26 BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 
necessary to effect a complete priming were less under such conditions 
or when the outlet lip of the discharge leg was so deeply submerged 
as to set up reverse action in the expulsion of the entrained air. 
(Since the tests referred to were made a system of relief valves has 
been installed in connection with the battery of seven siphons placed 
in the Huntington Lake dam in California, the discharge from which 
amounts to 5,000 second-feet. The description of these siphons fol- 
lows in another part of this paper. 
SIPHON AT GIBSWIL, SWITZERLAND. 
A siphon at Gibswil in Switzerland, built with a sloping instead of 
a vertical outlet leg, is described as consisting of a J-inch riveted 
steel tube tapering from 31.5 to 23.6 inches in diameter, the assumption 
being that the taper would tend to keep the water column from part- 
ing under the 52.48-foot head which was utilized. The inlet pipe 
was cut on a horizontal plane at the normal high- water surface and 
was incased in a reinforced concrete hood projecting 3.28 feet below 
normal water surface, so as to prevent the entrance of ice or float- 
ing debris. The air control, to break the action of the siphon and to 
prevent the water from being drawn down into the reservoir below 
normal surface, consisted of long narrow slots cut through three of 
the sides. When the water rises these slots are closed and siphonic 
action begins. A series of tests to determine over-all efficiency for 
this siphon gave a discharge of 98.9 cubic feet per second, but this 
was considered inaccurate and lower than the real efficiency of the 
siphon, because it was found that some of the air slots were not fully 
sealed. A maximum efficiency yielding 123.6 cubic fecit per second 
was determined as a more accurate assumption of the real capacity. 
Computation of the end area at 3.03 square, feet would give a corre- 
sponding velocity of 40.8 feet per second. The velocity due to 52.48 
feet head is V= V 2 gH = 58.06 feet per second and thus the efficiency k 
is Kg' nr = 0.70 as a coefficient of discharge. The computed friction loss 
in the pipe alone equaled 10.2 feet, which certainly is high and con- 
firms the statement made in another part of this paper that the in- 
crease in the length of the tube beyond 34 feet would reduce the 
value k. In addition, this tube had the added friction produced by 
building the draft tube on a slope and thus making it longer. 
In another siphon installation, also in Switzerland, the conditions 
to be overcome as the result of conflicting requirements of several 
plants and their water rights, were peculiar. There was a spinning 
and weaving mill operated by hydraulic machinery, the tail water 
*rom which, up to a maximum of 56.5 second-feet, was appropriated 
by a twine plant farther down the stream. The discharge from the 
first plant in excess of the 56.5 second-feet had to be led over a weir 
