506 WATER SUPPLY AND SEWAGE DISPOSAL 



and out through the SOIL PIPE (Fig. 313). The air and gases 

 within the drain and soil pipe are nearly certain to be warmer 

 and less dense than outside air. Explain clearly the circula- 

 tion of air through the drain pipe and its cause. 



594. The Trap. Every opening into the drain, whether it 

 be from sink, lavatory, bath, or closet should be sealed by 

 means of a trap. Most traps consist of a sharp upward bend 

 in the drain pipe just after it leaves the fixture. The water 

 settles into this bend and seals the outlet. Point out the 

 traps in the illustrations. 



Exercise 98. A Study of Traps 



Examine several fixtures in the schoolhouse or residence and 

 study carefully the traps. Notice the provision which is made for 

 removing obstacles from the bottom of the trap. Such openings 

 are called CLEANOUTS. 



595. Siphoning of Traps. When a large flow of water 

 passes through the trap, it sometimes happens that the water 

 completely fills the drain pipe beyond the trap and causes all 

 the water to pass over the upward bend, thus leaving the trap 

 unsealed. When this happens, the trap is said to have been 

 SIPHONED OUT. To prevent siphoning, an air vent is usually 

 connected at the highest point of the trap, the other end of the 

 vent pipe opening either into the soil pipe some distance above 

 or opening into the air above the roof (see Fig. 313). 



Exercise 99. A Study of the Siphon 



Place one end of a small clean rubber tube into a vessel of water 

 and hold beneath the surface of the water. Place the other end be- 

 tween the lips and suck out the air (see Art. 374). When the tube 

 is filled with water, close the end of the tube near your lips by press- 

 ing between the thumb and finger. Now lower this end to a point 

 lower than the level of the water in the vessel. Remove the pressure 

 from the tube. Does the water flow through the tube? If not, try 

 refilling the tube. Now carefully raise the open end of the tube, not- 

 ing the effect upon the ra'a of flow of water. Does the water con- 

 tinue to flow after the free end of the tube has been raised above the 



