22 



BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 



(e) The relation between the depth of submergence of the dis- 

 charge lip and the speed of priming "with various depths of water 

 on the throat and various conditions of air inlet. 



(/) Any additional relations which might become apparent. 



COMPUTATIONS FOR DESIGN. 



The calculations for the design of the models were varied to include 

 a uniformly enlarging cross section of the outlet leg, then decreasing 

 this to a uniform and finally to a converging section for the different 

 tests. The formula Q = kAy2gH was used, with A indicating the 

 area of the throat in this case, but which may be taken as the cross- 

 sectional area of the 

 outlet leg, using, of 

 course, the corre- 

 sponding coefficient 

 of discharge, de- 

 pending upon the 

 shape of the siphon 

 and the extent of di- 

 vergence or conver- 

 gence. 



The effect of a 

 properly expanding 

 outlet leg is to in- 

 crease the coefficient 

 of discharge, calcu- 

 lated to produce re- 

 sults showing it to 

 be greater than 

 unity, and in siphon 

 construction the ex- 

 tent of throat con- 

 traction is limited by 

 a maximum throat 

 velocity equal to 

 that produced by a 

 perfect vacuum. 



Assuming that the operating head is relatively large, so that the 

 outlet velocit}' will approach the highest throat velocity, the degree 

 of expansion need be small, and in fact it may be advisable to make 

 the outlet leg convergent, as was done on the final design of the in- 

 stallation of the Salt River siphons (fig. 8). 



Data as to the effect of such practice have not been published. In 

 the lower operating heads the outlet velocity may be low and it may 

 be desirable to construct the throat area in a contracted form to such 

 an extent that the throat velocity will approach the maximum 



Fig. 8. — Cross section of siphon spillways, Arizona Canal, 

 IT. S. Reclamation Service, near Phoenix, Ariz. Installed 

 to control the flow of canal at Arizona power plant. Xote 

 the converging outlet of the discharge chamber. Sketch 

 shows air-control valves removed. 



