MACHINERY. 441 



Water wheels, turning on a horizontal axis, are distinguished as 

 overshot, undershot, or breast wheels. The overshot wheel, receives 

 the water at, or near its top ; and is turned by the pressure of the 

 water, descending by the force of gravity. In practice, it is capable 

 of raising as much water as is employed in turning it, to about three- 

 fourths of the height of the wheel ; and the velocity of the circum- 

 ference should be between two and five feet per second. The undershot 

 wheel, receives the water near its lowest part; and is turned by the 

 impulse, or momentum, which the water already has on striking it. 

 It will raise an equal quantity of water to only one-third of the 

 height through which the water that turns it would have to fall, to 

 acquire the velocity with which it strikes the wheel. The breast 

 wheel, receives the water at some intermediate height; and the float 

 boards move nearly in contact with a surrounding enclosure called 

 an apron, so that the water can descend only by turning the wheel. 

 It is intermediate between the preceding kinds, in its power ; and 

 is best suited for moderate falls, not sufficiently high for an overshot 

 wheel. 



For great heights, and a small supply of water, the chain wheel is 

 sometimes used, consisting of an endless chain, passing continually 

 around two rag wheels, at the top and bottom ; and hung with buckets, 

 which, receiving the water near the top, descend on one side of the 

 circuit by its pressure. The horizontal wheel, is occasionally used, 

 with a vertical shaft, and oblique floats around its circumference, at 

 the foot. When these floats are placed within a hollow cylinder, 

 receiving the water from above, it is called a tub tuheel. In several 

 of these forms, the effect may be much diminished by the resistance 

 of back water, remaining at the foot of the wheel. Tide mills, are 

 usually undershot wheels ; resting on boats, moored in the current ; 

 and turned by the tide. 



3. Wind Power, is still occasionally used, in those localities 

 where water power is wanting ; although so uncertain in its continu- 

 ance, that its use is necessarily limited. It is ordinarily obtained by 

 means of the vertical windmill, turning on a horizontal axis, and 

 having arms with oblique sails, revolving in a vertical plane. In this 

 case, the sails should make an angle of only about 20 degrees, with 

 the weather, or plane of rotation ; that is, an angle of about 70 with 

 the axis ; and the axis should be capable of turning around horizon- 

 tally, as the wind shifts, in order that it may act in whatever 

 direction the wind blows. The Horizontal windmill, turns on a 

 vertical axis, and has its sails so arranged that they may catch the 

 wind and draw, during one-half of the revolution, but not during the 

 other. Windmills are used for grinding grain ; but oftener for 

 pumping water, either in salt works, or for draining. 



Animal power, is necessarily used more frequently than any other 

 kind; that of men, oxen, and horses, being the most generally 

 employed. Where mere physical force is required, that of oxen, or 

 horses, is preferable, from its being cheaper ; but where strength 

 must be combined with skill, or applied on a small scale, that of 

 man is alone available. The most favourable application of human 

 strength, is in drawing, rather than pushing ; and it is greatest when 

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