The _turbine_ is a horizontal water wheel, and is similar to the _hydraulic tourniquet_ or reaction wheel shown in Fig. 117. This consists of a glass vessel, M, containing water and capable of moving about its vertical axis. At the lower part there is a tube, C, bent horizontally in opposite direction at the two ends. If the vessel were full of water and the tubes closed, the pressure of the sides of C would balance each other, being equal and acting in contrary direction: but, being open, the water runs out and the pressure is not exerted on the open part but only on the opposite side, as shown in the figure A. And this pressure, not being neutralized by an opposite pressure, imparts a rotary motion in the direction of the arrow, the velocity of which increases with the height of the liquid and the size of the aperture. This description and the illustration gives an idea of the crude _reaction wheel_ invented by Barker about 1740; again a turbine is simply a centrifugal pump reversed, but the turbine is usually furnished with curved _guide vanes_ to guide the water as it enters the wheel. NOTE.—The _steam turbine_ has come into common use and competes in its economical performance with the simpler and less economical types of the steam engine; it is impelled by steam jets, the steam impinging upon vanes or buckets on the circumference of a rotating disc or cylinder. _In those turbines which are without guide blades_—i. e., which have a high fall—the discharged water still possesses a great velocity and the wheel is thereby deprived of a considerable part of mechanical power. This loss can, however, be obviated or lessened by using the energy of this discharged water to drive a second wheel. A construction of this sort has been carried out by Ober-Bergrath Althaus in the tanning mill at Vallendar near Ehrenbreitstein. The essential parts of the arrangement can be seen in Fig. 118. A E A is an ordinary reaction wheel with four curved revolving pipes and a fall of 124 ft., and B B is a larger wheel with floats which is set in rotation by the water issuing from A. A. _Since the two wheels turn in opposite directions, they must be connected together by a special form of wheel-work._ The outer wheel affords the additional advantage of serving at the same time as a fly-wheel, thereby giving a more uniform rate of motion to the whole machinery. [Illustration: FIG. 118.] Turbines are variously constructed, but all have curved floats or buckets against which the water acts by its impulse or reaction in flowing either _outward_ from a 1 central chamber, 2 _inward_ from an external casing, 3 _from above downward_, and, 4 from _below_; these constructions are either divided into _outward, vertical or central discharge wheels_.