were only one sheave in the movable block on B, the thrust of the ram would be 2W when lifting a weight W, and the motion of the ram would be only half that of the weight. For two sheaves at B the motion would be magnified four times, and so on exactly as on page 114.

The crane is slewed by hydraulic cylinders, arranged as shown in plan by Fig. 369, and in elevation in Fig. 370. C and C' are equal cylinders, D and D' the rams. A chain is fastened below C, passes over a pulley on the end of D, then round the sheave E on the crane post, over the pulley on the end of D', and is then fastened to a similar point below C'. Evidently if D be thrust out the crane is rotated clockwise, and D' is pulled in, and vice versa if D' be thrust out. There is no energy expended other than that required to overcome the friction,

if the pressure on the water forced out of D' be utilised, but if not then there is just as much energy used as if the ram C had been thrust out against the full load it could

overcome.

Differential Rams.-The last point we have mentioned is of great importance, and requires examination.

In all cases we have a constant head, h say, producing the pressure Po If then A be the area of the ram, a load PoA can be lifted at a very slow speed; if the load be less than PoA, the speed must be controlled by means of the frictional resistance, but in all cases the effort is POA, and during a lift y energy PoAy must be exerted. If the load be small the greater part of this energy is necessarily wasted in overcoming the increased friction which must be applied to keep the motion steady,

and it becomes of importance if possible to avoid this

waste.

We cannot prevent the waste, as in a steam engine, by cutting off the supply, since the water would not expand as steam does, but it can be decreased by the use of the differential ram.

Ac - AA = area of piston exposed to the water pressure in D.

When a heavy weight is lifted, C is open to supply and D to exhaust, and energy PoAcs is used per stroke. But when a smaller weight is lifted both C and D are connected to the supply, so that the ram moves out under a pressure,

and less energy is used per stroke; or, looking at it another way, a less quantity of water is used from the supply pipe, the quantity Acs enters C, but a quantity (Ac-A)s is forced from D into the pipe, so the quantity used is only As instead of Acs. This latter consideration shows also the gain of energy, for to force the water to the elevation h, or into the accumulator against the pressure Po or wh, requires the doing by the engine of an amount of work wh ft.-lbs. for every lb. of water supplied.

This device is also used in the Differential Accumulator, Fig. 372.

The central spindle is in two lengths of different diameters, and the cylinder slides on it, the water entering through the dotted channel through the centre of the spindle. The effective area is the difference of the areas of the two parts of the spindle, and thus the latter can be made stout, without requiring the weights W to be very large to produce the required pressure.

3. In the third type of pressure machine, the water drives an engine almost identical in its arrangement with a steam engine; no fresh principle is involved, and want of space forbids our entering into details. Hydraulic Brake. The hydraulic cylinder is often used simply as a brake.

Fig. 373 shows a common form; the rod A is connected to the body whose motion is to be controlled, and as the rod moves the water is driven

W

W

supply

Fig. 372.

by the piston B through the pipe C from one end of

C

A

Fig. 373.

the cylinder to the other.

In C is a cock D, by means

of which the value of F is made anything we please; there is now no useful resistance.

If the body is always to be brought up in about the same space, and is subject to the same forces, as a gun fired with its ordinary charges, then F will not require to be varied, and the pipe and cock may be dispensed with, their place being taken by a hole or holes in the piston itself, the resistance to flow through the orifices supplying the necessary retarding force.

Pumps. If a pressure machine be worked backward, the water passes from the exhaust pipe into the cylinder, and is then forced back into the accumulator or reservoir through the supply pipe. But a machine which does this we call a Pump, so that a pump is simply a reversed pressure

A

engine or motor, and the theory of its action is similar to that of the engine.

This kind of pump can be divided into two classes, viz. Lift or Bucket Piston Pumps, and Force, Plunger, or solid Piston Pumps.

Lift Pump.

Fig. 374 shows the construction of a lift pump. A is the suction pipe dipping in the water, its top can be closed by the valve a, B is the cylinder, C the piston or bucket, perforated to admit of the passage of fluid, but the holes can be closed by the indiarubber or leather valve b. The first few strokes remove the air from and thus lessen the pressure in A, so that the water rises in it to a height h, given

Fig. 374.

by (Po - P)/w, where Po is the atmospheric pressure on the water surface, and P is the pressure of the air under C; this height will not exceed 25 feet, since a perfect vacuum cannot be obtained Now as C descends the water passes through it, but cannot re-enter A since the valve a will shut (in Fig. 374 they are both shown open for clearness, but this cannot be during the working), then as C rises b closes and the water above C is lifted to the spout whence it flows, while the reduction of pressure under C allows the valve a to open and water to enter the lower part of B, following C up, so long as C does not rise more than about

25 ft. above the surface, as stated above.

If a somewhat higher lift be required it can be obtained by fitting a valve or valves to the top of the pump, as shown at cc in Fig. 375. The bucket is rising, is shut and a open, and с с are also open to allow the water to pass through them and by the pipe D to the required place. When the bucket descends the valves cc close, so the water cannot follow the plunger down, and prevent the valve lifting, as it would do if cc were not fitted. с с

A

Fig. 375.

are called the Head Valves, and b the Foot Valve.

Force Pump.-When a great lift is desired then the bucket valves do not work well, and it is better to have a solid piston or plunger.

Fig. 376 shows the arrangement.

The delivery pipe

D now leads from the same end as the suction pipe, and