508 WATER SUPPLY AND SEWAGE DISPOSAL 



when the trap is properly seated. When raised an inch or 

 two, the trap permits the water to rush directly into the dis- 

 charge pipe. If the trap be again dropped upon its seat, the 

 discharge pipe, now rilled with water, together with the two 

 arms, or passages of the trap, becomes a perfect siphon. The 

 discharge pipe and the right-hand passage in the trap form 

 the long arm of the siphon and the left-hand passage forms the 

 short arm. From our study of the siphon, it is evident that 

 the water will continue to flow through the siphon thus formed 

 till the water level sinks to the level of the opening in the side 

 of the trap, air there enters and destroys the siphoning action. 

 The tank then again fills to the height permitted by the 



AUTOMATIC FLOAT VALVE. 



598. The Float Valve or Automatic Cut-off. In many cases 

 other than the flushing tank it is desirable to have the height 

 of water in tanks automatically controlled. In such cases a 

 float, as shown in Fig. 314, is frequently used to operate the 

 cut-off valve. The float is a light, hollow, brass or copper 

 sphere. When the water is lowered, the float falls, thus per- 

 mitting the valve to open ; as the water rises again, the float is 

 forced upward until it closes the valve. 



V. DISPOSAL OF SEWAGE 



699. Disposal of City Sewage. In cities having sewer sys- 

 tems the final disposal of sewage gives the individual citizen 

 little or no worry. He has merely to connect his drain in 

 proper manner with the city sewer; the city is responsible 

 for the final disposal of the sewage. In many cases, city sew- 

 age is merely conveyed to, and emptied into, the nearest stream, 

 always polluting it more or less, depending upon the amount 

 and kind of sewage and the size of the stream. Our little 

 friends, the ever-present bacteria, however, at once begin their 

 work of decomposing the organic matter in the sewage and, 

 under favorable conditions and with sufficient dilution, most of 

 the polluting matter is soon destroyed and the stream again 



