water to be distributed uniformly over its surface in the manner shown in the figure. It is further necessary to group the filter-beds, pumps, sand-cleaning appliances, and other portions of the plant, so that the ends of economy and efficient administration may be served in the highest degree. It is always necessary that these features of the whole filtration system should be carefully kept in view in laying out the entire plant. =234. Design of Filter-beds.=—The preparation of the site for a group of filtration-beds also involves the consideration of a number of principal questions. In the first place, the depth required for the sand and underdrains will not be far from 5 feet, and there must be a suitable bottom prepared below the collecting-drains. Again, the depth of water above the sand may vary from 3 to 5 feet, making the total depth, including the bottom, of the filter proper about 10 or 11 feet, and this may represent the depth of excavation to be made. If the material on which the filter to be built is soft, it may be necessary to drive piles to support the superincumbent weight. The bottom must be made water-tight. This can be done either by the use of a layer of well rammed or packed clay, 1 to 2 feet in thickness, carrying 6 or 8 inches of concrete, or by a surface of paved brick or stone. If the sides of the filter-beds are of embankments with surface slopes, the latter may be protected in the same manner. If the sides are of walls of masonry, concrete is an excellent material to be used for the purpose. [Illustration: FIG. 7.—Sedimentation-basin and Filter-beds at Albany, N. Y.] [Illustration: Filtration-plant at Albany, N. Y.] In designing the sides of filters or of the piers projecting up through the sand for the support of the roof, in case there is one, it is imperative that care be taken to prevent water from flowing down through the joints between the sand and the sides of piers or the masonry sides of the filter-beds. There should be no vertical joint of that character, but the faces of masonry in contact with the sand should both slope and be made in steps, so that any settlement of the sand will tend to close the joint, while the steps will prevent flow. Nor should there be angles in which sand is to be packed; filleted corners are far preferable and should be used.