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%. Where the house sits back from the street an observation-hole (see Art. 47) should be placed at the fenceline, 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 20 to 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° 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 brick sewers that a trickling discharge from it will flow over the brick 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 with inlets should never enter the sewer at an angle with its axis greater than 45°, 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 continued by a curved invert in the bottom of said manhole (see Plate VIII, Fig. 5).

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, 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 sewer-main, or a small stream is there received, it may be necessary to continue the sewer to the inlet, not only not diminished in size but even enlarged into a bell mouth. It would be advisable to use an

increaser at the upper end of every inlet-connection, since, owing to the churning of the water in the inlet, a "standard orifice" will not pass more than two thirds the water which can be carried by a pipe of the same size.

ART. 41. MANHOLES, INLETS, FLUSH-TANKS, ETC.

The necessity for frequent connections between the air of the sewer and the outer air has been shown (Articles 28 and 29). As one means for this, and one which can always be adopted, manholes should be adapted to serve this end by having perforated covers. For this purpose, also, the more numerous they are the better. The other and greater necessity for their use, that of providing access to the sewers, should, however, have greater weight in fixing the distances which should separate them. It has been found in practice that a 6- or 8-inch sewer can be easily inspected and cleaned if this distance be not greater than 300 feet; a 12- or 18-inch sewer, when not more than 400 feet separates successive manholes. A sewer which can be entered may, for this purpose, have its manholes even 600 or 1000 feet apart; but the cost and difficulty of cleaning are thereby increased, owing to the distance the material removed in cleaning must be carried through the sewer. Ventilation also is not so well served by so great intervals. It is better to fix 500 or 600 feet as the maximum distance between manholes on lines of the largest

sewers.

Economy would suggest placing a manhole at each sewer intersection, where it would serve both lines. This is also desirable as permitting a curved junction between the sewerchannels. Where a curved bend is made in the entire sewer a manhole should be placed at each end of the curve unless the sewer is sufficiently large to be entered.

A manhole should be placed, in general, at each change

of line or grade, in order that every part of the sewer may be easily inspected.

Economy will set a limit to the number of manholes which may be introduced; the number of the breaks in the streetpaving caused by their covers it is also desirable to keep at a minimum. Principally for the first reason a manhole is sometimes omitted in small sewers when it would come less than 200 feet distant each way from another manhole, and a lamphole substituted. While the sewer cannot be inspected from this, a light can here be lowered into it to light up the sewer for inspection from the next manhole either way. Also a hose can be inserted at a lamp-hole for cleaning the sewer.

The use of flush-tanks has already been discussed (Articles 25-27). The grades of the laterals and the conditions of their use should be carefully examined 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 automatic flush-tanks 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 employed no special tanks need be built in such a case, but manholes at intervals along the line can be used for flushing.

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. As a general rule every dead end of a house- or combined sewer should be flushed frequently and some arrangement for this placed at each such point.

Inlets should be provided at frequent intervals throughout the area drained to receive the surface-water. In districts where the street traffic is considerable and where any great

depth of water in the gutters would inconvenience a large proportion of the population the inlets should be not more than 200 or 300 feet apart, while in residence districts they may be so situated as to require the run-off to flow for 600 or 700 feet over the surface. They should generally be so placed that all the run-off can reach them by flowing along the gutters only, and need not flow across the streets. The plan Plate V shows how this can be accomplished in most cases. Where this is impossible a culvert should be placed

under the street-pavement in line with the gutter.

Where street grades are continuous from one intersecting street to another inlets should be placed on street-corners. They are frequently placed at the gutter intersection; but a better plan in many cases, particularly on steep grades, is to place two openings, one just above each cross-walk, as this avoids the vehicle-trap caused by the ordinary corner inlet. Also an inlet should be placed at every point where two falling grades meet, and if this be between street intersections an inlet must be placed there on each side of the street.

In the majority of cities a large proportion of the inlets are provided with catch-basins-more than the best practice would warrant, in the author's opinion. The object of using a catch-basin is to retain there the silt and other heavy matter and not permit it to be carried into and deposited in the sewer. Catch-basins should be cleaned after every storm.

The objection to catch-basins is that several days sometimes must elapse-and several weeks usually do-between the beginning of a storm and the cleaning of the catch-basin; and during this time the organic matter which has been washed or thrown into the inlet, including horse-droppings, fruit and vegetable refuse, etc., is putrefying and frequently emitting objectionable odors. Such foulness is less offensive in the drains [storm-sewers] than in the catch-basins, which are situated at the sidewalks and where it is much more