[Illustration: FIG. 12.--The open fireplace as an early method of
heating.]

As the air is heated by the fire it expands, and is pushed up the
chimney by the cold air which is constantly entering through loose
windows and doors. Open fireplaces are very healthful because the air
which is driven out is impure, while the air which rushes in is fresh
and brings oxygen to the human being.

But open fireplaces, while pleasant to look at, are not efficient for
either heating or cooking. The possibilities for the latter are
especially limited, and the invention of stoves was a great advance in
efficiency, economy, and comfort. A stove is a receptacle for fire,
provided 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