arguments in favor of their standpoint are certainly weighty, but on the other hand the cost of treatment is considerable, and many towns could not afford a sewerage system at all if a plant for treatment also were necessary. A balance of benefits and evils, of what is desirable and what is possible, must be made for each case. "A question which we should be glad to have answered is this: To what extent must a polluted liquid be diluted in order to be safely used for domestic purposes? The answer, however, none can give. We do know this: It has been shown by actual experiment that the spores of some of the lower orders of vegetable organisms are very difficult to deprive of vitality; they may be frozen or heated to the boiling temperature, or they may be kept in a dry condition for years, and then, if placed in a favorable medium, become active and produce their kind. Admitting the presence of disease-germs in a liquid, the liquid may be diluted until the chance of taking even a single germ into the system is so small that it may be disregarded; and yet if the prevailing theory be true a single germ if taken might produce disastrous results. It is easy to push the demands for purity to an absurd extent; all reasonable precautions should be taken to insure purity, but there is a point beyond which it is foolish to attempt to go. In the present state of our knowledge we should, however, err on the side of safety, and the mere fact that chemical analysis fails to detect impurity should not be accepted as a guaranty that a water is fit to drink." (Nichols, Water-supply, Chemical and Sanitary.")

Along with our knowledge of the purifying action of the minute animals and plants has grown up a more definite knowledge of the causation of typhoid fever, cholera, and the other water-borne communicable diseases; and before it can. be positively affirmed that a sewage-polluted stream is safe for drinking after a few miles' flow it must be shown so definitely as to be beyond question by those whose special studies have fitted them for intelligent judgment that the

purifying agencies have practically eliminated the germs of the water-borne communicable diseases. Until such showing is clearly made the proposition that crude sewage ought not to be turned into running streams, ponds, lakes, or other bodies of water which either are or may be the sources of water-supplies must be considered as holding good." (Rafter and Baker, "Sewage Disposal in the United States.")

The above restrictions apply equally to ice which may be used for drinking-water, since it is known that bacteria are not wholly excluded from water by freezing, and that many varieties will live in ice for months.

The discharging of sewage into tidal waters involves the principles given as applying to discharge into rivers so far as creating a nuisance is concerned, and also the practical consideration of the movements of prevailing winds and tides. "In every case the outfall of the discharging sewer should be below the level of the water at all states of the tide, and be provided with a tidal valve, to prevent the ingress of sea-water. The position of the outfall should, if possible, be so chosen that the sewage will be always carried out to sea independently of the tides, and the possibility of its return avoided; and for this purpose advantage should be taken of any current that flows off or along the shore, the sewage being discharged into it, and thus carried away from the neighborhood of the town. If there is a current setting along the shore, then the seweroutfall should be placed at that extremity of the town which will prevent the sewage being borne along the whole sea-front. The prevailing winds also must be taken into account, so that floating matters may not be blown back toward the town." (W. H. Corfield, "Treatment and Utilization of Sewage."

From experiments conducted by the Metropolitan Sewerage Commission in 1898 in Boston Harbor, it was concluded, in the case of the Moon Island outlet, where sewage is stored and discharged on the ebb-tide and in addition about the same

amount is discharged continuously, that the area covered by a reservoir-discharge in three-quarters of an hour of 22,000,000 gallons is approximately 750 acres; when but 11,000,000 gallons is discharged at once this area is not more than 250 acres. In calm weather the sewage is offensively visible over twothirds of this area, but the odors are confined to a relatively small portion. By far the greatest amount of sewage is found in the upper two or three inches of the polluted area, and this largely disappears in two or more hours after the discharge, depending chiefly upon the force of the waves. A thin film of grease sometimes covers large areas, but is not accompanied by enough sewage to be detected. Within the polluted area sewage cannot be detected at a greater depth than 5 feet at the outlet and 2 feet near the edges of the area. When 35,000,000 to 40,000,000 gallons daily is continuously discharged the dilution is such that fifteen minutes after leaving the outlet, sewage constituted but 20 per cent of the surface water; 30 minutes after, 15 per cent; 45 minutes after, 5 per cent; and 60 minutes from the outlet but 4 per cent of the surface-water was sewage. The discoloration was evident for about 1 miles, and covered about 350 acres during ebb-tide and 300 acres during flood-tide.

Shores within one-half to one mile of sewer-outlets are apt to be polluted, and these outlets should hence be at some distance from any land, when possible.

It should be remembered that the water of dilution has been considered in the above discussion to be unpolluted; and that the same water swinging back and forth with the tide past a sewer-outlet will soon become grossly polluted. The actual dilution will be closely indicated by multiplying the actual cross-sectional area of the channel by the distance separating the positions occupied by a given sub-surface float at two successive ebb-tides, as compared with the sewage discharged in the same time.

CHAPTER III.

AMOUNT OF SEWAGE.

ART. 13. SEWERAGE CONDUITS.

THE object of a system of sewers is in general to conduct all excreta and fouled waters from the places of their origin to an appointed outlet, and as rapidly and continuously as possible. No part of the sewage should be retained in any portion of the system for any considerable time, either in its liquid form or in the shape of deposits upon the bottoms or walls of the conduits or their appurtenances; for such retention may permit of putrescence of the organic matter before it reaches the place assigned for disposal, the conduits thus becoming no better than " elongated cesspools." The insuring of this result with the greatest certainty and economy is the prime requisite in the design of a sewerage system.

The largest part of the system is made up of conduits of various size, shape, grade, material, and depth below the ground-surface. The two last are practical points to be considered later (Articles 37 and 45), but the size, shape, and grade are to be determined—approximately at least-by theoretical considerations. The data used in these considerations are (1) the amount and character of the sewage to be removed, and (2) the relative surface elevations and grades along the line of the proposed sewer-conduit. The latter are obtained by the instrumental field-work, to be discussed in Chapter VI. While the grade of the sewer need not be

that of the street-surface, it cannot depart far from this without greatly increasing the difficulty and cost of construction. The two grades will therefore be approximately parallel unless very good reasons to the contrary exist.

ART. 14. AMOUNT OF HOUSE-SEWage.

The obtaining of satisfactory figures for the amount of sewage is one of the most difficult tasks entering into the designing of a system. The sewage to be considered is of two entirely different kinds, from two totally different sources: house-sewage from dwellings, stores, factories, and other buildings, and storm-water from the streets, the groundsurface, and from roofs. The former is limited in quantity largely by the number of inhabitants and industrial establishments and the water contributed to the sewers by each. The latter is limited by nature's local limit of intensity of rainfall, the area tributary to the sewer, and the proportional run-off.

Considering first the house-sewage, this is almost entirely composed of water which has first been introduced artificially into the dwellings or establishments. Excreta and solids legitimately finding their way to the sewer comprise only a very small part of the sewage-from 5 to 15 parts in 10,000. There may be besides this comparatively small amounts of leakage of ground-water, roof-water, and flushing-water reaching the sewer. It would seem, therefore, that we may make a close approximation to the amount of house-sewage by using the water-consumption of the town in question. This can usually be obtained from the pumping records, or, in the case of a gravity supply, from a meter set in the main near the reservoir. Table No. I shows the rates for a number of cities of the United States at intervals of 10 years. This table shows the great difference between the per capita