In a water-pressure engine there exists the following principal parts: (r) the consisting of a supply-pipe leading from a reservoir to the working cylinder, which pipe, together with the reservoir, constitutes what is called the "pressure column ;" (2) the regulator, a valve which is capable of being adjusted to any required extent of opening; (3) the engine proper, consisting of a piston moving in a cylinder together with the for admitting and discharging the water from the cylin der; and (4) the consisting of a whose final outlet may be either at or below the level of the cylinder.
When a water-pressure engine is spoken of without qualification, a self-acting engine is generally meant—that is, an engine which differs from a mere hydraulic press, hoist, or crane in having distributing-valves for regulating the supply and discharge of the water, which valves are moved directly or indirectly by the engine itself, so that it is a machine which has a periodical motion, and which, having once been started, goes on of itself until it is stopped either by shutting the regulator and so stopping the supply of water, or by disengaging or otherwise stopping the valve-motion.
The water is admitted to the cylinder at high pressure and discharged at low pressure, consequently the work done on the piston causes a recip rocating action. The useful work is due to the difference of the pressure of adlni:;sion and the discharge, whether the pressure is (Inc to the of a column of water whose source is of more or less considerable height, or is artificially produced. In default of a natural head of sufficient power, the head is established in an accumulator of power, which is a body of water driven into a reservoir by forcing-pumps operated by steam. When the water is said to act by pressure, the pressure winch drives the piston is not simply the effect of the weight of a portion of water which] descends, but is the effect of the weight of sonic more or less distant mass of water transmitted through an intervening mass and diverted to any extentin direction or modified in the velocity of its action.
The motion in these machines never acquires a high velocity on account of tile non-elastic nature of water, which prevents its being used expan sively like steam or air, but, for the most part, on account of the kinetic energy of the water being wasted, with the result that hydraulic engines use as much water when running idle as when working at their full power. An engine is single-acting if the water acts on one face of the piston only, and double-acting if the water acts on each face alternately. The valves are sometimes worked by a small auxiliary water-pressure engine, and sometimes by hand, in which latter case the same valve may act as the regulator and the admission valve.
In the rotative class of engines the cylinders are either double- or single-acting, and the piston-rods drive a shaft by means of connecting rods and cranks. To diminish as much as possible the variations of the effort upon the crank-shaft, it is usual to have two, three, or four cylinders acting in succession; but a single cylinder will answer if the fly-wheel is of sufficient inertia.
Water-pressure engines were invented by11 in Hungary in 1749, and at nearly the same time (1753) in Germany by Winterschmidt, and later (1765) in England by \Vestgarcl. In 18°9 they were so improved by Von Reiehenbaeh as to answer their purpose more perfectly, being almost exclusively employed in mine-drainage. This limited application of the water-pr&sure engine is clue, on the one hand, to the necessity for a con siderable fall of water, on which its effectiveness depends, and, on the other hand, to the necessity for an intermediary mechanism to convert the slow reciprocating motion into a rotary motion, which, though causes loss of power and additional expense, is generally required. The pipe conduit system of water-distribution in cities from elevated reservoirs, with a pressure equal to a fall of from 3o to Go metres (too to zoo feet), opened up a new field for the extended use of such engines, but it was not known how to impart a greater velocity to the piston and how to change its slow reciprocating motion into the requisite rotary motion by means of some simple mechanism, until Ramsbottom of England pro duced his small rotary water-pressure engine as shown in Figures 3 and 4 (p/. 64). This motor consists of two vertical cylinders oscillating on a central axis, which operate one crank-shaft by means of two cranks at right angles to each other. Figure 9 shows how the surfaces of contact between the two cylinders and a central post, in which the inner trunnions are journaled, are made water-tight by two adjusting screws (in the hous ing frame) pressing the cylinders against the central piece, which is also connected with the frame. The water is conveyed to the cylinders through the central post and through the channels cast on the cylinders, first over and then under the piston, and is discharged. The alternate induction and eduction of the water from the centre post to the cylinder channels are effected by the oscillation of the cylinders. The Ramsbottom engine is largely used in England for the lighter industrial purposes, such as ope rating printing-presses, lathes, etc., and combines the advantages of sim plicity, compactness, safety, and economy.