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 = kAV2gH 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- 
/pipe in center of each siphon. 
5' pipe with 45" 
ell incenterof 
each siphon 
■ Q This end plugged 
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 velocity 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 clone 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, 
U. S. Reclamation Service, near Phoenix, Ariz. Installed 
to control the flow of canal at Arizona power plant. Note 
the converging outlet of the discharge chamber. Sketch 
shows air-control valves removed. 
