[Illustration: OUTSIDE WALL, READY FOR CONCRETE BACKING.] [Illustration: SEDIMENTATION-BASIN: SHOWING CONSTRUCTION OF FLOOR. [_To face page 294._] ] =Filters.=—The filters are of masonry, and are covered to protect them against the winters, which are quite severe in Albany. The piers, cross-walls, and linings of the outside walls, entrances, etc., are of vitrified brick. All other masonry is concrete. The average depth of excavation for the filters was 4 feet, and the material at the bottom was usually blue or yellow clay. In some places shale was encountered. In one place soft clay was found, and there the foundations were made deeper. The floors consisted of inverted, groined, concrete arches, arranged to distribute the weight of the walls and vaulting over the whole area of the bottom. The groined arch-vaulting is of concrete with a clear span of 11 feet 11 inches, a rise of 2-1/2 feet, and a thickness of 6 inches at the crown. It was put in in squares, the joints being on the crowns of the arches parallel with the lines of the piers, and each pier being the centre of one square. The manholes are in alternate sections, and are of concrete, built in steel forms with castings at the tops, securely jointed to the concrete. Above the vaulting there are 2 feet of earth and soil, grassed on top. The tops of the manholes are 6 inches above the soil to prevent rain-water from entering them. The drainage of the soil is effected by a depression of the vaulting over each pier, partially filled with gravel and sand, from which water is removed by a 2-inch tile-drain going down the centre of the pier and discharging through its side just above the top of the sand in the filter. In order to provide ready access to each filter, a part of the vaulting near one side is elevated and made cylindrical in shape, making an inclined runway from the sand-level to a door the threshold of which is 6 inches above the level of the overflow. [Illustration: FILTER BEDS SECTION OF FILTERS FIG. 4.] This sand-run is provided with permanent timber runways and with secure doors. [Illustration: Section. Elevation. FIG. 5.—ENTRANCE TO A FILTER.] The manholes of the filters are provided with double covers of steel plates to exclude the cold. The covers also exclude light. When cleaning the filters, light can be admitted by removing the covers. Supports for electric lights are placed in the vaulting, so that the filters can be lighted by electricity and the work of cleaning can be done at night, and in winter under heavy snow, without removing the covers. The electric lights have not yet been installed. The regulator-houses, the entrances to the sand-runs, and all exposed work are of pressed brick with Milford granite trimmings and slate roofs. The regulator-houses have double walls and double windows and a tight ceiling in the roof, to make them as warm as possible and to avoid the necessity of artificial heat to prevent freezing.