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 vertical the well is usually under the curbing or 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 cast iron. A stonetopped inlet is shown in Plate X, Fig. 5, an iron-topped one in Fig. 6.

Traps are frequently placed in catch-basins or the connecting-pipes to prevent the exit of sewer-air, unwisely the author thinks (see Art. 41). 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 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 if they were not provided. Their advantage over inlet catch-basins is that the

odors reach the outer air further from pedestrians, and that the difficulty and cost of cleaning is not so great. They should be used in sewers which carry house-sewage in exceptional cases only.

Inlet catch basins are generally preferable on lines of combined sewers where much heavy dirt reaches the inlet, or on storm-sewers where such dirt is washed in in very large quantities.

ART. 48.

INTERCEPTORS AND OVERflows.

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 tributary sewers which originally discharged into a near body of water, the interceptor shown in Plate XI, Fig. 1, may be used. This "leaping weir," it is believed, was first used by Baldwin Latham about 1876. 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 be deposited there.

An interceptor which meets this objection, but which may more properly be called a divertor, is shown in Plate XI, Fig. 2.* The flap-valve shown is closed by the rising of the float, which occurs when the amount of sewage becomes greater than it is desired that the house-sewer carry. The joints of the mechanism should be of bronze. A sewer does not offer the best conditions for the continued proper working

* See Engineering Record vol. xxxII, p. 41.

of any mechanism therein, but one so simple as this should give little trouble in its maintenance.

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 reliefsewers placed at intervals along its length and leading to near-by outlets.

An outlet 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, which is usually that which will carry sufficient storm-water to afford such dilution to the house-sewage as will render it unobjectionable to discharge this into an adjacent stream. The diversion into such a relief-sewer or relief outlet is ordinarily made by means of an overflow, constructed as shown in Plate XI, Fig. 3, or as in Fig. 4, where the relief-sewer was constructed after the smaller sewers had long been in use.

ART. 49. INVERTED SIPHONS; SUB-DRAINS; FOUNDATIONS.

Inverted siphons are usually circular in section, since always flowing full; usually of metal, since always under pressure, although the metal may be lined with brick or other material. The size required has already been referred to. When laid under water they should be so weighted or covered with earth or stone as to prevent their floating when pumped empty for inspection or cleaning, and should be absolutely tight. The inverted siphon is made sometimes to slope from both ends to a point near mid-length, sometimes with a vertical drop at one end, sometimes at both ends. The first

siphon is sufficiently large When not of such a size it

This will usually require

should be adopted only when the to permit the entrance of a man. should be straight from end to end. a shaft at one, sometimes at each, end, which may also serve as a manhole. It is in most cases advisable to place a catchbasin at the foot of such a shaft, although in place of this a basin in the bottom of an enlargement of the sewer just above the siphon is sometimes employed. A siphon with catchbasins is shown in Plate XI, Fig. 5, the valves on the ends of each siphon-pipe permitting either siphon to be closed to sewage and pumped out for inspection, while the other is in

use.

Unless a siphon under water is of large size and in tunnel or laid in a trench in a rocky bottom it should be protected from undermining by currents, or movement by shifting bottoms or channels. This protection is usually afforded by driving a row of sheet-piling on each side of the pipe, the space between these being in most cases excavated and filled with concrete. The softer the material in the bottom and the stronger the currents the deeper the sheeting should be driven. If the bottom is too hard to permit of driving sheeting, large stone rip-rap may be placed on both sides and over the siphon.

A sewer must sometimes pass either under or over an obstruction-such as a water-main, another sewer, etc.-by a siphon, either inverted or erect. The latter requires greater care in construction and constant attention to maintain a vacuum at the summit, and the former is in the majority of cases the preferable construction. Such a siphon is usually a few feet in length only and under but little head. A manhole should be placed over or near it when the sewer is 24 inches or more in diameter, since it will probably need more frequent cleaning than the other parts of the line. If the sewer is less than 24 inches diameter a manhole should be