56 
On Water Supplies suited to 
injection-pipe will discharge, as compared with a 3-inch, as 
2"^ is to 3^ ; that is, as 4 is, to 9. The water required to work 
.„ , . , 25 gallons X 4 „ 
it will thereiore be ^ — g =11 gallons per minute, 
and the quantity raised when the lift is, say, 10 times the 
fall = 4 gallons X 4 _ ^^^^ ^ little more than half a gallon 
per minute. It is best, for many reasons, when more water is 
required than one moderate-sized ram will supply, to arrange a 
battery of two or more. 
Rams are generally placed in vaults made by the river-side 
(Fig. 7), so as to be entirely protected from frost. They will 
work for years with very little attention ; they require no 
oiling, and the working parts wear very slowly. 
The price of a 3-inch ram put up complete is about 35Z., 
including the vault in which it is placed. 
When the falls exceed 30 feet, turbines of small power have 
to revolve at a destructive speed, and hydraulic-rams knock 
themselves to pieces very quickly ; it is then necessary to have 
recourse to water-pressure engines. These are either rotatory 
engines, in appearance and structure very like ordinary steam- 
engines, and acting precisely in the same manner ; or they are 
direct-acting pumping engines, in which the stroke is regulated 
by automatic valve-gear, worked without the intervention of 
rotatory motion. 
The hydraulic-engine differs from a rotatory single-cylinder 
engine only in having the pipes, ports and passages much 
larger in proportion to the volume of the cylinder than is 
required for steam ; this is in consequence of the greater 
density of water and the large amount of friction which results 
when it flows at high velocities. The slide-valves also have to 
be so arranged that the water can never be imprisoned in the 
cylinder, for water is incompressible, and if imprisoned, the 
engine must either stop or something must give way. As 
the motion of a piston is constantly changing from being sta- 
tionary to a high speed, and then becoming stationary agahi, 
the water which flows into the cylinder partakes of the same 
motion ; and therefore in order to avoid destructive shocks in 
the necessarily long supply pipe, it is desirable to place an air 
vessel close to the engine, and arrange means of keeping it 
properly supplied with air, if the water tends to absorb it, as is 
frequently the case. 
The ends of the cylinders also should have small passages, 
covered by valves opening outwards, communicating with the 
pressure-pipe. These, in ordinary working, stre kept closed 
by the pressure in the main ; but if from any cause undue 
