In large communities where houses are quite close together, the sewerage and waste water is disposed of by being allowed to flow into an outlet pipe, called a drain pipe. The drain pipe flows into a larger one and thence, by the force of gravity, to the sea, river, or to a specially prepared place called a filter bed. To prevent the escape of odors from the drain pipe, each pipe must be equipped with a trap - an S-shaped form - partially filled with water (Fig. 207). The water in the trap acts as a seal and prevents the gas from escaping into the house. The trap connects with a pipe, called a vent pipe, that runs to the roof. The object of a vent pipe is to prevent the water from being forced out.