General Science | Page 6

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.
The principle of hot-water heating is shown by the following simple
experiment. Two flasks and two tubes are arranged as in Figure 15, the
upper flask containing a colored liquid and the lower flask clear water.
If heat is applied to B, one can see at the end of a few seconds the
downward circulation of the colored liquid and the upward circulation
of the clear water. If we represent a boiler by B, a radiator by the coiled
tube, and a safety tank by C, we shall have a very fair illustration of the
principle of a hot-water heating system. The hot water in the radiators
cools and, in cooling, gives up its heat to the rooms and thus warms
them.
[Illustration: FIG. 14.--Hot-water heating.]
In hot-water heating systems, fresh air is not brought to the rooms, for
the radiators are closed pipes containing hot water. It is largely for this
reason that thoughtful people are careful to raise windows at intervals.
Some systems of hot-water heating secure ventilation by confining the
radiators to the basement, to which cold air from outside is constantly
admitted in such a way that it circulates over the radiators and becomes
strongly heated. This warm fresh air then passes through ordinary flues
to the rooms above.
[Illustration: FIG. 15.--The principle of hot-water heating.]
In Figure 16, a radiator is shown in a boxlike structure in the cellar.
Fresh air from outside enters a flue at the right, passes the radiator,
where it is warmed, and then makes its way to the room through a flue
at the left. The warm air which thus enters the room is thoroughly fresh.

The actual labor involved in furnace heating and in hot-water heating is
practically the same, since coal must be fed to the fire, and ashes must
be removed; but the hot-water system has the advantage of economy
and cleanliness.
[Illustration: FIG. 16.--Fresh air from outside circulates over the
radiators and then rises into the rooms to be heated.]
11. Fresh Air. Fresh air is essential to normal healthy living, and 2000
cubic feet of air per hour is desirable for each individual. If a gentle
breeze is blowing, a barely perceptible opening of a window will give
the needed amount, even if there are no additional drafts of fresh air
into the room through cracks. Most houses are so loosely constructed
that fresh air enters imperceptibly in many ways, and whether we will
or no, we receive some fresh air. The supply is, however, never
sufficient in itself and should not be depended upon alone. At night, or
at any other time when gas lights are required, the need for ventilation
increases, because every gas light in a room uses up the same amount
of air as four people.
[Illustration: FIG. 17.--The air which goes to the schoolrooms is
warmed by passage over the radiators.]
In the preceding Section, we learned that many houses heated by hot
water are supplied with fresh-air pipes which admit fresh air into
separate rooms or into suites of rooms. In some cases the amount which
enters is so great that the air in a room is changed three or four times an
hour. The constant inflow of cold air and exit of warm air necessitates
larger radiators and more hot water and hence more coal to heat the
larger quantity of water, but the additional expense is more than
compensated by the gain in health.
12. Winds and Currents. The gentlest summer breezes and the fiercest
blasts of winter are produced by the unequal heating of air. We have
seen that the air nearest to a stove or hot object becomes hotter than the
adjacent air, that it tends to expand and is replaced and pushed upward
and outward by colder, heavier air falling downward. We have learned
also that the moving liquid or gas carries with it heat which it gradually

gives out to surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat
with it, the process is called convection.
Convection is responsible for winds and ocean currents,