The third consideration which affects the plumbing system of the small house is whether it can draw upon a public water-supply, or whether it must secure its private supply from a well or a near-by stream or lake. A private source of supply generally means the erection of a storage tank. The best type of tank for this purpose is the pneumatic tank, which is installed in the cellar, and not in the attic, as was the old-fashioned tank. The water is pumped into this tank, and the air which is in it is trapped, so that the more water that is pumped into the tank, the more compressed becomes the air. This springlike cushion of air gives enough pressure to force the water to any fixture in the house. [Illustration:—PLUMBING SYSTEM USING ANTI-SYPHON TRAPS—] _Simplest Type of Drainage System_ On page 97 is represented the simplest type of drainage system that can be installed in the small house, but since it uses anti-siphon traps and no back-venting, it will not be possible to make use of it in all cities or towns which have plumbing rules prohibiting it. The average small house does not have room for more than one bath, a kitchen-sink, a set of laundry-tubs, and a toilet for the servant, generally placed in the cellar. For purposes of economy it is essential to place all of these fixtures on the same soil-line, the main pipe which extends vertically from the horizontal house-drain in the cellar up through the roof. If the bathroom is so located that the vertical line which serves its fixtures cannot serve the kitchen-sink or the laundry-tubs, then a special waste-line or small vertical pipe draining fixtures other than water-closets, must be carried up and through the roof, which is extravagant of material. As this waste-line will be only 2 inches in diameter, it is necessary to increase its diameter to 4 inches before projecting it from the roof, since it may become clogged in the winter with frost. But the main soil-line is 4 inches in diameter and needs no increaser on it. The main house-drain is also made 4 inches in diameter, and is generally laid under the cellar floor with a pitch of ¼ inch to the foot. At the junction of the vertical soil-line with it, and also at any other point where there is a marked change in direction, the house-drain should be equipped with clean-out holes, covered with brass screw-caps. Just where the house-drain leaves the house, a house-trap is installed (see illustration), and back of this an inlet for fresh air to permit the circulation of air in the system. The foundations should be arched over the house-drain where it passes through them, so that any settlement of the masonry will not come upon the pipe and cause it to be broken.