=Aeration.=—After leaving the meter, the water passes to the sedimentation-basin through eleven outlets. These outlets consist of 12-inch pipes on end, the tops of which are 4 feet above the nominal flow-line of the sedimentation-basin. Each of these outlet-pipes is pierced with 296 3/8-inch holes extending from 0.5 to 3.5 feet below the top of the pipe. These holes are computed so that when 11,000,000 gallons of water per day are pumped all the water will pass through the holes, the water in the pipes standing flush with the tops. The water is thus thrown out in 3256 small streams, and becomes aerated. When more than the above amount is pumped, the excess flows over the tops of the outlet-pipes in thin sheets, which are broken by the jets. [Illustration: GENERAL PLAN FIG. 2.] Regarding the necessity for aeration, no observations have been taken upon the Hudson River, but, judging from experience with the Merrimac at Lawrence, where the conditions are in many respects similar, the water is at all times more or less aerated, and, for the greater part of the year, it is nearly saturated with oxygen, and aeration is not necessary. During low water in summer, however, there is much less oxygen in the water, and at these times aeration is a distinct advantage. Further, the river-water will often have a slight odor, and aeration will tend to remove it. The outlets are arranged so that they can be removed readily in winter if they are not found necessary at that season. =Sedimentation-basin.=—The sedimentation-basin has an area of 5 acres and is 9 feet deep. To the overflow it has a capacity of 14,600,000 gallons, and to the flow-line of the filters 8,900,000 gallons. There is thus a reserve capacity of 5,700,000 gallons between these limits, and this amount can be drawn upon, without inconvenience, for maintaining the filters in service while the pumps are shut down. This allows a freedom in the operation of the pumps which would not exist with the water supplied direct to the filters. The water enters the sedimentation-basin from eleven inlets along one side, and is withdrawn from eleven outlets directly opposite. The inlets and aerating devices described previously bring the water into the basin without current and evenly distributed along one side. Both inlets and outlets are controlled by gates, so that any irregularities in distribution can be avoided. The concrete floor of the sedimentation-basin is built with even slopes from the toe of each embankment to a sump, the heights of these slopes being 1 foot, whatever their lengths. The sump is connected with a 24-inch pipe leading to a large manhole in which there is a gate through which water can be drawn to empty the basin. There is an overflow from the basin to this manhole which makes it impossible to fill the basin above the intended level. [Illustration: LONGITUDINAL SECTION ON _a-b-c-d-e-f-g-h_ FILTER BEDS PLAN AND SECTION OF FILTER NO. 2 FIG. 3.]