THE PULLEY. 



83 



Cog-wheels are most usually made with the teeth 

 on the outside or circumference of the wheel ; these 

 are termed spur-ivheeh. If the teeth are set on one 

 side of the wheels, they are termed crown- 

 wheels. When they are made so as to 

 work together obliquely, they are called 

 bevel-wheels, as in Fig. 62. vig. es. 

 Where the obliquity isi 

 Bevei-whe'eis. small, the motiou may be 

 communicated by means of the univer- 

 sal joint., as shown in Fig. 63. This is 

 commonly used in the thrashing-ma- 

 chine, where there is a slight change 

 in the direction of motion between the 



horse-power and the tlirasher. 



Universal joint. 



SECTION" V. 



Fig. 64. 



THE PULLEY. 



Let a cord fixed at one end pass round a movable 

 grooved wheel, and be grasped by the hand at the other 

 end ; then, in hfting any weight attach- 

 ed to the wheel, by drawing up the cord, 

 the hand will move with twice the ve- 

 locity of the weight. It will, therefore, 

 exert double the degree of force. This 

 operates precisely as a succession of le- 

 vers of the second kind, the fixed cord 

 being the fulcrum, and the cord drawn 

 up by the hand the power. It thus con- 

 stitutes one of the simplest kinds of the 



Pulley doubling the ^^ -n- n t 



force. pulley, l^lg. 64. 



