[Illustration: COMPUTED FRICTIONAL RESISTANCE OF DRAINAGE SYSTEM OF ONE FILTER WHEN OPERATING AT A RATE OF 100 M.M. PER HOUR, EQUAL TO 2,570,000 GALS. PER ACRE DAILY. STANDARD GRAVEL SECTIONS. FIG. 6.] [Illustration: PLACING THE FLOOR OF A FILTER.] [Illustration: BUILDING THE BRICK PIERS. [_To face page 298._] ] The main underdrains for removing the filtered water are of vitrified pipe surrounded by concrete and are entirely below the floors of the filters. Connections with the main drain are made through thirty-eight 6-inch outlets in each filter, passing through the floor and connected with 6-inch lateral drains running through the whole width of the filter. These drains were made with pipes having one side of the bell cut off so that they would lie flat on the floor and make concentric joints, without support and without having to be wedged. They were laid with a space of about 1 inch between the barrels, leaving a large opening for the admission of water from the gravel. The underdrainage system is so designed that, when starting a filter after cleaning, the friction of the sand is about 50 mm. at a rate of 3,000,000 gallons per acre daily, and the friction of the underdrainage system is estimated at 10 mm. This very low friction, which is necessary, is obtained by the use of ample sizes for the underdrains and low velocities in them. In the outlet and measuring devices moderate losses of head are not objectionable, and the sizes of the pipes and connections are, therefore, smaller than the main underdrains. The gravel surrounding the underdrains is of three grades. The material was obtained from the river-bed by dredging, and was of the same stock as that used for preparing ballast for the concrete. It was separated and cleaned by a special, cylindrical, revolving screen. The coarsest grade of gravel was that which would not pass round holes 1 inch in diameter, and free from stones more than about 2 inches in diameter. At first it was required to pass a screen with holes 2 inches in diameter, but this screen removed many stones which it was desired to retain, and the screen was afterward changed to have holes 3 inches in diameter. The intermediate grades of gravel passed the 1-inch holes, and were retained by a screen with round holes 3/8 inch in diameter. The finest gravel passed the above screens and was retained by a screen with round holes 3/16 inch in diameter. The gravel was washed, until free from sand and dirt, by water played upon it during the process of screening, and it was afterward taken over screens in the chutes, where it was separated from the dirty water, and, when necessary, further quantities of water were played upon it at these points. [Illustration: INLET TO FILTER BED. SECTION ON M-N-O REGULATOR CHAMBER LONGITUDINAL SECTION ON A-B-C-D REGULATOR CHAMBER SECTION ON K-L FILTER BEDS INLET VALVES AND REGULATOR CHAMBER INLET TO FILTER BED SECTIONAL PLAN ON P-Q