ing-station in order that they may be under the inspection of the engineer and that the deposits may be raised to the surface by power. If a steam-plant is used the screenings can be burned on specially prepared grates.

The Shone Ejector is a device for raising sewage which is actuated by compressed air. It is usually employed where a number of lifting-stations are needed, and the compressed air for all is supplied through iron pipes from one air-compressing station. While the prime motive power, steam, is employed indirectly, the efficiency of compressor, air-pipe, and ejector combined is greater than if a number of separate steampumps are used, with either separate boilers or a central steamplant, especially when the stations are numerous and widely scattered. For only two or three stations the economy of their use is doubtful.

At Margate, England, sewage-lifts are used, with city water under considerable pressure as a motive power.

At Aberdeen, S. Dak., two Worthington motors connected with a sewage-pump are driven directly by pressure of the water from an artesian well. The capacity is 3,500,000 gallons per day, lifted 23 feet.

The Adams Sewage-lift can be employed where the surface grade at some part of the system will admit of introducing a drop, either vertical or on a steep grade through a pipe under pressure, in the line of some sewer or sewers. The sewage in making this drop transfers its energy by the medium of compressed air through pipes to a lift-station. The more frequent application of the Adams lift, however, is in flat districts where city water is usually employed for compressing the air, the supply being controlled by a ball cock in a catch-basin at the lift-station.

From none of these lifting appliances is there any odor, under good management. They can therefore be placed at any convenient point. The small pumping-plants, the Shone

Ejector and Adams Lift, are usually placed in vaults beneath the surface, the larger plants above ground. The sewagepumping stations of London and Berlin are within the city limits, no odor whatever being perceptible near them.

ART. 43.

INTERCEPTING-SEWERS AND OVERFLOWS.

It often happens that a town lies in a valley and upon the slope on one or both of its sides, and that while the valley district is too low to sewer to the outlet by gravity the upper districts are sufficiently elevated to do so. In such a case it would be useless to carry all the sewage to a main lying in the valley and raise it all to a gravity outlet-line. Instead

a gravity-main should be run up each side of the valley at the minimum grade to receive all the sewage from higher up the hill, leaving only the sewage from below this to be pumped. Such a main is called an intercepting-sewer.

In some instances a combined sewer is provided with an outlet to the nearest watercourse, which is for storm-sewage only, it being intended that the house-sewage shall be received and conducted away by another sewer, which also is called an intercepting-sewer.

This term is also applied to a long sewer which passes down a valley and receives the sewage from several systems. or parts of systems to conduct it all to a common outlet.

It is frequently advisable, when the gravity-outlet must be below high tide, to locate an intercepting-sewer which can discharge above all tidal influence, that the effect of the sealing of the lower outlet may be felt by only a part of the system, the upper sections discharging through the free outlet of the intercepting-sewer.

It sometimes happens that a system must be extended further in a given direction than was anticipated, or that the amount of sewage contributed by a district becomes greater

than the sewers can carry. This can be remedied by running an intercepting-sewer across such gorged sewers at mid-length, intercepting the sewage from above and leaving the lower lengths to carry only their local sewage.

Where storm-water can find near outlets from many dis tricts to a stream or other body of water, at which outlets, however, the house-sewage should not be discharged, an intercepting-sewer may be run along and near the water to intercept the house-sewage and convey it to a satisfactory outlet or to a disposal grounds or works. By a construction of the sewers called an interceptor (see Art. 48) the housesewage and the run-off from light rains, which is the filthiest of storm-sewage, may be diverted to the intercepting-sewer, while the run-off from heavy storms will reach the nearer outlet. Mechanical contrivances for diverting the sewage are also used (see Art. 48).

Another method of obtaining similar results is that of putting storm-overflows in the combined sewers, a special storm-sewer taking the overflow sewage to a convenient outlet. The overflow is, in general, an opening in the sewer with its bottom elevated some distance above the sewer-invert. Until the sewage reaches the height of this overflow it remains in the combined sewer and flows to its outlet; when the quantity becomes such that the height of sewage flow is greater than this the surplus discharges through the overflow into the storm-water outlet. It is usually so arranged that this shall occur only when the dilution of house-sewage by storm-water has reached the point where the discharge of the mixture into a stream is free from all danger.

With either of these constructions the overflow or the interceptor should, if possible, be at such an elevation that it cannot be reached by floods or tides backing up the stormwater sewer.

In many cities, before any general sewerage system is con、 structed or even thought of, short conduits, both private and public, have been built, discharging at the point nearest to hand-usually a stream or lake. These are often built in the crudest manner, graded by eye, and generally larger or smaller than necessary. In other cases the sewers are well built and graded and of a size adapted to remove the stormwater, but the outlet is located where house-sewage should not be discharged, or the sewer is not sufficiently deep to permit of receiving all house-sewage, or it is a pipe sewer and is not provided with sufficient branches for house-connections. Such sewers can frequently be incorporated into the proposed system, and a saving made of the cost and the tearing up of the streets avoided. But a thorough examination of them should first be made to ascertain which ones can be so used and how.

If they are sufficiently large they should be entered and their condition learned as to size, grade, character of workmanship, etc. If the brick-work is very rough it may be desirable to clean it and plaster it with cement mortar. It may be cleaned by washing first with dilute muriatic acid, then with a solution of potash, and then with water.

No connection-pipes should be allowed to protrude within the sewer. If the junctions are not well designed they should be torn out and rebuilt. If necessary a sufficient number of manholes should be built to bring the intervals between them within the proper limits. If it is desirable to use an old circular sewer as a combined sewer the invert can be narrowed as shown in Plate VII, Fig. 7.

If the sewers are too small to be entered they should be examined thoroughly from the manholes by means of mirrors (Art. 68); pills (Art. 85) should be passed through them to

ascertain whether the bore is of uniform size and clear of deposits. Their size, grade, elevation, etc., should be learned by actual measurement. If they are not laid in straight lines, particularly those less than 12 or 15 inches in diameter, it is doubtful if they should be used, unless manholes and lampholes can be so judiciously located as to give straight stretches of sewer between them.

If a pipe sewer is too high for efficient service or at too flat a grade a trench may be sunk along its line and the pipe taken up, cleaned, and the good ones relaid at a lower level or better grade in the same trench. In the majority of cases this probably will be the best disposition which can be made of old pipe sewers.

Owing to the difference in character and volume of houseand storm-sewage a sewer not adapted for use as a house or combined sewer may often be used as a storm-sewer. It frequently happens that old combined sewers, or even the larger house-sewers, are admirably adapted to this use, and a separate system can then be built for the house-sewage.

If an old combined sewer, or storm-sewer modified into a combined sewer as explained above, can be used, except that the house-sewage should be discharged at a new and more distant outlet, this sewage can be discharged through an interceptor, or diverted by a mechanical regulator into an intercepting house-sewer, and the old outlet used to discharge the storm-water only.

But the efficiency of the system is of greater moment than small economies, or even large ones, and should not be sacrificed to them.