'The butterfly-valve, exhibited in fig. 14, varies from the two former, in having two semicircular flaps appended by hinges to a bar passing over the centre of the exca vated piston. This valve is peculiarly eligible, because if one part should be stiff and adhere to the piston, the other will play with an increased effect, though not equal to the action of both valves.
The simplest valve with which we are acquainted is the sphere, which is made of metal, and fits into a semi-spherical cavity on the top of the piston or block. When the piston (if it he on that) rises, the sphere falls into the socket; but when the piston is depressed, the rush of water from below forces the sphere upwards. The only inconvenience attendant upon this valve, which is shown at fig. 15, is, that its diameter, being nearly equal to that of the bore, leaves a very narrow passage for the water. This, however, might perhaps be obviated, by making an excavation in the pipe, as shown by the dotted lines, and by driving nails through, to obstruct the ball from rising too high.
These are the general principles of the valves in common use ; though we could enumerate a great variety, which have all been strongly recommended, but in prac tice proved very deficient. We shall, therefore, proceed with the detail of hy draulic machines, commencing with those which supply the place of pumps, by rais ing water to given heights. The most simple, and, perhaps, the most ancient, is the spiral pump of Archimedes. It con sists of a cylinder of wood, about a foot in diameter, and of any length at pleasure : on this a leaden pipe of any bore is wound from the bottom to the top, spirally.— When the bottom of the cylinder revolves in the water, (by means of a common winch handle at the top, and of a pintle in the centre of its base, which rests in a box or step for that purpose below) the reclined position, as shown in fig. 16, oc casions the water to enter the bottom of the pipe, and to be carried by the revolu tions of the cylinder completely up to the top, where it discharges into a vessel.
This, however, raises but a small quanti ty, though the height may be indefinite : therefore, where such a machine is in use, it will be found eligible to have the whole cylinder covered with various pipes, like the bands in a rope, whereby the quantity of water raised would be pro portionably increased with very little ad dition of power : the greatest resistance would arise from the friction upon the supporting axis, especially the lower one under the surface. Some of these ma chines have been worked in strong run nig brooks, by means of water-boards, the same as the great wheels in under shot mills.
The born-drian, so called from a number of segments passing from the circumfer ence of alarge flat cylinder to its centre, is an easy mode of raising water. The scoops or mouths, by turns, dip into the water, and as they rise cause it to pass up the horn or segment, until it is discharg ed into a trough placed under the end of the axis, which is hollow, and has its pin tle fastened to a cross, as seen in fig. 17. Such wheels usually work with water (or float) boards ; and some of them have projecting fins, from which rectangular buckets are suspended ; these dip into the water as the wheel turns, and succes sively discharge into a trough, by means of a pin at A, which causes every bucket, as it passes, to turn to a horizontal instead of an erect position. The latter invention is ascribed to the Persians. The reader will, no doubt, readily perceive, that a strong current, or other force, is needful to move machines so laden as the Persian wheel, it sometimes raising near a ton of water in each revolution ; and that no thing but the necessity for raising water could induce to so great a loss of power. When treating of MILLS, and of Pus:rs, as also of PNEUMATICS, with which HYDRAU.