230

Water-wheels: Pumps.

detail, and he then saw the reasons here stated for approving of the project. There was opposition with unfavourable judgments from many high quarters-as there had been before in regard to novel inventions, such as gas-lighting, locomotive engines on railways, steam navigation on the high seas, the electric telegraph, penny postage, &c.-but gradually the opposition ceased. The vessel referred to was afterwards well known as the Archimedes, so named by the last patentee of the screw-who erroneously thought that the propeller resembled in principle the screw of Archimedes, described in Art. 389. This experiment drew the attention of Government, and of engineers generally, to the subject, and a new construction of mercantile vessels and war ships in all countries has been the momentous result.

386. The operation called sculling, (which is the propelling of a boat or vessel by the use of a single oar, resting on a round-headed prop or nail at the stern, and made to vibrate from side to side,) is referable to this law of oblique liquid impact. In all positions, the surface of the oar pressing the water is turned obliquely backwards; hence the re-action of the water drives the boat forward. In China, vessels of more than 100 tons are moved by a single large sculling oar, which half the ship's company may be urging at the same time. A sculling oar may be regarded as a single vane of such a propelling wheel or water-screw as above described, made to sweep across, behind the vessel, alternately to the right and to the left.

The action of a fish's tail, or of the bending of an eel or snake in water, partly resembles that of the sculling oar. Many people believe that the tail of the fish is only the rudder of the body, and that the fins give it forward motion--as is true of a bird's tail and wings; but the fish's tail is in fact its great instrument of motion, while the fins serve chiefly to steady and direct the motion.

Hydraulic Machines; Water-wheels and Pumps.

387. In the progress of civilization many different means have been devised for the raising of water from depths to supply the lack of natural provision of this liquid, which is to the world of animal and vegetable life what the blood is to the body.

The first improvement on the simple bucket attached to a rope, and pulled up by the hand, was the use of an axle and winch, to lessen the toil of lifting the bucket, and to enable larger buckets to be used.

A further improvement was to have a succession of buckets fixed

The Archimedean Screw.

231

on an endless rope, which passes over two wheels, so that the buckets dip in the water and are filled as they are carried round the lower wheel, and discharge their contents as they pass over the top wheel. This is an old contrivance, and is still in use, especially for the dredging of harbours.

A bucket-machine of this sort, called the noria, is common in the East for irrigation purposes, and in these the buckets consist of a series of earthen pots simply.

388. Instead of buckets on such an endless rope or chain, there may be a series of flat discs of wood or metal drawn up through a large tube or barrel, like loose-fitting pistons, and raising a copious stream. This is the contrivance called the chain-pump, which used to be the only pump in use on board our large line-of-battle ships.

More simple still is the use of a rough endless rope of hair; this, carried rapidly up through a pipe, will bring a considerable quantity of water with it, which will be thrown off by centrifugal force into a reservoir at the top, where it passes over the upper wheel.

389. An ancient contrivance, by means of which water may be readily raised in large quantity to a moderate elevation, is the Archimedean screw. It is represented in fig. 100, and consists of a pipe open at both ends,

wound like a screw upon a sloping cylinder or shaft, and with its lower mouth dipping into the water. At each revolution of the barrel the lower turns of

Fig. 100.

the pipe are filled with water, which, as the cylinder continues to turn, gradually rises to the top, as if drawn up an inclined plane. There are usually three threads of the screw, wound at an angle of about 60° to the axis of the shaft, which should not be inclined at a greater angle than from 30° to 45° to the horizontal.

Archimedean screws are still much used in Holland for draining, and are commonly driven by windmills. They are a simple and economical means of raising water in large quantity to a moderate height, such as fifteen or twenty feet. In France they are sometimes made of five or six feet diameter, and turned by steam power. 390. The Persian wheel is the name given to a simple wheel, by which the streams in Persia are frequently caused, by their own action, to lift a part of their water into elevated reservoirs, from

232

The Persian Wheel: the Hydraulic Ram.

which it again flows in sloping channels to fertilise the fields and gardens. A large water-wheel may be placed so that the stream shall turn it, while buckets around its circumference are filled as they sweep along below, and are emptied into a reservoir as they pass above-or instead of buckets, the spokes of the wheel may themselves be made hollow, and curved as in fig. 101, so that their extremities may dip into the water below and receive a quantity of it, to run along them as they rise, and be discharged into a reservoir at the

centre.

