width in
the smoke chamber immediately diminishes in rising until it joins the
flue at the flue's own area.

The sectional diagram indicates a cast-iron damper built in the throat.
This is not necessary, for it contributes nothing to the efficiency of
the fire itself. Its one great advantage is that by furnishing the mason
with an unalterable form, it forces him to build the throat properly
rather than in one of the wrong ways that his own judgment might
dictate. Such a cast-iron damper also forms a support for the flat arch
of brick over the opening if bricks are used. If the damper is not built
in, it is necessary to use an iron supporting bar to carry this flat
arch. Then too, in case the damper is not used, there is lost the
advantage of being able quite re