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