Fig. 101.

391. Of modern water-raising engines the most common and important are the lifting and force pumps. These, being dependent upon atmospheric pressure, wili be described in the following section. The only other machine we need here describe coming under the present head, is the self-acting contrivance now in common use, and known as the

Hydraulic Ram. It has been often observed that while water is running through a long pipe, if a cock at the extremity be suddenly shut, a smart shock is produced there. The reason is, that the momentum or shock-giving power of a moving liquid is the same as that of a solid of the same weight moving at the same rate. Then, as a fluid presses equally in all directions, a leaden pipe of great length may, near the extremity, be widened, or even burst in this experiment. The employment of this forward pressure of an arrested stream for raising water was first suggested by Montgolfier in the end of last century; and the arrangement of parts contrived to render it available has been called, on account of the shock produced, the hydraulic or water-ram. The ram (fig. 102) may be described as a sloping pipe in which a stream flows, having a valve at its lower end which the action of the stream is made to shut at intervals and so arrest itself automatically; a small tube rising from near that end towards a reservoir above, to receive a portion of the water forced up at each interruption. The water allowed to run for a certain time, in a pipe ten yards long, two inches wide, and sloping six feet, acquires momentum enough to shut the valve, a, and force about half a pint of itself into the air-vessel of a tube leading to a reservoir forty feet high. The stop-valve, a, is made so heavy, that the stream must run for a certain time to acquire force

Barker's Mill: the Turbine.

233

enough to shut it; and in the instant of its shutting, a portion of the advancing water passes upwards through the other valve, h, towards the high reservoir. The water in the main pipe, then

becoming stagnant, no longer has power, by its weight alone, to keep the valve, a, shut; this, therefore, falls open and the stream begins again, to be arrested after a time as before; and as long as the supply of water lasts, the action of the apparatus continues. The action of a water-ram has been compared to the beating of an animal's pulse. The upright tube has usually at the bottom an air-vessel or airmatrass, b, which, by the elasticity of the confined air, converts the interrupted gush first received, into a nearly uniform current towards the reservoir. The supply of air to this vessel requires to be renewed from time to time by the contrivance called a shifting-valve, as part of the confined air is continually passing away with the water through the ascension pipe.

Fig. 102.

392. Water-pressure engines may be called the reverse of the hydraulic machines we have just mentioned. In these we have water standing at a height made, by its mere weight, to act as a source of motive power. One of the simplest of these is what is called

Barker's Mill.-This consists of an upright tube, a b (fig. 103), with a funnel at the top, into which water pours

from a reservoir. The water fills the tube, ab, and its two arms, b c and b d, in each of which, near the end, there is an opening from which the water spouts, and by its re-action, or the unbalanced pressure on the interior of the tube opposite to the opening, pushes the arm in the contrary direction. Then as the two holes are on opposite sides of the arms, both cooperate to whirl the axis round, and thereby to turn a millstone above, or to do any other work.

393. The Turbine wheel resembles

Fig. 103.

Barker's mill in principle, although differing in form. It has below, instead of the two arms with spouting apertures, a cylindrical drum,

234

The Centrifugal Pump.

f(fig. 104), close at top and bottom, divided into a number of curved channels as outlets for the descending water, all pointing in the same way, as shown in the sectional view, g. The reaction of the

water as it glances off the curved blades turns the cylinder. This arrangement is equally efficient for large or small falls of water, and has the advantage of being extremely simple, requiring no valves nor internal parts, but only curved vanes or walls of division. They are extensively used as a source of motive power for mills or any kind of machinery, and are exceedingly economical where a plentiful supply of water is available, seeing that they may be made of almost any power.

Fig. 104.

394. Appold's centrifugal pump is a contrivance which may be called the reverse of the turbine wheel. It is simply a turbine placed with its axis horizontal, and having an opening round the axle by means of which the water is admitted to the curved vanes. On being rapidly whirled, the water is, by centrifugal force, moved towards the circumference of the wheel, and may thus be forced up a vertical tube with great efficacy if the height be moderate.