of the curve (Art. 32). It may exceed this by a small amount-say 10%-to allow for incorrect apportioning of densities. If it does not at least equal it changes in the extent of the different areas should be made sufficient to give this total and at such points as the engineer's best judgment dictates.

The second subdivision is that into drainage districts. For this purpose a carefully prepared contour-map of the city or area to be sewered is necessary. Each district is to contain all the territory draining into one main sewer, together with that main down to its outlet or junction with the intercepting or outlet sewer. Under some plans of sewer assessments this subdivision is necessary for other than engineering purposes. For house-sewers it can usually be best made after the designing of the sewers is completed. For stormsewers, however, it should be made after the lines are located, but before the sizes are determined upon, to facilitate calculation of the latter.

ART. 34. LOCATING THE SEWER-LINES.

Unless this location is already occupied by gas- or waterpipes or a street-railroad, house and combined sewers are in most cases located in the centres of streets or alleys, the cost to the householders on each side for house-connections being thus made equal. In some cities the sewers are located under the sidewalks, there being a line on each side of the street. This plan, which is used at Washington, D. C., quite extensively, is usually adopted in the case of wide streets, since there the cost of the extra line is less than that of the additional lengths of house-connections required by a single sewer. From a financial standpoint the double line is cheaper when the cost of a minimum-sized sewer (6- or 8-inch) of a length equal to the average house-lot frontage is less than the cost

of a house-connection of a length equal to the distance between the two sewer-lines. Another advantage of side sewers is that the street-paving need not be torn up in making house-connections. A serious disadvantage is that the distance from the upper end of each line to the point where the sewage flow is self-cleansing in volume and velocity will be double that when but a single line is laid. Also the roots of shade-trees are apt to cause serious trouble by entering the pipe-joints. Probably the best method of avoiding both these last objections and that of the continual tearing up of the street-pavement is to lay the sewer in the street centre and at the same time carry each house-connection to the curb.

Where a city has alleys intermediate between the streets it may sometimes be advisable to carry the sewers through these rather than through the streets, the principal argument for this being that less valuable paving is destroyed and less obstruction caused to traffic by the work of construction. On the other hand the house-connection will be longer, and both the cost increased and the grade in such connection decreased, if the distance from the house to the street centre is less than that to the alley centre, as is generally the case. Moreover, the paving in an alley should be equally as good as that in a street, and the unevenness consequent on sewer construction is exceedingly apt to contribute to the diseasebreeding slovenliness in what is often at its best an elongated Gehenna. Again, in a narrow alley the space available for piling the excavated dirt is so contracted that the cost of construction is frequently increased by a very appreciable amount on this account. On a side hill, however, it may often be advisable or even necessary to locate sewers in the alleys for the drainage of houses on the lower sides of streets above.

Sewers should be laid in continuous straight lines, as far as possible.

No turn greater than a right angle should be made at any one point by any sewer less than 24 inches in diameter, and any turn whatever made by such a sewer should be in a manhole, by means of a curved channel. For sewers larger than 12 or 15 inches it is advisable to use two manholes in making

than 24 inches in diameter may be laid on curves, since they can be entered for inspection or cleaning.

Each lateral sewer should take the most direct course to its main, each main the most direct course to its outlet, and the number of mains should be as few as possible. This serves both economy and sanitary efficiency.

The dead ends should be made as few as possible, even at some expense of additional excavation, but not by reducing mean velocities below 2.5 feet per second; nor is it ordinarily serviceable to unite the upper ends of sewers flowing in opposite directions.

House-sewers should be carried within reach, as regards both horizontal distance and grade, of every lot in the sewered district.

Storm-sewers should have as few branches as can be

made to reach all the street-inlets, to better insure which such inlets should be located previous to the location of the sewerlines.

It is generally advisable to avoid crossing private property where possible, since legal complications and delays might result from such crossing. This will frequently be impossible, however, particularly near outlets.

The sewer-lines can usually be laid out directly upon a contoured working map, an approximate rough estimate of the necessary size and consequent minimum slope of each sewer being made, that deep or shallow cutting may be avoided. The direction of flow should be indicated by

arrows.

In the separate system the storm-water sewers should usually be placed on one side of the street centres, the housesewers being placed in the centres. The two should never be placed one above the other in the same trench unless in contact with each other or connected by masonry.

ART. 35. VOLUME OF HOUSE-SEWAGE.

Since the grade of a sewer is limited by its size, and the size is determined by the grade and consequent velocity, but to even a greater extent by the maximum volume of sewage to be carried, this last must be determined before either the limiting grade or size can be decided upon. If the maximum rate of water-consumption be taken at 175 gallons per day per capita the maximum volume per second to be carried by 175DA a sewer (in cubic feet) is in which D density 7.48 X 86400'

of population and A = the area in acres.

Beginning at the summit of each lateral, it is clear that it is unnecessary to calculate the capacity required for any section of sewer until the point is reached where the volume of

sewage to be carried exceeds the capacity of the smallest sewer used at the given grade. For an 8-inch pipe flowing half full with an average velocity of 2.5 feet per second this

volume is about (

(3.1416 × 16 × 2.5 – )0.4363 cubic feet

2 X 144

=

per second, which would be contributed as a maximum flow.

X X

by a population of (0.4363 × 7.48 × 86400 =)1611,

175

about 40 acres having a density of population of 40.

or

At the point where the sewage from the tributary population exceeds the capacity of the sewer its size must be enlarged to the next market size of pipe or the next size of brick sewer convenient for construction.

The allowance for leakage into the sewer of ground-water, which should be a small proportion of the sewage proper, may be added at intervals, according to the engineer's judgment, based on such data as he is able to obtain.

In calculating these volumes it is advisable to begin with the furthermost lateral sewer first; where this joins another the contributions of both are to be added to determine the flow below that point, and in tracing down this line as each branch is encountered its contribution must be calculated and added. Decision having been made, after a study of the topographical map, as to the line of sewer into which each section of undeveloped territory will drain when sewered, the sewage which this area will ultimately contribute should be placed at the heads of the volumes of flow in this line.

An excellent method of making these calculations is shown on page 122. The sewerage-map Plate No. III was

used for this table.

In this case it is seen that the capacity of an 8-inch pipe at the minimum grade was reached at the junction of the Newcastle and Budd Street sewers, but the line down Budd has 50 as its grade, and no increase of size is yet necessary.