he furnace must be gotten from outside, and the source must be pure,
above the ground level, and free from contamination of any kind.[16]
(5) The cold-air box and ducts must be clean, protected against the
entrance of vermin, etc., and easily cleaned. (6) The air should not
be overheated. (7) The hot-air flues or tubes must be short, direct,
circular, and covered with asbestos or some other non-conducting
material.

[Illustration: FIG. 14.

A HOT-AIR FURNACE.

The cold air from outside comes to the COLD-AIR INTAKE through the
cold-air duct, enters the furnace from beneath, and is heated by
passing around the FIRE POT and the annular combustion chamber above.
It then goes through pipes to the various registers throughout the
house. The coal is burnt in the fire pot, the gases are consumed in
the combustion chamber above, while the heat eventually passes into
the SMOKE FLUE. The WATER PAN supplies moisture to the air.]

=Hot-water System.=--The principles of hot-water heating are very
simple. Given a circuit of pipes filled with water, on heating the
lower part of the circuit the water, becoming warmer, will rise,
circulate, and heat the pipes in which it is contained, thus warming
the air in contact with the pipes. The lower part of the circuit of
pipe begins in the furnace or heater, and the other parts of the
circuit are conducted through the various rooms and halls throughout
the house to the uppermost story. The pipes need not be straight all
through; hence, to secure a larger area for heating, they are
convoluted within the furnace, and also in the rooms, where the
convoluted pipes are called _radiators_. The water may be warmed by
the low- or high-pressure system; in the latter, pipes of small
diameter may be employed; while in the former, pipes of a large
diameter will be required. The character, etc., of the boilers,
furnace, pipes, etc., cannot be gone into here.

=Steam-heating System.=--The principle of steam heating does not
differ from that of the hot-water system. Here the pressure is greater
and steam is employed instead of water. The steam gives a greater
degree of heat, but the pipes must be stronger and able to withstand
the pressure. There are also combinations of steam and hot-water
heating. For large houses either steam or hot-water heating is the
best means of warming, and, if properly constructed and cared for,
quite healthy.[17]

FOOTNOTES:

[16] Great care should be taken that the air