=Parts of a system.=--The four parts of a septic-tank installation with subsurface distribution of the effluent are outlined in figure 19: (1) The house sewer from house to tank; (2) the sewage tank consisting of one or more chambers; (3) the sewer from tank to distribution field; (4) the distribution field, where the sewage is distributed and wasted, sometimes called the absorption field. These parts will be discussed in the order named, although the last should have the first consideration. [Illustration: Fig. 19.--Parts of a septic-tank installation.] [Illustration: Fig. 20.--Setting line and grade for house sewer. To the observer at A the top edges of the grade boards appear as one; the half-driven nails are set to line.] =House sewer.=--The length will vary with the slope of the ground and position of buildings, well, and distribution field. Fifty to 100 feet is a fair length; a greater is still more sanitary. Wherever possible the house sewer should be laid straight in line and grade. Figure 20 shows how this work may be done. Suppose the distance from A to E be 100 feet; that grade boards be set 25 feet apart crosswise of the trench at A, B, C, D, and E; that the ground at A be 4 feet lower than at E; that the top of the sewer be 2-1/2 feet below the surface of the ground at A and 4-1/2 feet below the surface of the ground at E; the fall of the sewer between A and E is 2 feet (4 + 2-1/2 - 4-1/2 = 2). If the fall in 100 feet be 2 feet, in 25 feet it is one-fourth as much, or 6 inches. Hence, grade board B is 6 inches higher than grade board A, C is 6 inches higher than B, and so on to E. The top edges when all the boards are set with a carpenter's level and fastened in position should be in line. The grade thus established may be any convenient height above the top of the proposed sewer, and the measuring stick used to grade the pipe is cut accordingly. This height is usually a certain number of whole feet. Fixing the line of the sewer is a mere matter of settling nails in the top edges of boards A and E directly over the center of the proposed sewer and tightly stretching a fish line or grade cord; nails should be set where the cord crosses boards B, C, and D. If the cellar or basement contains plumbing fixtures, the house sewer should enter 1 to 2 feet below the cellar floor. If all plumbing fixtures are on the floors above, the sewer may enter at no greater depth than necessary to insure protection from frost outside the cellar wall. Digging the trench and laying the pipe should begin at the tank or lower end. The large end of the pipes, called the hub, should face uphill, and the barrel of each pipe should have even bearing throughout its length. Sufficient earth should be removed from beneath the hubs to permit the joints to be made in a workmanlike manner.