BRIDGE, CANAL, DOCK, HARBOUR, &c., these matters have been briefly touched upon.
Hydraulic machinery has been brought to its present efficient state by many successive improvements. It may be remarked however that, on account of the high degree of perfec tion which, within a few years past, the steam engine has attained, the employment of hy draulic machines for raising great quantities of water, or as first movers with respect to extensive works of any kind, has of late con siderably diminished. Yet, where the circum stances are favourable, as when a supply of water for working the machine can be readily obtained; the latter, from being less expensive in their construction, are still preferred to the former. • The principal hydraulic machines Which are in use for domestic or general purposes are the Siphon, the Screw of Archimedes, the va rious kinds of Pumps, the Hydraulic Press, and Wheels turned by water. For the first two see SIPHON and SCREW PROPELLER. The others will beLLOW briefly described.
The common pump is a machine for raising water by the pressure of the atmosphere. It consists of a cylindrical body, or barrel, from the lower part of which a tube descends into the water contained in the well or reservoir. In the interior of the cylinder is. .a Moveable piston, surrounded with leather, in order that it may be water-tight, yet capable of moving up and down with freedom. The piston is perforated, and the orifice is covered above by a valve which opens upwards : a similar valve at the bottom of the cylinder, or barrel covers the upper extremity of the tube which leads to the well. The piston being raised by means of the handle; the air contained in the tube tends by its elasticity to occupy the lower part of the .cylinder, which it enters by forcing up the last-mentioned valve ; and its elasticity diminishes in consequence of its occupying a greater space than before. Hence the air exerts on the surface of the water within. the tube a less pressure -than that which the ex ternal air exerts ppon the water in the well; and the water consequently rises in the tube to a certain height. The valve then falls over the orifice, and, :the piston being depressed, the air contained between it and the bottom of the cylinder will be condensed; in which state it will force up the piston valve, and escape at the top of the pump. The valve
then falls ; and, if the piston be again ele vated, the. water will rise higher in the tube, for the same reason as before. The operation of raising and depressing the piston being re peated a few times, the water will at length enter into the cylinder through the valves ; after which it will, at each stroke of the pis ton, be forced through the spout.
. The lifting pump is frequently similar in construction to the common pump above de ' scribed; but the lower valve is always below the surface of the water in the reservoir, and the piston is so when depressed to bottom of the cylinder or barrel. On raising the piston, the water above it is lifted up, and the pressure of the external atmosphere forces the water of the reservoir to enter into the cylinder. Then, by successive depressions and elevations of the, piston, the water is at length raised to the top of the pump, and dis charged by the spout.
The forcing pump is one in which the water, when raised in the barrel,. is driven through an orifice in its side by the depres sion of the piston, which is solid, or without a perforation ; it is also, in general, provided with an air-vessel, into which the water is forced, and whence, by the elasticity of the condensed air, it is made to issuo through a pipe inserted in the upper extremity.
In the chain pump a chain, carrying a num ber of flat circular pistons, passes round a wheel at the upper, and sometimes also at the lower extremity; each piston, as it goes over the wheels, being in part received in the inter vals between the radii. The wheel being put in motion, the pistons descend in a barrel on one side, and enter frqm below into another on the ascending side, when pushing the water before them, they raise, it into the servoir, from whence it escapes, by a pipe. Pumps of this kind are frequently fixed in in clined positions; and it is when the inclina tion of the barrel is about 21 degrees, the distance of the pistons from one another being equal to their_ diameter, that the greatest quantity of water is raised.