but if the lateral will flow continuously it is desirable to have the two surfaces of flow at the junction as nearly at the same level as possible.

When a sewer increases in size or changes its form of cross-section, it should do so gradually so as to minimize the loss of head. If the vertical diameter increases, the top of the sewer should continue at the same gradient and the invert drop on a slope an amount equal to the increase in diameter. If the horizontal diameter increases and the vertical decreases, leaving the area of cross-section the same, theoretically the centers of gravity of the successive crosssections might continue on a uniform slope, the invert rising; but it is desirable to continue the invert on a uniform slope instead, both to give additional fall to compensate for the loss of head, and especially because, if the sewer should flow less than half full, the rise in the invert would dam back the sewage.

Most of the above statements are based on the assumption that the normal velocity of flow in the main is theoretically not more than 3 or 4 feet per second. If greater than this, and especially if greater than is desirable, these precautions are unnecessary; in fact, obstructing the flow in these and other ways may be advantageous. But the possibility of abrasion or rupture of the sewer by too great agitation of the sewage must not be overlooked. Another objection to agitation is that by it suspended organic matter will be thrown up upon the sewer walls, where it may remain and putrefy.

ART. 25. HOUSE AND INLET CONNECTIONS

The connections between the sewers and houses and stormwater inlets are of an importance second only to the sewer mains. Any defect in one of the connections, while limited in the range of its effect, is fully as detrimental within that range to the proper working of the system as a defect in the main itself. Since the house-connections are subject to extreme fluctuations of discharge and hence to stoppages, as also to the formation of grease deposits, it is desirable that

they be equally as accessible as sewer mains for both inspection and cleaning, and also that their grade and alignment be given equal care in both the design and the construction. They should, if possible, be given a uniform grade of not less than 2 per cent. Where the house sits back from the street, an observation-hole at the property-line is desirable and one should be placed wherever there is a change in the line or grade. There should also be a hand-hole in the pipe just after it enters the cellar. The junction with the sewer should be made by means of branches, either Y or T. It should never be made in pipe sewers by breaking a hole into the shell and inserting a pipe. If the sewer be larger than 24-inch a T is advisable, both because this offers easier inspection of the house-connection from its lower end, which inspection can be made by a person entering the sewer, and because the branch can be placed entirely above the ordinary level of the sewage, which position it should occupy when possible, so as to cause no interference with the sewage flow. When the sewer is too small to admit a man (which size will also not admit of raising the branch entirely above the ordinary sewage flow without giving it too steep a pitch), a Y branch is preferable, because this will retard the flow less than a T, and because the house sewage will enter the sewer at a less angle with its flow. The vertical angle which the branch makes with the horizontal should not ordinarily exceed 45 degrees in small sewers, because of the interference with the flow and of the splashing caused by a vertical drop of sewage into their relatively small stream, and because of the danger that the weight of the house-connection may break in the crown of the sewer.

It is well to so place the branch in masonry sewers that a trickling discharge from it will flow over the surface for the least possible distance, that deposits from such discharge may be avoided. In the case of combined sewers this would call for placing the branch but a short distance above the invert, but it should be given such a grade as to bring it higher than the crown of the sewer when it reaches the cellar.

Some engineers always use T branches, more always use

Y branches, for house-connections; but the practice here recommended seems to best utilize the advantages and avoid the disadvantages of each.

The connections from inlets should never enter the sewer at an angle with its axis greater than 45 degrees, on account of the great disturbance to the flow which would be occasioned. Where possible, and particularly in small sewer mains, a manhole should be placed where each connection enters the sewer, and the connection be continued by a curved invert in the bottom of said manhole.

It is difficult to calculate the proper size for a storm-water connection; but, since there is little disadvantage in having it larger than is actually required except the cost, and these connections are so short that this is not considerable, while the effect of too small a pipe may be disastrous, it is advisable to make the size fully ample to discharge all the run-off from the heaviest storms. A 12-inch pipe is probably the smallest which should ever be used; while a 24-inch may be required if the sewer lies near the surface (thus giving little fall to the connection) and if the tributary area is large. Where considerable undeveloped territory drains into the head of a sewermain, or a small stream is received there, it may be necessary to continue the sewer to the inlet, not only not diminished in size but even enlarged into a bell mouth.

A bell-mouth construction is advisable for all inlet connections, and the upper few feet must be larger than the remainder of the connection if the latter is to run full, since the velocity of flow into the entrance will seldom be as great as that nearer the sewer. Theoretically the best construction would consist of a bell-mouth entrance and a pipe tapering nearly or all the way to the sewer. But this construction would be difficult and expensive, and is seldom, if ever, adopted. A flaring entrance, however, is practicable, and the entire length of the connection should be of as large a diameter as that required to admit the water to the connection under the pressure due to the head from connection mouth to street surface.

ART. 26. USE OF OLD SEWERS

In many cities, before any general sewerage system has been constructed 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 storm-water, 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. may be cleaned for this purpose by washing first with dilute muriatic acid, then with a solution of potash, and then with water.

It

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 Fig. 33.

If the sewers are too small to be entered, they should be examined thoroughly from the manholes; "pills" (wooden balls a few inches less in diameter than the sewer) should be floated through them to ascertain whether the bore is of uniform size and clear of deposits, or this be ascertained in some

other way. 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 can be so located as to give straight stretches of the sewer between them.

If a pipe sewer lies too high for efficienct service or at too flat a grade, a trench may be dug down to it 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 that can be made of old pipe sewers. But if it requires much additional excavation to recover the pipes it will be a waste of money to do so.

Owing to the difference in character and volume of house and storm sewage, a sewer not adapted for use as a separate or combined sewer may often be used as a storm sewer. It frequently happens that old combined sewers, or even the larger separate 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 sewer, and the old outlet used to discharge the stormwater 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.

ART. 27. PUMPING SEWAGE

WHEN PUMPING IS NECESSARY

Pumping house sewage is employed continuously where (1) A section of a city is too low to drain by gravity to any acceptable outlet. This section is then provided with a system of its own, draining to one point, at which it is pumped