30 
BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 
and 1,400 second-feet at the intake and outlet ends respectively. The 
siphons are to operate under a head of 12 feet, at an average velocity 
of about 11 feet per second, assuming 0.50 as a coefficient of discharge. 
The structure is of re- 
inforced concrete 
throughout, with a 
detached air chamber 
for the regulation of 
the siphons. The air 
control has not given 
satisfactory results 
because of its isola- 
tion ; otherwise the 
structure is a good ex- 
ample of the combi- 
nation of facilities 
for the control of su- 
perdrainage on side- 
hill canal or where 
melting snows or ob- 
struction below the 
structure might cause 
an abnormal rise of 
the water level in the 
canal at the site of the 
siphon. 
A structure to regu- 
late the Arizona 
Canal of the Salt 
River project, at the 
Arizona Power Plant, 
is shown in figure 8. 
The area of the throat 
section is 10 square 
feet, flaring to a maxi- 
mum section of 15 
square feet at a point 
10 feet above the lip 
of the outlet, where it 
tapers to the same 
form and cross section 
as that of the throat. This is the only case of the tapering draft tube 
known to the writer and it is to be regretted that there is no informat- 
ion as to the efficiency of the structure. Such a series of tests as 
would give results enabling the engineering profession to ascertain 
