and which consists in intercepting. sewers and pumping. But before discussing these points in detail, we must decide what is to be done with the sewage, and where it is to be carried.

FINAL DISPOSAL OF THE SEWAGE.

There are in use now in various parts of the world three methods of disposing of the sewage of large cities, where the water-carriage system is in use.

1st. Precipitation of the solid parts, with a view to utilizing them as manure, and to purifying the streams.

2d. Irrigation.

Neither of these processes has proved remunerative, and the former only clarifies the sewage without purifying it; but if the time comes, when, by the advance in our knowledge of agricultural chemistry, sewage can be profitably used as a fertilizer, or if it should now be deemed best to utilize it, in spite of a pecuniary loss, it is thought that the point to which we propose carrying it will be as suitable as any which can be found near enough to the city, and at the same time far enough away from it.

The third way is that adopted the world over by large cities near deep water, and consists in carrying the sewage out so far that its point of discharge will be remote from dwellings, and beyond the possibility of doing harm. It is the plan which your Commission recommend for Boston. The work will require a large sum of money, but no larger than has been expended by other cities for the same purpose; only two-thirds as much as the city of Frankfort-on-the-Main has lately appropriated for their sewers, and a small sum when we consider the benefits which will come from it.†

POSITIONS OF SEWER OUTLETS.

In order to ascertain the best position for the outlet of a main intercepting sewer for that part of the metropolitan district south of Charles river, extensive experiments have been made during the past summer, with floats placed at six different points in the harbor. It has been very difficult to follow the courses of them all, for many successive days, as they became rapidly dispersed, and were often driven by wind and storm. A chart has been prepared, giving the courses of all these floats; but the multiplicity of lines would only confuse the reader, and therefore only the general results are given. They may be concisely stated as follows: Starting from Commercial Point, City Point, and Charles

*See Appendix F.
† See Appendix A.

river, below West Boston bridge, sewage would be deposited in large quantities even if discharged on the ebb tide, as it would return in considerable quantity by the next flood.

Starting from Castle island or Moon island, sewage discharged on the flood would be deposited, to a considerable degree, on the flats of the Charles and Mystic rivers, or on Dorchester and Quincy bay flats. Discharged at and immediately after high tide, it would generally go as far as Bell Buoy or Boston Light, with a certainty of not being a source of nuisance by the returning tide.

Of the two points just mentioned, Castle island is objectionable, for three reasons:

1st. It is not remote enough from what will be soon a densely populated part of the city.

2d. The discharge of such a mass of sewage into such a thoroughfare as the main ship-channel should be avoided, provided another available point could be chosen.

3d. It can hardly be expected that sufficient land could be obtained from the United States government for the erection of the necessary works on Castle island, if, indeed, permission could be got to use the island at all. In South Boston sufficient land for a reservoir and pumps would be got only at great cost.

If the sewage were discharged into the channel of Dorchester bay, probably no offence would arise for some years; but the time would almost certainly come when the accumulation of organic matter would become so great as to render complaints quite frequent; for much of the sewage would pass backward and forward without getting farther than Spectacle island; and no inconsiderable quantity would be likely to deposit on the flats.*

For the cities lying north of Charles river the problem is more simple. The experience of the English cities Weymouth and Hastings leads us to conclude that sewage cannot be discharged on an ocean-beach without its being the source of a nuisance. With us the only available point where sewage would be carried from the northern outlet out to sea, by a rapid current, is Shirley gut.

THE INTERCEPTING SYSTEM.

The plan recommended is to discharge all the sewage from that part of Boston situated between the Charles and Neponset rivers at the north end of Moon island.

The main interecepting sewer is to be located in nearly a direct line from Cottage Farm station to the Neponset river,

* See Appendix G.

near Savin hill, to cross this river by a siphon, and thence to be built along Squantum beach and across Squantum point to the end of Moon island.

The general course of this sewer will be understood from the following list of streets through which it is to pass, viz. : Beacon street from Cottage Farm station to St. Mary street; thence to the junction of Tremont and Cunard streets; thence in Windsor, Madison, Hunneman, and Yeoman streets and Norfolk avenue and Clapp and Mt. Vernon streets to Dorchester avenue; thence in nearly a direct line to Neponset river north of Savin hill. The sewage is here to be raised by pumping, and passes by a siphon under Neponset river into the main outlet-sewer.

The grade of the sewer at Cottage Farm station is to be one foot below low tide. The fall, or inclination, is to be one in twenty-five hundred, or twenty-five inches per mile. The sewer is to be circular, nine feet in diameter from Cottage Farm station to Albany street; nine feet by eleven (equal to a circle ten feet in diameter) thence to the pumping station; thence to the outlet at Moon island, at first ten feet by twelve (equal to a circle of eleven feet in diameter), and finally twelve feet by thirteen; thus enlarging the storage capacity of the outfall sewer.

