234 SCIENTIFIC AGRICULTURE. 



These few elementary powers are, the lever, the wheel and 

 axle, the pulley, the inclined plane, the wedge and the screw. 



The lever is a straight bar placed upon a supporting point 

 called a fulcrum, with the resistance which i's to be overcome, 

 at one end, and the power applied, at the other. 



The wheel and axle is somewhat more complex than the 

 lever ; it consists of two concentric wheels, one of which is 

 larger than the other, and both revolving on a common axis. 

 This power acts like a succession of levers, and is therefore a 

 a modification of the lever. 



The pulley consists of a flat disc, with a groove on the edge, 

 through which a rope passes, and a hole in its centre, through 

 which a fixed axis passes, on which it revolves: when several 

 pulleys are combined, they constitute a system of pulleys, or a 

 compound pulley. The power of a system of pulleys increases 

 in proportion to the number of pulleys employed. 



The inclined plane, as its name implies, consists merely of a 

 plane surface, with one of its ends higher than the other, so 

 that the plane forms an angle with the horizon. 



The wedge may be considered as two inclined planes with 

 their bases placed together, and their apices forming an acute 

 point. The power of the wedge depends upon its relative 

 length compared with the width of its base, or upon the de- 

 gree of taper from the base to the point. 



The screw is the sixth mechanical power, and may be con- 

 sidered a continuous spiral wedge, or a modification of the in- 

 clined plane. The power of the screw depends upon the rela- 

 tion between its circumference and the distance between its 

 threads. 



OBJECTS AND ADVANTAGES OF MACHINERY. 



No actual power is ever generated by machinery ; force and 

 velocity may be gained, but they are always gained at the 

 expense of the motive power applied to work the machine: 

 the power and force must always be in exact proportion to 



