16 BULLETIN 831, TJ. S. DEPARTMENT OE AGRICULTURE. 
These were of comparatively small dimensions, however, and of a 
rather low degree of efficiency — estimated at from -10 to 50 per cent. 
A higher state of development was later obtained in France, where 
reinforced concrete was used and more efficient functioning effected by 
proper proportioning of the essential parts of the structure. Fear 
of the possibility of freezing weather interfering with the operation 
of the siphon spillways is said to be the principal reason why Ameri- 
can engineers failed to adopt this type of structure, but a modifica- 
tion of the design led to the overcoming of this difficulty and to their 
rapid adoption at a number of points throughout the United States. 
The lowering of the intake leg well below the normal water surface, 
and the draining of the outlet sealing basin to prevent the possibility 
of clogging at that point, did away with the ice menace and also 
prevented the possibility of debris collecting or lodging in the throat. 
There should be little argument against the use of the siphon spill- 
way on canals or in connection with reservoirs where absolutely auto- 
matic control without the maintenance of mechanically operated de- 
vices is desired, and where there must be close regulation of the rise 
and fall of the water surface. Economy of cost, space, and rapidity 
of control greatly argue in favor of such structures, especially where 
laws require close regulation of the pond level, as in Italy where 8 
inches is the limiting range. It is also most efficient where immedi- 
ate response to sudden rises in water level is essential, because of the 
fact that it is brought into action to ite full capacity with the rise of 
only a few inches, whereas the overfall spillway is not fully effective 
until the danger point is reached and then is dependent upon the 
stored head for acceleration of velocit}\ 
The use in this country of siphons as spillways is much more re- 
cent than it is in Europe, some of the earliest examples being con- 
structed in 1910 on the New York State barge canal. 1 Since then 
they have been used on the fore bay of a hydroelectric plant of the 
Tennessee Power Co. and on the power canal of the Mount Whitney 
Power & Electric Co. in California. They have been used still more 
recently on several projects of the United States Reclamation Service 
and on a number of irrigation and power projects in California, 
the notable cases being the Orland, Salt River, Yuma, and Sun 
River projects of the Reclamation Service; the South San Joaquin 
Irrigation District ; the Sweetwater Dam near San Diego ; and the 
Southern California Edison Co. at Huntington Lake, Calif. 
As has been shown, the head utilized to produce flow in an over- 
flow spillway is figured from the surface of the water above the spill- 
way to its crest, although the actual head available, but which is not 
utilized, is much larger, being the total head from water surface 
1 Engineering Record, July 30, 1910, and Oct. S, 1910. Eng. News, Nov. 17, 1910. 