The siphon under Neponset river is to be six feet in diameter and fifteen hundred feet long. It is designed to be of wrought iron, properly protected from the action of salt water. Chambers are to be built at each end of it for connecting a second siphon. The results of further surveys and borings at this point may show that it would make a more permanent and less expensive structure to build a brick tunnel laid in Portland cement with iron ribs to strengthen the masonry.

There is to be built at the outlet on Moon island a reservoir to hold twenty-five million gallons, which is somewhat more than the usual amount of sewage now discharged in twenty-four hours. The discharge into the sea is to take place at each tide for the first two or three hours after high water.

It is proposed to erect at the pumping-station three engines, of 145 horse-power each. This is a very liberal provision for the present; but, in view of the free use of water after the completion of the Sudbury aqueduct, it is thought best to make it. The lift will be from twenty to thirty-three feet. The grade of the outfall sewer is at this point fourteen feet, and at the outlet eight feet above low

water.

The sewer from Cottage Farm station to the Neponset

river is to drain all that part of the city which lies between it and the Charles river and the harbor on the north; also all that part of the city south of it and at the same time below grade forty. It will be large enough to drain twenty square miles, and to take the sewage from a population of 750,000. Its capacity is three hundred and forty-five cubic feet per second, or over two hundred and twenty million gallons per day.

The size of the outfall-sewer is sufficient to carry the sewage from a population of one million, and also one-fourth of an inch rain-fall per day from an area of thirty square miles. Its capacity is four hundred and forty-five cubic feet per second, or over two hundred and eighty million gallons per day.

In determining the sizes of the sewers, we have assumed the amount of sewage to be seventy-five gallons per day per inhabitant; but, as the maximum flow would be at the same rate as if this were all to be discharged in twelve hours, we have fixed the sizes large enough to discharge double that amount, and, in addition to the above, an amount of storm water at the time of the maximum flow of the sewage equal to onefourth of an inch of rain-fall per twenty-four hours. The large capacity of the sewers fixed upon will be sufficient to carry off or impound for the time a still greater rain-fall per hour. It should, however, be here stated that this plan contemplates the construction of storm-overflows at suitable points, so as to prevent in times of very great or long continued rains a surcharging of the sewers.

It is also to be understood that the natural water-courses in Dorchester, Roxbury, Brookline and Brighton, particularly Stony brook and Muddy brook, are to be kept open, and free and clear from sewage contamination, and that their channels are to be straightened and to discharge as at present into the Charles river; for no sewers, could be built at reasonable cost large enough to carry off these waters in case of storms. If by the growth of the city it should ever become necessary to cover them or other natural watercourses, great care should be taken that their character as drains for the soil be maintained without allowing any ordinary sewage to be discharged into them, but only occasionally an excess of storm-water.

The sewage from those portions of the city situated south of the main intercepting sewer, and above the plane of forty feet above low water, can be intercepted by a high-level sewer and delivered into the outfall-sewer without pumping. The district is not now so densely populated as to require this; and for the next ten or fifteen years the sewage from

such portions as require sewers can be delivered into the lowlevel sewer and pumped.

The principal branch of the main sewer is to be located in Albany street, from Yeoman street to Kneeland street, and at this point to receive a lateral to be built in Kneeland, Federal and Broad streets, and Atlantic avenue to Central wharf; from the junction of Albany and Kneeland streets it is to be built in Kneeland, Elliot and Pleasant to Boylston, and thence to junction of Arlington and Marlborough streets and in Marlborough to Parker street. At the junction of Marlborough and Arlington streets it is to receive a branch to be located in Arlington, Beacon, Brimmer, Pinckney and Charles streets to Leverett street.

The location of this branch intercepting sewer on Marlborough street is recommended on account of economy in construction, and the saving of much annoyance to the public by obstructing Beacon street for so long a time as would be necessary, to say nothing of the danger of accidents to the water-mains there.

The desirableness, however, of keeping the tide more perfectly out of this district, the ground-water of which fluctuates in consequence of it from one to three feet at every tide, and any future examinations and negotiations which may be made with the owners of the property on the north side of the mill-dam, may satisfy the city that it would be better to rebuild the sea-wall there, making it water-tight, and then construct the sewer inside of it.

It is recommended to close that portion of the Roxbury canal west of Albany street. If this is done, the expense of a siphon for the sewer in Albany street will be saved; and the value of the land reclaimed will more than compensate for the expense of filling it, beside greatly improving the sanitary condition of this immediate vicinity.

The sewage from South Boston is to be intercepted by a sewer to be built in Dorchester avenue, A street, and First street to Second street. It is thought that for the present it will not be necessary to build this sewer farther than from its junction with the main in Dorchester avenue to B street, where it will intercept a large sewer, which now discharges into the South bay.

The sizes of the branch intercepting sewers will vary from four feet in diameter up to seven and one-half feet; their minimum inclinations are to be one in two thousand, or thirty-two inches per mile.

The inclination of all the sewers is to be sufficient to give a minimum velocity of about two miles per hour, and thus prevent any deposit of sediment in them.