=Concrete work.=--Before excavation for the tank is begun, two wooden forms should be built for shaping the inside of the settling and siphon chambers. In most instances the ground is fairly firm, so that the lines of excavation may conform to the outside dimensions of the tank, the back of the walls being built against the earth. The forms may be made of square-edged boards, braced and lightly nailed, as shown in Figure 26. The forms should have no bottom. If it is desired to lay the sides and covering slab in one operation, the top of the forms must be boarded over. All pipe and manhole openings should be accurately placed and cut. The faces of the forms may be covered with paper or smeared with soap or grease to facilitate removal later. [Illustration: Fig. 26.--Forms for concrete work--how to use them] 1 Make the forms as shown and to the dimensions required by Figure 23 and the table on p. 29; nails to be driven from the inside and left projecting for drawing with a claw hammer. 2. Excavate to lines 6 or 8 inches, as may be required, outside of the forms and to the depths required for both chambers. 3. Pour settling chamber floor and place form thereon. 4 Pour settling chamber walls to level of siphon chamber excavation, inserting submerged outlet pipe at the proper height. 5. Block siphon and short pipes to correct line and grade, and fill with concrete around the trap. 6. Pour siphon chamber floor, and place the form thereon. 7 Continue pouring all walls to their full height, inserting the inlet pipe when the concrete reaches that elevation. 8. Do not remove forms till the concrete is hard; with favorable weather, forms for walls only may be removed in 1 to 2 days; forms supporting a cover slab should remain 1 to 2 weeks.