NOTE.—“Wherever a stream flows from a higher to a lower level it is possible to erect a water motor. The amount of power obtainable depends on the available head and the supply of water. In choosing a site the engineer will select a portion of the stream where there is an abrupt natural fall, or at least a considerable slope of the bed. He will have regard to the facility of constructing the channels which are to convey the water, and will take advantage of any bend in the river which enables him to shorten them. He will have accurate measurements made of the quantity of water flowing in the stream, and he will endeavor to ascertain the average quantity available throughout the year, the minimum quantity in dry seasons, and the maximum for which bye-wash channels must be provided. In many cases the natural fall can be increased by a dam or weir thrown across the stream. The engineer will also examine to what extent the head may vary in different seasons, and whether it is necessary to sacrifice part of the fall and give a steep slope to the tail race to prevent the motor being flooded by backwater in freshet time. In designing or selecting a water motor it is sufficient to consider only its efficiency in normal working conditions. It is generally quite as important to know how it will act with a scanty water supply or a diminished head. The greatest difference in water motors is in their adaptability to varying working conditions.”—ENCYC. BRIT. [Illustration: FIG. 110.] _Water wheels_ are large vertical wheels driven by water falling from a higher to a lower level: they are motors on which the water acts, partly by weight, partly by impulse. _Turbines_ are wheels, generally of small size compared with water wheels, driven chiefly by the impulse of the water. Before entering the moving part of the turbine, the water is allowed to acquire a considerable velocity; during its action on the wheel this velocity is diminished, and the impulse due to the change of momentum drives the turbine. Roughly speaking, the fluid acts in a water-pressure engine directly by its pressure, _in a water wheel chiefly by its weight causing a pressure_. _A flutter-wheel_ is shown in Fig. 110. This is a water wheel of moderate diameter placed at the bottom of a chute so as to receive the impact of the head of water in the chute and penstock. Its name is derived from its rapid motion, the effect of which is to cause a commotion of the water like “the fluttering” of a fowl. _Impact Wheels._—The simplest and most imperfect of the horizontal wheels are the so-called impact wheels or impact turbines, such as shown in Fig. 111. [Illustration: FIG. 111.] They consist of 16 or 20 rectangular blades fastened to the wheel at an inclination of 50° to 70° with the horizon. The water is brought on through a race of 40° to 20° inclination, so that it strikes at about right angles upon the blades.