having a continuous grade; either one should discharge from both directions into the other or they should cross, the one above the other.

At junctions the surface of the sewage in the contributing sewer should never be designed to be lower than that in the other; that is, if they are both branch sewers the centre of the tributary should not be below the centre of the intercepting sewer; if the larger is a main the centre of the smaller should not be lower than a point two thirds the diameter of the larger above its invert. It would be still better to place the invert of the tributary above the sewage-surface in the interceptor, particularly when the former drains but a small district; but where the total fall possible is slight none of it need be utilized for this purpose.

Difficulty will sometimes be found in so arranging the comparative depths of storm- and house-sewers that the house-connections can pass under or over the former. In some cases this may be impossible, and it may be necessary to place a house-sewer on each side of the storm-sewer.

Reference to the data of locations and depths of gas- and water-pipes and other existing sub-surface systems should be constantly made and the sewers so designed as to interfere with them as little as possible.

On the profile of each sewer-line the elevation of all transverse sewers should be indicated and a cross-section of the sewer shown. On the finished profile it is well to indicate the thickness and material of the sewer-walls and of all manholes, lamp-holes, and other appurtenances. The materials may be indicated by colors, as red for brick, brown for sewerpipe, etc. The grade, length, and size of the sewer between each two manholes should be given in figures, as well as the exact elevation of the invert at each change of grade.

ART. 38. INVERTED SIPHONS.

Since the ordinary sewer is designed to flow only to full, while an inverted siphon, being under a head, will flow full bore, the velocity in the latter will be only to that in the sewer laid to the hydraulic gradient, if they are of the same size. On account of the difficulty of access and repairs it is especially necessary that the velocity of flow in the siphon should be at least as great as that in the ordinary sewer, that deposits may be prevented. This can be attained only by reducing the size of the siphon-pipe. Moreover, this velocity should be had from the beginning of the use of the system; and therefore this size should be designed to give sufficient velocity to the sewage from the first. This first sewage flow may be doubled or trebled as time passes, and the increase may then be provided for either by giving sufficient fall to the siphon originally to produce the greater velocity necessary or by additional siphon-pipes. Usually at least two siphonpipes are laid at the first, that while one is being emptied and cleaned the other may be used. The friction-head in the inverted siphon will be greater than if the sewer were laid to the hydraulic gradient, and consequently the gradient must be steeper. The difference in elevation of the two ends of the siphon should be equal to the fall required by a sewer of the same size flowing full and of the length of the entire siphon (which is not the horizontal distance between its ends) to pass the given amount of sewage.

The velocity of flow in an inverted siphon is entirely independent of the fall therein, but depends upon the quantity of sewage, since all of this must, but no more can, pass through it. If the fall in the inverted siphon is not sufficient the sewage will back up the sewer until sufficient head is obtained to produce the required velocity. Hence to prevent

this the fall in the siphon itself should be made great enough to create the velocity which will be required by the largest quantity to be passed at any time.

An inverted siphon may at times be necessary for passing under some obstruction in the street-as a large conduit of one kind or another, but this should be avoided where possible.

For details of inverted-siphon construction see Articles 49 and 77.

ART. 39. 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 the house-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 the health of residents thereon, if this ground-water could be lowered along the trench 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 (see Arts. 75 and 76).

sewers.

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 These are ordinarily laid in a narrow trench in the bottom of, and at one side or in the centre of, the sewertrench. Their use for construction drainage will be considered in Part II. When properly designed for this purpose 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 subdrainage 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 sub-drains at the same point. It may happen, however, that the necessity for sub-drains is not foreseen when the seweroutlet is being built; or the place where they will be neces sary 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 drain-pipe 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 one of the methods of pumping (see Art. 42).

If the sub-drain is necessary for construction purposes only it may be led to a sump-well where a pump is stationed, and 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 house-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 or 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. 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 near water

course.

Pipe

ART. 40. HOUSE- AND INLET-CONNECTIONS.

The connections between the sewers and opposite houses and storm-water inlets are of an importance second only to the sewer-mains. Any defect in one of the connections, while