A well-equipped house is nowadays expected to contain efficient apparatus for supplying plenty of hot water at all hours of the day. There is little romance about the kitchen boiler and the pipes which the plumber and his satellites have sometimes to inspect and put right, but the methods of securing a proper circulation of hot water through the house are sufficiently important and interesting to be noticed in these pages. In houses of moderate size the kitchen range does the heating. The two systems of storing and distributing the heated water most commonly used are--(1) The _tank_ system; (2) the _cylinder_ system. THE TANK SYSTEM is shown diagrammatically in Fig. 192. The boiler is situated at the back of the range, and when a "damper" is drawn the fire and hot gases pass under it to a flue leading to the chimney. The almost boiling water rises to the top of the boiler and thence finds its way up the _flow pipe_ into the hot-water tank A, displacing the somewhat colder water there, which descends through the _return pipe_ to the bottom of the boiler. Water is drawn off from the flow pipe. This pipe projects some distance through the bottom of A, so that the hottest portion of the contents may be drawn off first. A tank situated in the roof, and fed from the main by a ball-cock valve, communicates with A through the siphon pipe S. The bend in this pipe prevents the ascent of hot water, which cannot sink through water colder than itself. From the top of A an _expansion pipe_ is led up and turned over the cold-water tank to discharge any steam which may be generated in the boiler. A hot-water radiator for warming the house may be connected to the flow and return pipes as shown. Since it opens a "short circuit" for the circulation, the water in the tank above will not be so well heated while it is in action. If cocks are fitted to the radiator pipes, the amount of heat thus deflected can be governed. [Illustration: FIG. 192.--The "tank" system of hot-water supply.] A disadvantage of the tank system is that the tank, if placed high enough to supply all flows, is sometimes so far from the boiler that the water loses much of its heat in the course of circulation. Also, if for any reason the cold water fails, tank A may be entirely emptied, circulation cease, and the water in the boiler and pipes boil away rapidly. THE CYLINDER SYSTEM