84 



MECHANICS, 



Fig. 65. 



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 sin- 

 gle pulleys for increasing powder, the most 

 common of which, and least liable to be- 

 come deranged by the cord being thrown 

 off the wheels, is shown in Fig. ^fi. In 

 this and in all similarly constructed pul- 

 leys, the weight is as many times great- 

 er than the power as the number of cords 

 which support the lower block. If there 



be six cords, as in 

 the figure, the weight 

 will be six times the 



Fig. 66. 



^i^ljgiij/ 



Pulley of six-fold 

 power. 



power. 



Where a cord is 

 over a single fixed wheel, 

 as in Fig. 66, or over two or more 

 wheels, as in Fig. 67, no power is 

 gained, the moving force being the 

 same in velocity as the weight. 

 Such pulleys are sometimes, how- 

 ever, of use by altering the direc- 

 tion of the force. The latter is 

 applied with advantage to unload- 

 ing or pitching hay by means of a horse power, saving 

 much time and labor, as shown in Fig. 67. The head 

 of the fork {Fig. 68) is about 28 inches long, and is 

 fitted with steel prongs 20 inches long. The rope at- 

 tached at a passes over the pulley above, by which the 



Pulley with no increase of 



