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. 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.]