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Three types of water wheel which were in universal use before the advent of the steam engine were the undershot wheel, the overshot wheel, and the breast wheel (Figure 30). In the undershot wheel the water stored back of a dam is let out near the bottom of the dam and strikes the under side of the wheel, so that the top of the wheel turns toward the dam. In the overshot wheel, the water flows over the wheel striking the paddles or buckets on top and on the forward side, so that the wheel turns forward. In the breast wheel, the water strikes the paddles half way up the wheel on the rear side and drives the wheel in the same direction as that of the undershot wheel.

When we speak of water power we are apt to think of the water as actually furnishing the energy. As a matter of fact, it is not water but gravity that drives the wheel, the water being merely the medium that gravity acts upon. By having the water drop from a great height, its velocity is greatly increased and the power it imparts to the wheel is much higher. In mountainous regions it is easy to obtain a high head of water and thus generate a great deal of power from a relatively small stream. However, the ancient type of wheel with its paddles or buckets has now practically passed out of existence, being superseded by the Pelton wheel for high heads and the turbine for low heads of water.

FIG. 30.—UNDERSHOT, OVERSHOT, AND BREAST WHEELS