WATER-POWER and WATER-PRESSURE ENGINES. The weight and the impetus of a stream of water are frequently employed in mechanics as a source of motive power for the machinery used in industrial operations, in the form of the various kinds of water-wheels; and of late years the law by which water and other similar (practically) incompressible fluids transmit pressure in every direction, and tend to assume a constant level in any reversed syphon, has been made to serve the purposes of mechanics by furnishing the power exerted by the so called hydraulic cranes and the water-pressure engines. The term water-power, therefore, simply refers to the force developed by the water in its natural fluid state; and it acts either by its weight, its impact, or by its power of transmitting an effort exercised upon any part of its circumference. In an overshot wheel the water acts almost exclusively by its weight, which is applied on one side of an unbalanced wheel; in an undershot wheel the water may often exercise power by reason of the velocity with which it is animated when it strikes the floats ; in a reaction wheel the power is produced by the escape of the water from a pipe reacting upon the air surrounding the apparatus; and in the hydraulic cranes, the power is obtained by forcing water into a raised cistern, from whence it is able to act, as workmen say " with a head," upon the underside of pistons bearing loaded surfaces. Virtu ally it is the weight of the water which produces the useful effect in all these cases ; but the interferences with its action are so numerous as to justify the popular distinctions above referred to.
The various descriptions of WATER-WHEELS are described under that head ; at present it is intended only to notice the water-pressure engines used in mining operations, and the hydraulic cranes ; because they constitute applications of water-power of so peculiar a nature, as not to justify their being included under the more generally known classification of water-wheels or engines. The mode of their applica tion is also different from that of the ordinary forms of water-engines; the latter being usually employed for the purpose of driving machinery or mill work, the former for raising water, or for hoisting weights only.
The water-pressure engines are constructed upon the principle of collecting, in a tube of a certain height, a quantity of water, and in allowing that water to escape when it has produced the desired effect. This is accomplished by placing the underside of a piston, moving in a vertical cylinder, in communication with a column of water, and in cutting off that communication when the piston has arrived at the head of its stroke. The pressure of the column of water acts, in fact,
to raise the piston, to which the pump rods are attached; and the alternate downward motion is effected by the weight of the pump rods themselves, in the same manner that the pump rods of the Cornish eigines work : the bottom of the cylinder is placed in communication also with the outflow, so as to allow the water to escape after it has done its work. The passages for the water are opened and closed by a series of tappets and equilibrium valves of a peculiar description, in order to avoid any abrupt hydraulic jar from the change in the con ditions of flow in the descending main; these details are, however, of too complicated a nature to allow of their being represented here, but they may be studied in the notice of the water-pressure engine at Huelgoat in the Annalea des Mines,' or in Burst's GE ologie appliqués.' In the best engines of this description the useful effect obtained is usually about 0.45 of the power exerted ; though it has been stated that in the pumps lately executed at Freyberg, as much as of the real power has been used. Mountainous districts are the most favourable for the establishment of the water-pressure engines; for it is only in them that the necessary conditions for their economical working occur naturally. These are, that a sufficiently copious supply of water should exist at a considerable height above the seat of the piston ; and that a free discharge for both the water which has served as the motive power and for the water raised should exist. The Huel goat engine is placed at a distance of 360 feet from the surface, and it raises the water from a mine 754 feet below the level of the cylinders; the diameter of the piston is 3 feet 43 inches, its height 9 feet, and the length of the stroke 7 feet 61 inches ; it makes, when in full work, 51 strokes in a minute, and raises through the total height of 754 feet, in one lift, 396 gallons per minute ; there are two cylinders, but the galleries are not sufficiently advanced to keep them constantly at work : the descending and outflow pipes are 15 inches in diameter, the pump barrel is 18 inches in diameter, and the ascending pipe is 101 in diameter. M. Rcichenbach has executed for the salt springs of southern Bavaria a great number of these water pressure engines ; and it may be desir able to add that the one at 11Isang is set in motion by a fall of water 328 feet in height, and that it raises, in one lift, not less than 1364 cubic feet of water.