Between the pipes which carry the water into the house and those which carry the sewage out are placed the house fixtures. There is a pipe leading to each fixture which carries the water to dilute the sewage, and a pipe leading from each of these fixtures carrying the sewage to the main house drain. The admission of sewer gas into the house through the fixtures is prevented by the trap which contains water known as the seal. The trap is of no value unless it is properly sealed. The trap is simply a bend in the pipe, as shown in the illustration, in which water is contained. This water keeps the sewer air from coming back into the house. A system of house plumbing, in order to meet the requirements of hygiene, must be properly trapped. There is a small trap for each fixture and in some cities an additional outside trap is required which protects the entire system. [Illustration: bowl and trap] When some fixture loses its seal then there is great danger of sewer air coming into the house. The main causes for traps becoming unsealed are: the forcible ejection of the seal by the momentum of the flush; loss of seal through siphon; the evaporation of the water when the fixture is not in use; and by capillary attraction as when waste paper or some similar material is left in the trap and projects through. Siphonage is caused by a large volume of water passing down through a vertical pipe with which the fixture is connected. The loss of the seal by evaporation may be prevented by frequent use of the fixture, or if the fixture is not to be used for some time the trap may be filled with oil or some substance that will not evaporate. The loss of the seal from momentum may be averted by providing proper flushing tanks, while the protection from capillary attraction may be effected by not allowing anything to remain in the trap which will exert this capillary attraction on the water in the trap. Prevention of siphonage is accomplished by providing proper ventilation for each trap so there may be a column of air which may be drawn upon in lieu of the seal. Care should be taken that the seal does not become foul. The fouling of the seal is the result of back pressure from the sewer and may be avoided by providing a vent pipe through which the air may pass from the sewer and thus relieve the pressure on the seal. The sewage is carried from the house by the house sewer into the street sewer, from the street sewer into the district sewers, and then into the large trunk sewers. There are also relief sewers, storm sewers, under drains, and intercepting sewers. The entire system is known as the sewerage system. The amount of water required to carry away the sewage varies in different cities. In the smaller towns it may be as low as fifty gallons a day per capita, while in some of the larger cities it may be as much as 200 gallons per capita per day.