=Vent Pipes and Their Branches.=--The purpose of vent pipes, we have seen, is to prevent siphoning of traps and to ventilate the air in the traps and pipes. The material of which vent pipes are made is cast iron. The size of vent pipes depends on the number of traps with which they are connected; it is usually two or three inches. The connection of the branch vent to the trap must be at the crown of the trap, and the connection of the branch vent to the main vent pipe must be above the trap, so as to prevent friction of air. The vent pipes are not perfectly vertical, but with a continuous slope, so as to prevent condensation of air or vapor therein. The vent pipes should be extended above the roof, several feet above the coping, etc.; and the extension above the roof should not be of less than four inches diameter, so as to avoid obstruction by frost. No return bends or cowls should be tolerated on top of the vent pipes. Sometimes the vent, instead of running above the roof, is connected with the soil pipe several feet above all fixtures. [Illustration: FIG. 23. LEADER PIPE.] =Rain Leaders.=--The rain leader serves to collect the rain water from the roof and eaves gutter. It usually discharges its contents into the house drain, although some leaders are led to the street gutter, while others are connected with school sinks in the yard. The latter practice is objectionable, as it may lead the foul air from the school sink into the rooms, the windows of which are near the rain leader; besides, the stirring up of the contents of the school sink produces bad odors. When the rain leader is placed within the house, it must be made of cast iron with lead-calked joints; when outside, as is the rule, it may be of sheet metal or galvanized-iron pipe with soldered joints. When the rain leader is run near windows, the rules and practice are that it should be trapped at its base, the trap to be a deep one to prevent evaporation, and it should be placed several feet below the ground, so as to prevent freezing. CHAPTER VIII =Plumbing Fixtures= The receptacles or fixtures within the house for receiving the waste and excrementitious matter and carrying it off through the pipes to the sewer are very important parts of house plumbing. Great care must be bestowed upon the construction, material, fitting, etc., of the plumbing fixtures, that they be a source of comfort in the house instead of becoming a curse to the occupants.