SPILLWAYS FOR RESERVOIRS AND CANALS. 25 



charge leg and under the discharge lip, provided the outlet leg was 

 sealed at that point and that the air becoming rarefied would encour- 

 age an increase in this flowing head until full siphonic action was 

 accomplished. The tests on the laboratory model indicated a tend- 

 ency to develop a back pressure in the crown by this rise of the water 

 surface at the inlet, and the suppressed effort of the air to escape 

 under the discharge lip when the seal was complete. 



In fact, it was shown that with the discharge lip submerged 0.1 to 

 0.2 foot, and fully out of water, resulting in the tube being unsealed, 

 the siphon primed easier and quicker than when the outlet lip was 

 submerged to increasing depths to effect more complete seal. The 

 tests on the Yuma siphons indicated that this is probably truf, but 

 the head on the discharge lip during the Yuma tests was not reduced 

 sufficiently to disclose its action with sealing heads of 1 foot or under, 

 or when flowing freely into air. The Ocoee River installation of the 

 Tennessee Power Co., however, does bear out the tests of the small 

 models, although it has been argued that the shape of the outlets of 

 these siphons in itself forms a seal. Conclusive proof that a sealing 

 basin is not necessary on siphon spillways or that the efficiency of the 

 structure is not decreased by the absence of it, would lead to a great 

 reduction in their cost and would permit of the elimination of another 

 weak point in freezing weather. 



(c) The placing of air inlets and the submergence of the outlet 

 end are so closely related that the points are difficult of separation 

 in summing up the results. One question to be solved is to determine 

 the proper depth of submergence of the discharge lip to produce maxi- 

 mum siphonic action, and another is to determine the shortest period 

 of time necessary to produce complete action under the same condi- 

 tions. To enable the plotting of a curve to show these relations, various 

 conditions were produced by the manipulation of improvised air in- 

 lets, made possible by utilizing the tubing for connecting the gage 

 glasses. With all air inlets or outlets closed over the throat section, it is 

 intended that the siphon will come into full action just as the water 

 flows over the crest. The condition as developed with a complete 

 seal of the siphon tube and with the water rising in the inlet and 

 outlet legs produced an air compression in the tube which tended to 

 retard the increase of head on the overflow crest, and consequently 

 required a longer period of priming because of the resulting decreased 

 flow of water necessary to produce expulsion of the confined air in the 

 tube. The shortest period of priming was obtained by using the 

 gage-tube connection at the throat as an automatic air valve, allow- 

 ing all compression in the siphon to be relieved when the water rose 

 to produce it, and then to be instantly closed when a reverse pressure 

 was started. 



(/) This suggested that the delays in priming were shortest when 

 a relief valve was provided ; that the heads on the crest of the throat 



