216 



SEWER DESIGN 



shown) of an annular intaking limb, and a discharging limb 

 at the top of which is an annular lip or mouthpiece, the bottom 

 of which is tapered to less diameter. The discharging limb 

 terminates in a weir-chamber which when full to its overflow- 

 point just seals the limb. Over the crest of the weir is a small 

 siphon whose function is to draw the water from the weir- 

 chamber and thus unseal the siphon. At the lower end of the 

 small siphon is a dam or obstruction to retard its breaking. 



FIG. 62. 



The main siphon is brought into action (on the tank being filled) 

 by means of a small stream of water flowing over the annular 

 mouthpiece and falling free of the sides of the discharging 

 limb. As soon as the lower end of the discharging limb has 

 been sealed by filling the weir-chamber the falling stream of 

 water gathers up and carries out with it a portion of the con- 

 tained air, thus producing a slight rarefaction. 



" This rarefaction causes the water to rise in the intaking 

 limb higher than in the basin outside, and hence increases the 

 stream of water flowing over the mouthpiece, which in turn 



