drain out the basin by means of a pipe at the bottom connected with the sewer. The gratings of catch-basins should be removable or the basins should be provided with manholeopenings, and the wells be sufficiently large to be entered for the inspection and cleaning of the connection-pipes.

When the inlet-opening is in the curb, the well with its catch basin (if one is provided) is generally placed under the sidewalk, and access to it is through a manhole-opening in the sidewalk. There is a great variety of inlet-tops for such construction, both cast iron and stone being used. The latter, where not too expensive, is usually preferable, being neater, more durable, and usually more like the contiguous sidewalk material than is cast iron. In some cities reinforced concrete is used instead of stone.

Traps are placed in many catch-basins or the connectingpipes to prevent the exit of sewer air through the inlet; unwisely, the author thinks (see Art. 19). The outside trap is usually a running or P pipe trap. Many varieties of inside trap have been designed, both fixed and movable. The former should not prevent access to the connection-pipe and hence should be at least 15 inches from its opening. Traps with movable parts should be as simple as possible in construction and any trap should compel the outflowing water to make the least possible number of angular changes of direction.

Instead of placing a catch-basin at each inlet, it is sometimes preferable to place silt-basins along the line of the sewer at intervals of 1000 feet or more, with a manhole over each for ventilation and cleaning. These are particularly applicable to flat grades of storm sewers in the separate system. They consist of an enlargement of the sewer, and a depression of a foot or more in its invert, into which the heavier silt is washed and from which it can be removed more easily than when deposited along a stretch of sewer. These, however, should not be used to encourage deposit, but only when deposits would occur along the sewer beyond them if they were not provided. Their advantage over inlet catch-basins is that one takes the place of several such basins, and the difficulty and cost of cleaning is not so great.

Only in exceptional cases should they be used in sewers which carry house sewage. Inlet catch basins are preferable on lines of combined sewers, or on storm sewers where heavy dirt is washed in in very large quantities.

ART 43. OTHER STRUCTURAL DETAILS

Interceptors. The best form of interceptor to be employed is determined largely by the character of the system at the point of interception. If the house sewage is to be intercepted from sewers that originally discharged into a near body of water, the "leaping weir" interceptor, Fig. 52, may be used. The exact

length of opening required in the invert can be only approximately determined. It may be made smaller than is thought necessary and cut to the right size, which is ascertained by trial, after the sewer is in use. It will also probably be desirable to increase the length from time to time as the amount of house sewage increases. The principal objection to this form of interceptor is that, although the storm-water may leap the opening, much of the sand and other heavy matter carried along the invert of the combined sewer will fall into the small intercepting sewer and may be deposited there.

An interceptor which meets this objection, which is called

a regulator, is shown in Fig. 53. The valve shown is closed by the rising of the float, which occurs when the amount of sewage reaching the intercepting sewer becomes greater than it is desired that it should carry. The joints of the mechanism should be of bronze. A sewer does not offer the best conditions for the continued proper working of any mechanism therein, but one so simple as this should give little trouble in its maintenance. Several other designs of automatic mechanism for accomplishing this purpose have been employed.

Relief Sewers. When a sewer, because of improper designing or of changed conditions, becomes too small to carry all the sewage coming to it, the excess above its capacity may be diverted to and carried by a relief-sewer or -sewers. A relief-sewer may cross under and receive the excess from several gorged sewers, or a single sewer may overflow into several relief-sewers placed at intervals along its length and leading to nearby outlets.

An outfall sewer main to combined sewers is sometimes provided with overflow outlets at several points, to avoid increasing the size of the main beyond the smallest necessary dimension, or to limit the amount reaching a treatment plant to from two to

five times the dry-weather flow. The diversion into such a relief-sewer or relief-outlet is ordinarily made by means of an overflow, as in Fig. 54, where the relief-sewer was constructed after the smaller sewers had long been in use.

Inverted siphons are usually circular in section, since always flowing full; usually of metal, since always under pressure. The size required has already been referred to. When laid under water, they should be so weighted or covered with concrete or