[Illustration: GENERAL VIEW OF VAULTING, UNDER CONSTRUCTION. [_To face page 302._] ] [Illustration: FIG. 9.] The sand was obtained from the river at various places by dredging. It was first taken up by dipper-dredges, and brought in scows to a point in the back channel a little north of the filter-plant. It was there dumped in a specially prepared place in the bottom of the river, from which it was lifted by a hydraulic dredge and pumped through a 15-inch pipe an average distance of 525 feet to points selected, and varied from time to time, on the flats north of the filters. The water containing the sand was then put through screens having meshes which excluded all stones 5 mm. in diameter and over, and was then taken into basins where the sand was deposited and afterward carted to the filters. Two ejector sand-washing machines, shown in Fig. 9, are provided at convenient places between the filters. In them the dirty sand is mixed with water, and is thrown up by an ejector, after which it runs through a chute into a receptacle, from which it is again lifted by another ejector. It passes in all through five ejectors, part of the dirty water being wasted each time. The sand is finally collected from the last ejector, where it is allowed to deposit from the water. Water is admitted to each filter through a 20-inch pipe from a pipe system connecting with the sedimentation-basin. Just inside of the filter-wall is placed a standard gate, and beyond that a balanced valve connected with an adjustable float to shut off the water when it reaches the desired height on the filter. These valves and floats were constructed from special designs, and are similar in principle to valves used for the same purpose in the Berlin water-filters. Each filter is provided with an overflow, so arranged that it cannot be closed, which prevents the water-level from exceeding a fixed limit in case the balanced valve fails to act. An outlet is also provided near the sand-run, so that unfiltered water can be removed quickly from the surface of the filter, should it be necessary, to facilitate cleaning. The outlet of each filter is through a 20-inch gate controlled by a standard graduated to show the exact distance the gate is open. The water rises in a chamber and flows through an orifice in a brass plate 4 by 24 inches, the centre of which is 1 foot below the level of the sand-line. At the nominal rate of filtration, 3,000,000 gallons per acre daily, 1 foot of head is required to force the water through the orifice. With other rates the head increases or decreases approximately as the square of the rate and forms a measure of it. With water standing in the lower chamber, so that the orifice is submerged, it is assumed that the same rates will be obtained with a given difference in level between the water on the two sides of the orifice as from an equal head above the centre of the orifice when discharging into air.