52. No privy-vault, or cesspool for sewage, will be permitted in any part of the city where water-closets or a school-sink can be connected with a public sewer in the street. 53. School-sinks must be of cast-iron, not more than two feet in depth, connected at the upper end with the Croton supply, and at the lower end with a drain leading to the street-sewer, and provided with an outlet at the lowest point and on the bottom so as to admit of a complete discharge of the contents whenever the outlet is opened and the sink flushed with water. 54. The sink must be set so that the flange will be at least two feet below the yard surface, to prevent freezing. It must be at least ten feet from any window, or as near that distance as practicable. 55. The waste-pipe from a hydrant-sink in the yard must be properly trapped, especially where it discharges into a school-sink, a privy-vault, or cesspool, or the house-drain. 56. Open light and air courts must be properly drained. 57. When a privy-vault or cesspool must necessarily be used, and the water-supply of the premises is from a well, they must be at least fifty feet from the well; and the privy-vault must be absolutely tight. _Explanatory Remarks._ 2. =As direct as possible.= To insure an uninterrupted flow. When a pipe has to pass an obstruction (a beam or post) an =offset= is used (Fig. 2). 3. The =soil-pipe= is that which conveys the contents of water-closets to the house-drain: the =waste-pipes= are intended to carry other refuse fluids only. The =house-drain= is the large pipe which receives the contents of the soil and waste pipes and conveys them outside the house. A =trap= is a bend (with or without an enlargement) of the pipe, intended to retain a sufficient amount of the fluids that enter it to occlude the pipe and prevent the backward flow of air. The effective =water-seal= is represented by the total depth of water in the trap, minus the inside diameter of the pipe, and should be at least one inch. [Illustration: Fig. 2.—Offset.] [Illustration: Fig. 3.—S-trap.] [Illustration: Fig. 4.—Half S-trap.] [Illustration: Fig. 5.—D-trap.] [Illustration: Fig. 6.—Bag-trap.] Traps are of various kinds, and many are patented. The accompanying figures represent the principal types. Fig. 3 is the S-trap. Fig. 4 is the half S-trap. There is also a three-quarter S-trap. Fig. 5 is a D-trap, formerly much used, but now mostly out of date, because it retains dirt. Fig. 6 is a =bag=-trap. It has no advantage over the S-trap, and uses more material. Fig. 7 is a =bottle=-trap, much used in Boston. This also retains dirt. There are varieties of it, the distinguishing feature of all being that the fluids enter them at the bottom and flow out at the top. [Illustration: Fig. 7.—Bottle-trap.] [Illustration: Fig. 8.—Bell-trap.] [Illustration: Fig. 9.—Running-trap.] [Illustration: Fig. 10.—Mason’s trap.]