WATER WHEEL. In hydraulics, an engine for raising water in large quan tities. Also a wheel turned by the force of running water. The kinds most known are : the undershot wheel, and the overshot wheel.
The force on an overshot water-wheel is that of the weight of the water fulling on it, the power being to the effect as three to two. The force on an undershot wheel is the velocity of the body of wa ter, the power being to the effect as three to one, or half the other.
The particles of fluids are found to flow over or amongst each other with less friction than over solid substances ; and as each particle has weight, it fol lows that no quantity of homogeneous fluid can be in a state of rest, unless every part of its surface is on a level. As the particles of all liquids are heavy, any vessel containing a liquid will be carried towards the earth by the two fold motion, with a power equivalent to the weight it contains, and, if the quantity of the fluid be doubled, tripled, &c., the influence will be doubled, tri pled, &c.
The pressure of fluids is, therefore, simply as their heights—a circumstance of great importance in the construction of pumps and engines for raising water. As atoms of liquids fall independently, if a hole be made in the bottom of the vessel the liquid will flow out, those particles directly over the hole being discharged first. Their motion causes a momentary vacuum, into which the particles tend to flow from all direc tions, and thus the whole mass of the water, and not merely the perpendicu lar column above the orifice, is set in motion.
When water flows in a current, as in rivers, it is in consequence of the incli nation of the channel, and its motion is referable to that of solids descending an inclined plane ; but, from want of cohe sion among its particles, the motions are more irregular than those of solids, and involve difficult questions. The friction between a solid and the surface on which it moves can be accurately ascertained ; but this is not the case with liquids, one part of which may be moving rapidly and another slowly, while another is sta tionary.
In rivers and pipes, the water in the centre moves with greater rapidity than at the rubbing sides, so that a pipe does not discharge as much water in a given time, in proportion to its magnitude, as theoretical calculations would lead us to suppose. As water, in descending, fol lows the same laws as other falling bo dies, its motion is accelerated ; in rivers, therefore, the velocity and quantity dis charged at different depths would be as the square roots of those depths, did not the friction against the bottom check the rapidity of the flow.
The same law applies to the spouting of water through jets or adjutages.
One of the most useful forms of water, which is that of French invention, called the turbine, is a species of hydraulic en gine, employed to a considerable extent in modern times, as a prime mover for machinery. It is considered to be pre ferable to ordinary water-wheels, in situ ations where the height of the fall is great and the quantity of water not very considerable. The annexed cut repre sents an example of a turbine, or hori ontal water-wheel. The water is intro duced into a close cast-iron vessel a, by the pipe b, connecting it with the reser voir. Here, by virtue of its pressure, it tends to escape by any aperture which may be presented ; but the only aper tures consist of a series of curved float boards ff, fixed to a horizontal plate g, mounted upon a central axis h, which passes upwards through a tube connect ing the upper and lower covers, c and d, of the vessel a. Another series of curv ed plates e e, is fixed to the upper sur face of the disk d, to give a determinate direction to the water before flowing out at the float boards, and the curves of these various parts are so adjusted as to render the reactive force of the water available to the utmost extent in produc ing a circular motion. The machinery to be impelled is connected with the axis h.