24 BULLETIN" 831, U. S. DEPARTMENT OF AGRICULTURE. 



height of the throat, and this coefficient is based on the velocity at 

 the throat (V). 



The friction loss in the outlet leg depends upon the material 

 and class of workmanship therein, and is further dependent upon 

 the cross section of the chamber with regard to the throat, which is 

 assumed as constant and equal to the area of the throat. 



RESULTS OF TESTS. 



The writer has stated in another part of this paper that the only 

 tests made to determine the losses on the different parts of the 

 siphon are those of the small laboratory models, so that in summing 

 up the results reference will be made to the tests of overall efficiency 

 on working models built to discharge large volumes of water where- 

 ever such information is available. Some points have been brought 

 out incidentally in these larger tests, indicating the value which may 

 be placed on the deductions drawn from the laboratory work. 

 Taking these points up in the order in which they are listed in a former 

 part of this paper, the following is a summary : 



(a) The theoretical and actual loss of head in the various parts of 

 the structure as determined from the tests were not consistent for the 

 various tests nor for the different models, but were of sufficient ac- 

 curacy to warrant the use of the standard formulas until some more 

 reliable data can be developed. The standard formula for the loss 

 at entrance head 0.50H V for the type of opening for which the 

 formula was developed ran both high and low in the tests, and ma}'" 

 be considered as holding good as an average, so far as any develop- 

 ments in the laboratory results are concerned. Friction loss in the 

 structure was indicated as being negligible in the larger sections of 

 the tube, and was heaviest at the throat or contracted section. 

 It was so small as to be neglected in the results. 



{h) The varied shapes of the discharge lip did not seem to affect 

 the total efficiency, and since all of the models were of uniform design 

 at the intake end, nothing developed in the tests at that point or in 

 the bends from which to draw conclusions. No data from models of 

 larger siphons are available with which to compare these. 



(c) The total efficiency for the various models for different air-inlet 

 conditions ran 0.84, 0.98, and 0.983 for the three sets of tests when 

 grouped and averaged. Similar tests on larger models, but without 

 the introduction of varying air-inlet conditions, ran from 0.644 to 

 0.805, and in a number of other siphons in this country and in Europe 

 coefficients of discharge ranging from 0.70 to 0.82 have been found. 

 These points will appear in the descriptions of the individual cases 

 hereinafter taken up. 



(d ) The commonly accepted theory has been that the Aoav of water 

 over the crest of the siphon would exhaust the air through the dis- 



