It will at times be necessary to use larger conduits than even an eight-inch pipe. Up to a diameter of fifteen inches, it is cheapest to use pipes, but for eighteen inches or more, brick-work is cheaper; and at that size--a considerable regular flow of water being insured--the slight roughness of brick-work offers no serious objection. The use of oval or egg-shaped sewers will rarely be necessary under the circumstances that we are considering; but there may be exceptional conditions where the covering-in of a brook, or storm-water course, cannot be avoided; and in such cases the volume of water may vary so greatly that there will at times be a mere thread of a stream, and at times a torrent. Here the oval form is the best, as concentrating a small flow within a narrow and deep channel, and still giving the capacity needed for exceptionally large volumes. All bricks used for sewers, man-holes, &c., should be of the very hardest quality, and true in form. The general rule is to be kept in mind, that the thickness of the wall of a brick sewer should not be less than one-ninth of the inner diameter; that is to say, that up to a diameter of three feet the thickness of the wall should equal the width of a brick,--four inches. This applies to circular sewers only: the oval form, being less strong, calls for a wall of a thickness equal to one-eighth of the largest diameter. Connecting drains leading from houses to the sewer are to be made at private cost; but they should be made in accordance with plans furnished by the public authority, and by a workman acceptable to that authority. The householder might be permitted to take the responsibility of the finishing of his drain, but for the fact that the working of the public sewer calls for the largest amount of water in proportion to the amount of solid matters that it is possible to secure, and thus makes it imperative that this drain should be absolutely tight, so that the liquid parts of the house outflow shall not trickle away through its joints, leaving only the more solid parts to flow into the public sewer. Properly graded and smoothly jointed, a four-inch pipe will carry more water than even the largest boarding-house or country hotel is likely to discharge. There is, however, a tendency in all house-drains to become filled in the early part of their course by the accumulation of grease and solid matters caught in the grease. Where no form of grease-trap is used, there is a certain argument in favor of the use of six-inch pipes for the upper part of house-drains. The use of a grease-trap, however, should always be insisted upon; and with its aid these obstructing matters will be retained, and the outflow may be perfectly carried by a four-inch pipe.