The lateral tiling should be of three-inch agricultural tile (see Fig. 31), laid with a space of one-quarter inch between each length and with a piece of tar paper or a half-collar of larger diameter pipe, as shown in Fig. 32, placed over the joints to prevent clogging of the pipe with earth. In the heavier soils the lateral lines of sub-surface tiling are sometimes set in trenches eight to fourteen inches deep and about twelve inches wide, filled with broken stone or gravel placed around the tiling to within two or three inches of the ground surface, as shown in Fig. 32. This allows the effluent to seep away more readily, but while of advantage in those soils the provision is not necessary in the more porous soils. [Illustration: FIG. 31.—Photograph of Tile laid as if for Sewage Disposal.] It is generally found that a sufficient length of sub-surface tiling should be laid to provide for not more than one to three gallons of effluent per day for each linear foot of tiling. In sandy soils there should be at least thirty to forty feet of tiling for each person served by the sewer, with six feet of space between the lines of tiling. This length per person should be increased up to seventy or eighty feet for the more compact sandy or gravelly loams, or the lighter clay loams, with the lateral tiling spaced four feet apart. It is not considered feasible to attempt to dispose of sewage by sub-surface irrigation in soils which will not care for effluent when the greater lengths of tiling per person, as stated above, will not prevent the appearance of effluent on the surface. If, however, after the installation of a sub-surface system in a rather heavy soil, it is found that proper seepage of the effluent does not occur, the lateral branches may sometimes be lengthened and the system then found to operate satisfactorily. [Illustration: FIG. 32.—Sub-surface Tiling with Broken Stone or Gravel Surrounding Pipe.] The lines of lateral tiling should be laid with the invert, or bottom of the pipe, inside, from six inches to one foot below the surface, as shown in Fig. 27. They should be parallel with the contours or at right angles with the slope of the field, and should have a gradient or fall of one-sixteenth of an inch to the foot when laid in sandy soil or sandy loam, and of not more than one thirty-second of an inch to the foot when laid in the heavier loams. To obtain such gradients for the sub-surface tiling it is sometimes necessary to lay out the trenches along irregular or curved lines, as shown in Fig. 33. The tiling should be laid near the surface, as stated, and never deeper than twelve inches. The temperature of the sewage will prevent its freezing even in very severe winter weather, especially when the ground is covered with snow. [Illustration: FIG. 33.—Sub-surface System on Irregular Ground.]