— SPILLWAYS FOR RESERVOIRS AND CAHAL3. 13 
They are installed as regulatory devices on numerous canal systems, 
where they work perfectly, because in such capacity they are kept in 
working condition by more or less frequent use of the operating' 
parti, On spillway crests, where they have a limiting range of possi- 
bilities and their operation is less frequent, tending to make them 
less sensitive to the reaction of theoretically determined pressures, 
tliey are not as serviceable, unless the mechanical equipment necessary 
to operate them is more complicated and correspondingly costly. 
The moving parts must be maintained in the same perfect condition, 
as that upon which their design was based if they are to function as 
they are intended, and such ideal conditions are rarely approached 
in actual spillway operation. Most types depend upon some system 
of counterweights to effect adjustment after they have been tilted 
by the action of the water pressure beyond a certain point. Or they 
may be so arranged that the water pressure on the gate or flashboard. 
will cause them to react upon a cylindrical weight in such a manner 
as to make it ascend an incline by means of energy transmitted to 
the ends of the cylinder through ropes or cables; or others are ar- 
ranged so that the tilting of the gate transmits power to the shaft 
from which counterweights are hung, such transmission being 
through chains or cables. Various methods are provided by which 
the introduction of cams or eccentric gearing may result in the com- 
pensation of the accelerated movement which might cause the slam- 
ming clown of the gates or their too rapid rise. 
An automatic form of spillway or floodgate has been installed on 
the Cedar River, near Nashua, Iowa. The design is one of the many 
patented types, but seems to be of simple operation. Figure 3 in 
section shows an outline of the structure, which is mounted on the top 
of a masonry dam 17 feet high and used for the generation of electric 
power. Each panel of the gate is 46 feet long and is so set that it 
will store 7 feet of water above the crest of the dam. To one end 
of a walking beam there is attached a reinforced concrete counter- 
weight and a bar connects the other end with the floodgates. The 
gate is hinged at the bottom so that when the water rises above a 
predetermined elevation the gate is forced down and the counter- 
weight is raised. As the height of water increases the pressure still 
further lowers the gate. When the gate opens the leverage between 
the counterweight and fulcrum increases, while it decreases between 
the fulcrum and gate hinges, and in this way overcomes the increased 
weight of water at every stage of gate opening. 1 
ROLLING DAMS OR BARRIERS. 
Rolling dams or barriers have had their term of popularity and a 
number of them were installed on dams in this country. Possibly 
1 Description from Engineering News-Record, Aug. 9, 1917. 
