In laying down water-pipes, in the streets and to houses, it is very important to make the distance between them and all drains and gas-pipes as great as possible. Suction of gases or liquids may occur into leaky pipes, even though these contain water, and still more when they are empty; and disease has occasionally been traced to this source. Thus if sewers and water-mains are laid in the same trench, foul matters which have escaped into the soil from the former may be sucked into the latter. This may happen if the water-mains are leaky, even when they are running full. Experiments have shewn that the flow of water causes a partial vacuum and insuction at the defective points. During intermissions of supply when the mains are partially or entirely empty, the danger of leakage into them is still greater. Coal-gas has been similarly sucked into water-mains. The pipes bringing the water to a house may be kept constantly filled with water, or only for a limited time once or twice a day. The intermittent system of supply necessitates the provision of cisterns or water-tanks, in which water can be stored in the intervals of flow of water. With a sufficient and properly-distributed public supply of water, no cistern ought to be required. =Cisterns.=—Cisterns for the supply of potable water may be made of iron, slate, stone, glass, glazed earthenware, or brick lined with Portland cement. Other materials have been used, as timber, lead, and zinc. _Timber_ is inadvisable, as it easily rots; _lead_ is very objectionable, owing to the possible solvent action of the water on it. _Zinc_ or galvanized iron cisterns are also acted on by soft water; but they may be used with most waters. Galvanized iron is iron coated with a thin layer of zinc. Iron cisterns soon rust; but this may be prevented by giving them a coating of boiled linseed oil before they leave the foundry. _Stone cisterns_ are too heavy for use, except in basements. _Slate cisterns_ are good, but are apt to leak; the points of leakage have occasionally been stopped with red lead, which is attacked by the water, and thus lead poisoning results. If the slate is set in good cement (not mortar, as this makes the water hard), it is a good material for a cistern. Every cistern should have a _well-fitting lid_, always kept closed, to avoid the entrance of dust of various kinds, or even dead cats, birds, etc. Noxious gases may be absorbed by the stagnant water. The cistern should be _easy of access_. If it is indoors, the cistern room should be well ventilated; and in any case the cistern should be periodically visited and cleaned out. When the cistern is full, a ball-tap prevents any further flow of water; and if this does not act properly, an overflow pipe carries off the excess of water.