COMMUNICATION OF MOTION IN MACHINES. 59 



to another; and much of the skill of the engineer 

 consists in his adapting certain methods to his par- 

 ticular purposes. 



Sometimes a simple cord, or a cord with pulleys, 

 may be used. Levers, either simple or combined, 

 and either straight or bent, are employed to com- 

 municate, and also change the direction of the 

 motion. Rods also are employed, which may be 

 carried to a great distance by being connected 

 together. 



But of all the methods of communicating mo- 

 tion, that by means of wheels is the most frequent. 



Wheels may be made to turn each other even 

 by the simple contact of their surfaces when 

 pressed together ; or their circumferences may be 

 formed into brushes with short thick hair, which 

 enable them to turn each other with considerable 

 power ; or they may have cords or straps of leather 

 or chains, passing from one to another, and then 

 sometimes there are points or protuberances on 

 the rims of the wheels. 



The most usual method, however, of making 

 wheels drive each other is by means of teeth. 

 These are either cut into the substance of which 

 the wheel is formed when it is metal; or formed at 

 the same time as the rest of the wheel when this is 

 cast metal ; or formed by inserting them when the 

 wheels are of wood. 



The proper method of shaping the teeth of 

 wheels, so as to communicate the motion equably, 

 and with as little friction as possible, is a matter of 

 very great nicety, and has given rise to much 

 study among mechanics. The ends of the teeth 

 should be curves, but not parts of circles. They 

 may be formed of the curve called the epicycloid; 

 or of the involutes of circles, which are curves de- 



