To guard against the loss of seal through siphonage "nonsiphoning" traps have been invented, that is, the traps are so constructed that the seal therein is very large, and the shape of the traps made so that siphonage is difficult. These traps are, however, open to the objection that in the first place they do not prevent the fouling of the seals by back pressure, and in the second place they are not easily cleansable and may retain dirt in their large pockets. The universal method of preventing both siphonage and back pressure is by the system of vent pipes, or what plumbers call "back-air" pipes. Every trap is connected by branches leading from the crown or near the crown of the trap to a main vertical pipe which runs through the house the same as the waste and soil pipes, and which contains nothing but air, which air serves as a medium to be pressed upon by the "back-pressure" air, or to be drawn upon by the siphoning, and thus preventing any agitation and influence upon the seal in the traps; for it is self evident that as long as there is plenty of air at the distal part of the seal, the seal itself will remain uninfluenced by any agitation or condition of the air within the pipes with which the trap is connected. The vent-pipe system is also an additional means of ventilating the plumbing system of the house, already partly ventilated by the extension of the vertical pipes above the roof and by the fresh-air inlet. The principal objection urged against the installation of the vent-pipe system is the added expense, which is considerable; and plumbers have sought therefore to substitute for the vent pipes various mechanical traps, also nonsiphoning traps. The vent pipes are, however, worth the additional expense, as they are certainly the best means to prevent siphonage and back pressure, and are free from the objections against the cumbersome mechanical traps and the filthy nonsiphoning traps. CHAPTER VII =Plumbing Pipes= =The House Drain.=--All waste and soil matter in the house is carried from the receptacles into the waste and soil pipes, and from these into the house drain, the main pipe of the house, which carries all waste and soil into the street sewer. The house drain extends from the junction of the soil and waste pipes of the house through the house to outside of the foundations two to five feet, whence it is called "house sewer." The house drain is a very important part of the house-plumbing system, and great care must be taken to make its construction perfect. _Material._--The material of which house drains are manufactured is extra heavy cast iron. Lighter pipes should never be used, and the use of vitrified pipes for this purpose should not be allowed.