to the outlet; and even if it settled in the sewer, it would be less offensive there than near the sidewalks. In the latter event the sewer would need to be cleaned to remove the deposit; but it is found impracticable to intercept all suspended matter in catch-basins, and if the sewer grade is flat the matter carried on would be deposited in it and, although less in amount, would still necessitate a cleaning of the sewer. If not cleaned before it is filled with sediment, a catch-basin ceases to act as such; also the contained matter gives off odors and the basin is much worse than useless. Moreover, catch-basins are usually cleaned with shovels only, and sufficient filth is left upon the sides and bottom to become noticeable by its odors.

For these reasons the universal use of catch-basins is, in the author's opinion, not to be advised, but rather the inlet should be so designed that all material shall at once reach the sewer. Also he would make the inlet connection without a trap, that it may assist in the ventilation of the sewer; and if the sewer and its appurtenances are properly designed, constructed, and maintained there will be very few instances where any odor can be detected at the inlet.

There may well be locations where catch-basins are desirable, as where the wash from a steep hillside is intercepted, or for other reason a large amount of coarse soil finds its way to the inlet; and there the catch-basin will need to be large, that only a small proportion of this may reach the sewer, and should be cleaned after every heavy shower. A small catch-basin is worse than useless in most locations. Also catch-basins are often desirable where the sewer grades are very flat, giving a velocity of less than 2.5 feet per second, especially if the sewers are on the combined system.

Several engineers have become so convinced of the objectionable features of catch-basins already existing in systems under their charge that they have filled in such basins, changing them to plain inlets only.

Flush-tanks. A flush-tank consists of a well or underground tank, generally of masonry, placed at the dead end of a sewer, or occasionally at other points. The grades of

laterals and the conditions of their use should be examined carefully to determine where frequent flushing will probably be needed. In some cases, such as where a flat grade on a long line of small sewer is unavoidable, it may be desirable to place flush-tanks, either automatic or for hand-flushing, at intervals of 800 to 1000 feet along its length, the tanks being placed at one side of the sewer and discharging into it through a short connecting-pipe. If automatic appliances are not to be employed, however, manholes at intervals along the line can be used for flushing by hand.

All the local conditions should be examined, that advantage may be taken of any opportunities for flushing offered by springs, streams, or any available sources of water; and, in general, decision made as to the places and methods of flushing. But in the great majority of cases the water main will prove the most satisfactory source of water.

Where a dead end is but temporary and the sewer is to be extended later, it may terminate in a manhole, so located as to be permanently serviceable, and this manhole used for flushing the sewer. Where two sewers flow in different directions from a common summit, one flush-tank may be made to serve for both, as a matter of economy in construction; but generally the two sewers can terminate a considerable distance apart, and the cost of laying a sewer across the intervening space will be greater than the cost of the second flush-tank.

ART. 21. SUB-DRAINS

Very frequently storm sewers are placed at such a short distance from the surface that they cannot be utilized for draining damp cellars, particularly since a cellar should be connected with no sewer whose crown is above its level, from danger of back-water when the storm sewer flows full. Ordinarily a separate sewer is below the cellar-level; but this should not be utilized as a drain, both because the amount of sewage may thus be too largely increased; and still more on account of the danger from sewer air, which would have free access

through the drain should the trap-seal evaporate during a drought, which it is very apt to do, and from the cellar this air might permeate the entire house.

From a sanitary point of view the drainage of wet soils is almost, if not quite, as important as the sewerage and should not be neglected. The mere opening of sewer trenches tends to drain the soil, even after they are refilled. But in many cases it is extremely desirable to provide other and more positive drainage.

It is almost impossible to make a perfectly tight sewer without great expense, and when laid in wet ground, sewerjoints may admit in the aggregate large quantities of water. This could be prevented and the land adjacent drained, to its great improvement and that of the health of residents thereon, if this ground water could be lowered along the line of the sewer by some means.

During construction in wet ground, much trouble will be experienced, even when the pumping facilities are ample, by water rising and flowing over newly laid inverts, to their permanent injury.

These difficulties can each and all be met in most cases by the use of sub-drains-that is, drains laid a little below the sewers. These are ordinarily laid in a narrow trench in the bottom of, and at one side or in the center of, the sewer-trench. When properly designed to faciliate construction, their size will in most cases be sufficient for the continuous drainage of the land and also for cellar-drainage. The instances will be very few, however, in which any approach to an accurate estimate can be made of the amount of sub-drainage which will be required in a system. But provision should always be made for sub-drainage wherever the soil is wet, for permanent drainage if for no other purpose.

The water flowing into such drains must have some outlet, and the most natural course would be, when the sewage is disposed of by dilution, to place the outlets of sewers and subdrains at the same point. It may happen, however, that the necessity for sub-drains is not foreseen when the sewer

outlet is being built; or the place where they will be necessary may be so far from this outlet that a great length of otherwise useless drain-pipe must be laid to reach it; also the amount of ground water may be so much greater than was anticipated, in spite of all investigations, that the drainpipe near the outlet will not carry it all. In any of these cases another outlet may be desirable or necessary. This can frequently be found by leading the sub-drain in a special trench to a near storm sewer or natural watercourse. In some cases, however, special means must be resorted to, such as pumping.

If the sub-drain is necessary for construction purposes only, it may, during construction, drain to a sump-well where a pump is stationed; and be broken and sealed at several points after construction is completed. (This last will be necessary, as otherwise the drain would continue to lead the ground water to this point, which might become permanently and dangerously water-soaked.)

Although the sub-drain is in most cases smaller than the sewer, it must be laid at practically the same grade. The objection to flat grades in separate sewers does not apply to these so urgently, however, since the water flowing through them, after construction is completed at least, is usually free from suspended matter likely to cause deposits. The size and position, then, are the only elements of the general design to be decided upon. The size it will not be advisable to make less than 6 inches at the outlet for long stretches, but for stretches of a few hundred feet only and through ground but moderately wet 4- or even 2-inch pipe may be used. Pipe larger than 10 or 12 inches is seldom used in any but exceptional cases. If a larger would be required (and instances can be named where the sub-drainage from a small town would more than fill a 36-inch pipe) special methods may be employed; such as dividing the sub-drainage system into small sub-systems, each having its own outlet, which may, when constructed under a storm sewer, discharge into the sewer immediately above it, or which may be at a nearby watercourse.

ART. 22. INVERTED SIPHONS

"Inverted siphon" is the name given to a stretch of sewer that lies entirely below the hydraulic gradient. It is not really a siphon, and the siphon principle is in no way involved in its operation.

A sewer should fall continuously throughout its length, but sometimes this is impracticable. For instance, if a stream

is to be crossed and the hydraulic gradient lies above its bed, this would require that the sewer be supported in the water or above it. The former is generally impracticable, for drift wood or ice might carry it down stream, or it might obstruct the stream seriously. Carrying it above a narrow stream is frequently done, but if the stream be wide or navigable it may be too costly or interfere with the water traffic. The only alternative in many cases is to lay it in the bed of the stream, below the hydraulic gradient.

Where a sewer line is intersected by a large underground structure, such as a transit subway or large storm sewer, and the bottom of this is lower than the lowest practicable hydraulic gradient for this sewer, the sewer must generally be placed