The house-drain, or the pipe which carries the wastes from the house to the point of final disposal, is generally made of vitrified tile, and in ordinary practice is five inches inside diameter. The lower end of this drain discharges into a cesspool, or settling tank, or into a stream, as local conditions permit. This house-drain should be carefully laid in a straight line, both horizontally and vertically, for two reasons. In the first place, the velocity of flow in a straight pipe will be greater, and therefore the danger of stoppage will be decreased, and in the next place, if a stoppage does occur in the pipe, it can be cleaned out better if the pipe is straight than if it is laid with numerous bends. Such a pipe should have a grade of at least one quarter inch to a foot, and this is conveniently given by tacking a little piece of wood one half inch thick on one end of a two-foot carpenter's level and then setting the pipe so that with this piece of wood resting on the pipe at one end and the end of the level itself on the pipe at its other end, the bubble will be in the middle. Figure 60 shows the carpenter's level in position on a level board, which rests on the hubs of three pipes. The joints of this pipe should be made with Portland cement mixed with an equal part of sand, and the space at the joint completely filled. When nearing the house, it is very desirable that a manhole should be built so that if a stoppage occurs, it may be cleaned out without taking up the pipe. In city houses a running trap is always inserted just outside the house with a fresh-air inlet on the house side of the trap, as shown in Fig. 61. But for a single house this is not necessary, and it is wiser to omit the running trap. The soil-pipe begins at the trap or at the cellar wall and runs up through the roof of the house, so that any gas in the drain or soil-pipe may escape at such a height as not to be objectionable. Through the cellar wall and up through the house the soil-pipe should be of cast-iron, which comes in six-foot lengths for this special purpose. Y's are provided by which the fixtures are connected to the soil-pipe, and the top of the pipe is covered with a zinc netting to keep out leaves and birds. This soil-pipe weighs about ten pounds per foot and is almost always four inches inside diameter. The length necessary is easily computed, since it runs from the outside cellar wall to the point where the vertical line of pipe rises and from that point in the cellar extends to the roof. Such a pipe may be estimated at two cents a pound with something additional for the Y's. [Illustration: FIG. 61.--Water-supply installation.]