The different methods by which water can be made to perform mechanical work are: First, by its weight; second, by shock, as when a stream of water impinges at right angles on a moving surface; third, by action or impulse, as when an unconhned stream of water meets a moving surface, the relative velocity having no portion at right angles to the surface, but gliding along and ultimately leaving the surface; fourth, by reaction, as when a stream of water enters, flows through, and ultimately leaves a moving pipe or channel, which it completely fills; and fifth, by a combination of two or more of the above methods of action. The classification of the motors may be as follows: (a) Water wheels (the water acting on the outside of the wheel) are either undershot, breast, or overshot wheels; (b) turbines (an arrangement where the water acts through the inside of the wheel) are either on the axial or the radial flow system, and may work either by reaction and impulse combined or by pure impulse alone. Water power is useful for any industry requiring slow-moving, regular power, such as corn-grinding, ore-crushing, chemical mixing, etc.

Tide motors may he on two systems: in the former, the tidal waters rush through a small opening into a reservoir, actuating a turbine which is fixed in the opening, and the ebb water rushes out through another opening (the first opening being closed by a penstock or shutter) actuating another turbine.

The cost of the reservoir, which is practically a tidal clock, is very great. In the other system, a series of wooden gates hanging from a frame are set in motion by the rise and fall of the waves, and their motion is conveyed by cranks and rods to an engine. Tidal motors, especially the latter form, are only available for purposes not requiring regularity, such as pumping water for keeping a reservoir replenished.