HYDRAULIC MACHINES. Under this head we propose to notice various machines which are at times employed in lieu of pumps for raising water, or which have been proposed for that purpose. The most simple, as well, perhaps, as the most ancient (next to the bucket and windlass), is the Cochlion Water Screw of Archimedes. It consists of a cylinder of wood, of about a foot in diameter, and of any length at pleasure, and on this a leaden pipe of any bore is wound from the bottom to the top. When the bottom of the cylinder revolves in the water (by means of a common winch handle at top, and of a pintle in the centre of its base, which rests in a bore or step for that purpose below,) the reclined portion, as shown in the following figure, occasions the water to enter the mouth of the pipe, and as by its gravity it naturally occupies the lowest part of the pipe, whilst by the revolution of the cylinder the orifice of the pipe is gradually elevated, and a different portion of the pipe occupies the lowest position, the water advances progressively along the pipe, always occupy ing the lowest portion of the bends or turns of the pipe, until it at length reaches the top of the cylinder, and is discharged into a vessel. This, however, raises but a small quantity, although the height may be indefinite; therefore, when this machine is used, it will be found eligible to cover the whole surface of the cylinder with a number of pipes laid close together, or, what is a better method, and is that which is usually adopted, is to wind a number of spiral feathers round the cylinder, standing out from it at right angles like the square threads of a screw, and covering these feathers with an exterior case closely fitting in every part. These machines were formerly in great repute ; but, owing to their liability to become choked with weeds and mud, they are not often employed at present.
The figure on page 720 represents what is called the horn-drum ; it is formed of a number of segments passing from the circumference of a large flat cylinder to its centre. This affords an easy mode of raising water. The mouths or scoops by turns dip into the water, and as they rise cause it to pass up the horn or segment, until it is discharged into a trough placed under the end of the axis, which is hollow, and is formed into a number of separate compartments, each communicating with one of the horns or segments. One disadvantage
of this machine is that it raises water no higher than the axle, and is therefore only applicable in situations where the water is required to be raised to an inconsiderable height. This circumstance renders it necessary to construct it of double the diameter of wheels that discharge their water at their tops. This machine, however, might be altered to do so likewise, by confining the scoops to near the periphery of the wheel, and discharging them by means of lateral valves, to be opened by coming against a contrivance fixed at the top of the wheel for that purpose. The Persian wheel, represented in the following engrav ing, is free from the defect of the last described. In this machine a number of rectangular buckets aaa a are hung upon strong pins b b b b, fixed in the side of the rim, the diameter of which must somewhat exceed the height to which the water is required to be raised. As the wheel turns, the buckets on the right hand descend into the water, where they are filled, and return up full on the left hand, until they arrive at the top at k, where they strike against the end a of the fixed trough m by which they are overset, and thus discharge the water into the trough, from which it may be conducted by pipes to wherever it is required ; and as each bucket gets over the trough, it falls again into a perpendicular position, and so goes down empty until it comes to the water at o, where it is filled as before. On each bucket is a spring r which, as it passes over the edge of the trough, elevates the end of the bucket above the level of its mouth so as to discharge the whole of the water into the trough. These springs are likewise useful in preventing the concussion of the buckets against the trough. This machine, as well as the horn-drum, is frequently driven by means of floats attached to the opposite side of the rim.