(2) _The cylinder system_ is very effective. In this system a metallic cylinder, capable of withstanding a pressure of 20 lbs. to the square inch, is placed in the kitchen or bathroom between the cold and hot supplies, its contained water being heated by circulation from the boiler, hot water ascending and cold descending. On the top floor of a house is a cistern from which cold water is supplied. Both the supply pipe and escape pipe for hot water may become frozen during frost. Then the supply of water is stopped, and the boiler and reservoir may boil dry. This would not occur without some indication in unusually vigorous boiling. Boilers sometimes explode, and cylinders sometimes explode. This can be effectually prevented by (3) _A Double-cylinder apparatus_ one within another. In this the water in the outer cylinder supplied from the main cistern can only be heated to 212° F., and the water in the boiler and inner cylinder supplied from a lower feed cistern can only be heated to 214° F., on account of the small head of water. Two escape pipes give free communication with the atmosphere. (4) In the _tank system_, which being cheap, is usually adopted in poor houses, the tank is placed high up in the system. The hot water branch pipes are usually taken from the flow-pipe between the boiler and tank. Hence when the supply fails, as during frost, the tank is drained empty, the circulation of water ceases, and the system is changed from a circulation system to a high-pressure one. Safety valves cannot always be relied on to prevent explosions. If they lead to the lighting of fires in frosty weather, when pipes are frozen, they may cause explosions. Explosions from frost only occur when both pipes are blocked. Incrustation of the boiler and pipes increases the danger of explosions; hence the necessity for their periodical cleaning. [Illustration: Cylinder and Tank systems] CHAPTER XXV. HOUSE DRAINAGE. =The Removal of Impurities.=—In order that health may be maintained in any inhabited house, it is essential that the impurities produced by animal life should be removed. These impurities may be divided into two classes—the first including the gaseous and volatile products evolved from the lungs and skin; and the second, the liquid excretion from the kidneys, and the solid from the bowels. The former are got rid of by efficient ventilation and by cleanliness; the latter, ought to be as quickly removed, but require more elaborate arrangements to ensure this. The =average daily amount= of solid excreta is about 4 ounces, and of fluid excreta about 50 ounces for each adult male. Taking all ages and both sexes into consideration, the amount per head is about 2-3∕4 ounces of fæces and 32 ounces of urine. When dried, the daily fæces amount to 1·04 oz., the daily urine to 1·74 oz., so that the manurial value as well as the possible polluting power of urine is much greater than that of the fæces.