Fig. 4 gives an illustration of the forms to be used in constructing the walls for concrete tanks, the cut at the left showing a view of the form to be used when the tank is constructed either partly or wholly above the natural ground surface, or below the surface in loose soils, and the cut at the right showing a view of the form to be used when excavation for the tank is made in rock, hardpan, or clay. The top width of the walls should be 8 inches, and the bottom width should be 8 inches plus 1½ inches for each foot of height. Thus, for a wall 6 feet high the bottom width should be 17 inches,—the inside face of the wall having a batter of 1½ inches per foot of height. This batter is necessary, when the tank is constructed below the ground surface, to withstand the lateral earth pressure when the tank is empty. If the tank is to be constructed above the ground surface, the outside wall should be battered and the inside wall made vertical, since the pressure which the wall must withstand is then only from the liquid within the tank. The partition wall between the settling tank and siphon chamber should be 10 or 12 inches thick at the top, depending on its height, and should have a batter on both sides. [Illustration: FIG. 4.—Forms Used for Building Side Walls for Concrete Tank.] To set up the forms for the concrete walls, stakes 2 inches by 4 inches and about 2½ feet long are first driven on each side of the bottom of the wall, and 6 inches away from the wall as laid out, at intervals of 2 feet. Pieces of scantling, 2 inches by 4 inches and with a length equal to the height of the wall, are then placed in upright position and securely nailed to these stakes. The inner scantling are then inclined and temporarily fastened at the top by a short nailing piece to the outer row so as to leave an opening of 10 inches between each pair of scantling. Additional stakes are then driven from 2 to 4 feet from the wall on each side, as shown in the illustration, and braces 2 inches thick and 3 inches or 4 inches wide are nailed to these stakes and to the upright and inclined scantling. One-inch boards are then lightly nailed to the scantling, as shown, the boards making up the inside face of the form being placed in sections of two feet in order to afford opportunity for thorough tamping of the concrete as the form is being filled. The concrete is then placed between the boarded sides of the form in 6–inch layers and well rammed.