device for utilizing the power of flowing or falling water. The Norse wheel is the oldest type known. Despite its name it probably originated in the Middle East, where the swift stream required by this type of wheel is common. The Norse wheel has a vertical shaft directly connected at the top to a millstone; the lower end of the shaft, with vanes or paddles attached, dips into the flowing stream. In the 1st cent. B.C.
a horizontal shaft came into use; the wheel attached to it had radial vanes around its edge. Among the early forms of this wheel are the overshot wheel, used where water falls from a height, striking the vanes from above; the breast wheel, employed where the height of the water is less than the height of the wheel so that the water strikes the wheel about midway; and the undershot wheel, usable where the water flows more or less on a level but with a swift current and strikes the vanes on the under part of the wheel. One of the first uses of the steam engine was to drive a pump that raised water into a millpond whose spillway drove a water wheel. Today the water wheel has been largely replaced by the turbine
. See hydraulic machine
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