Of horizontal wheels: In the proper turbine (from Ital. turbino, a whirlwind), the water passes either, first, vertically down through the wheel between fixed screw blades, which give it a spiral motion, and then strikes similar blades attached to a movable spindle, but placed in the opposite direction, so that the impact of the water communi cates a rotatory motion to the blades and spindle, or second, a modification of the fore going is to pass the water from the center horizontally outward through fixed curved blades, so as to give it a rotatory or tangential motion, and thereby cause it to act on the blades of the wheel which revolves outside.
In the reactionary wheel, which is in principle almost identical with Whitelaw and Stirret's wheel, previously described under the article BARKER'S MILL, the water is admitted at the center of the wheel from below, passes to the circumference between curved blades of the wheel, and escapes by tangential orifices at the circumference, there being valves made to open more or less, according to the quantity of water and to the power required.
The vortex wheel of prof. Thomson takes in the water after descending through tubes, at the circumference, where, by means of fixed blades, it acquires a tangential motion, and then passes through between the curved arm of the wheel, and escapes at the center. As the two last described wheels work always under water, they are not liable to be obstructed by back-water, or to have their power lessened thereby more than what is due to the diminished fall, and they are understood to yield a good percentage of power, sometimes stated at 75 per cent; hut all turbines are somewhat delicate, and liable to be choked by leaves or twigs, unless the water be carefully strained. Although only a few
horizontal wheels have been described, their name is legion, and it would take a book to mention them all, or to describe their respective merits.
The reciprocatory hydraulic engine works exactly on the same principle as the ordi nary non-condensing steam-engine. The water, under considerable pressure, is admitted at one end of a cylinder, the exit valve at that end being simultaneously closed, while it is shut off from the other end, and the exit valve there opened; and so the alternating action of the valves and of the piston goes on continuously. To work smoothly and effectively,ithe piston ought to be of large diameter, in proportion to the length of stroke, and to go slowly; otherwise the quick jerkin is apt pp pt to shake and to injure the engine; and generally it is better to have two cylinders and pistons together, as that enables them to work more equally, and to turn the crank without the working of a flywheel.
Both the turbine and the reciprocatory engine have been made use of as water meters.
The turbine and the reciprocatory engine have the being advantage of bein able to take the use of a fall much greater in height than the diameter of the largest wheel that can dae made; but for all ordinary falls, a good breast or overshot wheel, or even an undershot, is, on the whole, generally considered better.