─────────────────┬───────────────────────────────────────────────────── Slope │ Diameter of Sewer in Inches ─────────────────┼─────────────────┬─────────────────┬───────────────── │ 8│ 10│ 12 ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.005 │ 80│ 90│ 100 .0075 │ 55│ 65│ 80 .01 │ 45│ 55│ 70 .02 │ 20│ 30│ 35 .03 │ 15│ 20│ 24 ─────────────────┴─────────────────┴─────────────────┴───────────────── Engineers do not agree upon the advisability of the use of automatic flush-tanks, some believing that they are a needless expense that can be avoided by hand flushing, and others feeling that a flush-tank should be placed at the upper end of every lateral. These diverse opinions are the result of different experiences in different cities. =62. Siphons.=—There are two forms of siphons used in sewerage practice, a true siphon and an inverted siphon. A true siphon is a bent tube through which liquid will flow at a pressure less than atmospheric, first upwards and then downwards, entering and leaving at atmospheric pressure. An inverted siphon is a bent tube through which liquid will flow at a pressure greater than atmospheric first downwards and then upwards, entering and leaving at atmospheric pressure. In sewerage practice the word siphon refers to an inverted siphon unless otherwise qualified. Siphons, both true and inverted, are used in sewerage systems to pass above or below obstacles. True siphons are seldom used as they must be kept constantly filled with liquid.[40] Accumulated gas must be removed in order to prevent the breaking of the siphon which results in the cessation of flow. By the breaking of a true siphon is meant the stoppage of siphonic action due to the accumulation of air or gas at the peak of the siphon. Since the rate of flow of sewage fluctuates widely it is extremely difficult to control the flow so that a true siphon may be completely filled with liquid at all times.