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 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 hot-water heating.