54 
071 Water Supplies suited to 
require changing from time to time. To make a ram work to 
the best advantage, it is necessary to adjust the stroke of the 
outer valve C to the particular conditions of fall and lift, and to 
effect this there is always a special provision on the spindle of 
the valve. The less the stroke of the valve, the faster it will 
beat, and the best point can be determined by measuring the 
quantity of water delivered under various lengths of stroke. 
Fig. 7. — Section of Reservoir with Injection-pipe fitted with Flap-valve. 
When the water supply is scanty, the stream should be 
dammed, and a good-sized reservoir (Fig. 7) should be made 
by the side of the stream for the injection-pipe A to start from. 
The upper end of this pipe is, in all cases, fitted with a flap- 
valve opening outwards, and provided with a chain, so that it 
can be opened or closed at pleasure. But with a scanty flow the 
supply would, in time, become exhausted, the outer valve c would 
refuse to shut, and the water would dribble away through the 
ram to waste, without doing any work. To avoid this, the flap- 
valve at the upper end of the injection-pipe is attached to a 
float, just buoyant enough to keep the flap up, when quite sub- 
merged, but not of sufficient floating power to lilt it when 
closed. A second and more powerful float is attached to the 
valve by a slack chain, or by means of a sliding rod, so adjusted, 
that when the water rises to its full height, it will pull the valve 
open suddenly, allow the water to rush down into the ram, and 
set it going at once. As the water falls, the slack attachment of 
the large float allows it to sink without affecting the valve, 
which is kept open by the smaller float till the water reaches its 
lowest level again, and permits the valve to close. 
The same object can be accomplished by means of a single 
float sliding up and down upon a vertical bar between stoyts. 
