ded with a definite inlet for air and a definite outlet for
smoke, and able to radiate into the room most of the heat produced
from the fire which burns within. The inlet, or draft, admits enough
air to cause the fire to burn brightly or slowly as the case may be.
If we wish a hot fire, the draft is opened wide and enough air enters
to produce a strong glow. If we wish a low fire, the inlet is only
partially opened, and just enough air enters to keep the fuel
smoldering.

When the fire is started, the damper should be opened wide in order to
allow the escape of smoke; but after the fire is well started there is
less smoke, and the damper may be partly closed. If the damper is kept
open, coal is rapidly consumed, and the additional heat passes out
through the chimney, and is lost to use.

9. Furnaces. _Hot Air_. The labor involved in the care of numerous
stoves is considerable, and hence the advent of a central heating
stove, or furnace, was a great saving in strength and fuel. A furnace
is a stove arranged as in Figure 13. The stove _S_, like all other
stoves, has an inlet for air and an outlet _C_ for smoke; but in
addition, it has built around it a chamber in which air circulates and
is warmed. The air warmed by the stove is forced upward by cold air
which enters from outside. For example, cold air constantly entering
at _E_ drives the air heated by _S_ through pipes and ducts to the
rooms to be heated.

The metal pipes which convey the heated air from the furnace to the
ducts are sometimes covered with felt, asbestos, or other
non-conducting material in order that heat may not be lost during
transmission. The ducts which receive the heated air from the pipes
are built in the non-conducting walls of the house, and hence lose
practically no heat. The air which reaches halls and rooms is
therefore warm, in spite of its long journey from the cellar.

[Illustration: FIG. 13.--A furnace. Pipes conduct hot air to the
rooms.]

Not only houses are warmed by a central heating stove, but whole
communities sometimes depend upon a central heating plant. In the
latter case, pipes closely wrapped with a non-conducting material
carry steam long distances underground to heat remote buildings.
Overbrook and Radnor, Pa., are towns in which such a system is used.

10. Hot-water Heating. The heated air which rises from furnaces is
seldom hot enough to warm large buildings well; hence furnace heating
is being largely supplanted by