THE PULLET. 



61 



municated by means of tlie universal joint, as shown in 

 fig. 63. This is commonly used in the thrasking-machine, 

 where there is a slight change in the direction of motion 

 between the horse-power and the thrasher. 



THE PULLEY. 



Y\^. 64, 

 (illll ilill/'/// 



Pulley doubling tlie 

 foice. 



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 lifting 

 any weight attached to the wheel, by 

 drawing up the cord, the hand will move 

 with twice the velocity of the weight. 

 It will, therefore, exert double the degree 

 of force. This operates Fig. 65. 

 precisely as a succession of 

 levers of the second kind, 

 the fixed cord being the 

 fulcrum, and the cord drawn 

 up by the hand, the power. 

 It thus constitutes one of the simplest kinds 

 of the pulley, fig. 64. 



The wheel is called a sheave ; the term 

 pulley is applied to the block and sheave ; 

 and a combination of sheaves, blocks, and 

 ropes is called a tackle. 



There are various combinations of single 

 pulleys for increasing power, the most com- 

 mon of w^hich, and least liable to become 

 deranged by the cord being thrown off the 

 wheels, is shown in fig. 65. In this and in 

 all similarly constructed pulleys, the weight p^ii^f six-fold 

 is as many times greater than the power as i^^^^- 

 the number of cords which support the lower block. If 

 there be six cords, as in the figure, the weight will be six 

 limes the power. 



