The scoops are, in fact, large troughs fixed at one end on au axis, and connected at the other to a balance beam, to which an alternate motion is communimted from some extraneous motive power; so that the scoop is made to dip into the water when the beam is at its lowest point, and to discharge the water over the line of its own fixed axis when the btain rises. These engines are applicable to lifts of from 12 to 15 feet, and they certainly work with very economical results. The bucket wheels are, in principle, very similar to the norias, and are used in Switzerland, and in some puts of Germany, for irrigation purposes; they consist of a number of small buckets, arranged on the periphery of a wheel made to revolve in water, no that the buckets become filled at the bottom of the revolution, and discharge their contents at the top of the same. Time dynamical results of these wheels' is not very satisfactory, but they do not require frequent repairs, nor do they require attendance ; conditions which make them desirable for the class of work they are usually' applied to. Persian wheels have been ocasionalty used for raising water from the iuteriors of corer-dams, but are now almost entirely abandoned; they consist of a series of helicoidal channels formed in largo wheels, which are made to dip in the water, and as the wheel revolves the water finds its way to the centre of the wheel, where an outlet is provided for it. It is very rarely that the lift of these wheels eiceeds 12 feet. The dash wheels are, however, the machines of this description of the most common use, and in the polders of Holland they may be seen in Anent every direction in councetion with windmills, whilst in our own fens they are as frequently employed with steam as a motive power. They consist of a series of floats fixed on the periphery of a wheel made to revolve in the water with great velocity, and the tangential force thus applied to the water itself in sufficient to raise it to a height of nearly 20 feet. There 6 a very efficient wheel of this description at work In the beeping Fen, near Spalding, set in motion by a steam engine of 80-horso power; the wheel is 23 feet in diameter, with floats measuring Si feet by 5 feet, moving with a velocity of 6 feet per second on the average; the quantity of water discharged 6 165 cubic feet per second, raised 5 feet. At Ten-mile bank, near Littleport, in the Isle of Ely, there is a wheel of 40 feet in diameter also moved by steam.
The Rotary, or Centrifugal, pumps are those in which a rectilineal vertical motion in communicated to the water to be raised, by means of a wheel, bearing a series of fixed arms rotating at a high velocity in a close drum, and receiving its supply through the apertures in the side of the drum close to the axis. The shape of the arms has a very material influence upon the useful effects of these pumps ; and, after many experiments, it has been found that the most advantageous form to be given to them is to curve them backwards to the directiou of the movement, so as to form a tangent to the circumference of the wheel at the points where they intersect it. Amongst the most valuable forms of these centrifugal pumps for drainage purposes, those maim. factured by Mr. Appold, or by Messed. wynno and Co., are those most generally known; and perhaps it may be desirable to add that the latter work with the smallest loss of power compared to the useful effect with small lifts : Appold'a pumps, however, present more satis factory results when higher lifts are required. All these engines are
inferior to the best forms of force-pumps when large volumes of water have to be raised to great heights ; and their use would appear, in their present forms at least, to be most advantageously limited to lifts of about 30 or 40 feet vertical as a maximum.
The discussion of the motive powers to be applied for the raising of water by pumps, or other machines, will be more conveniently reserved to WATER SUPPLY.
In Air-Pumps the objects to be obtained are twofold : —1st, to exhaust the air under a receiver, for the sake of obtaining a vacuum ; and 2nd, to condense the air no as to produce an increased pressure upon the enclosed surface. The former object is sought to be obtained in philosophical experiments, or in chemical or physical investigations; the condensation of air has lately been applied to many important operations in the arts of construction. The exhaustion of the air is effected by means of one or more pistons working through air-tight cylinders, and provided with a series of valves which allow the air to pass from the receiver to the exterior, but without admitting any return ; so that the air of the interior becomes more and more rarifled in proportion to the effort empleed, and the perfection of the machinery. No instrument of thus description has yet secured a perfect vacuum, though from what has been said in the article Ain-Pumr, some remarkable results seem to have been attained, approximating very closely to that condition. As the mechanism of air-pumps for exhaustion has been described in the article thus referred to, it will suffice lucre to state that in machinery they are principally used for the purpose of overcoming the resistances opposed by the steam left in the cylinders of steam engines, and for removing the contaminated air in or in diving-bells. The condensing air-pumps, in their turn, are made with valves opening inwards only, so as to allow air io be forced auto the receivers without being able to returu. They are employed for the purpose of forcing air into diving-bells, into blowing machinery of mince, and latterly into the cylinders used for making the foundations of bridges upon running sands. In the latter case the compression of the air reaches to the extent of an effort equal to at least three atmos pheres, and time workmen are obliged to carry on their operations in as medium of that density. A description of the machinery of this description used in the Rochester bridge, was given some years since by Mr. Hughes, who executed the works of that remarkable monument, and the system has since been applied with success in the cases of rho bridges of Kiel over the Rhine, and of the railway bridge over the Theiss in Hungary. Coulomb appears to have been the first person to introduce thin system in a tatmorre addressed to the Academie des Sciences in 1778 ; and the French engineers have able° that p erica! frequently employed it in hydraulic, or in mining operations, when the pressure to be overcome has not exceeded the above quoted limit of three atmospheres.