102 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 78. 



ference, turns by its band a smaller wheel, or spindle, b, of half an inch, sixty 

 times for every revolution of a c. 



When the wheel is intended to revolve in the same direction -^^ith tlie one 



from which it receives its motion, the band is attached as in Fig. 76; but 



•when it is to revolve in a contrary direction, the band is crossed, as hi Fig. 77. 



In many wheels power is communicated by means of a weight apphed to 



the ch-cumference. 



In the tread-mill (Fig. 78) a number of persons 

 stepping upon the circumference of a wheel cause 

 it to revolve. Similar maclunes are often adopted in 

 ferry-boats, moved by horses, and called "horse- 

 boats." 



In most water-wheels, power is obtained by the 

 action of water apphed to the circumference of the 

 wheel, which is caused to revolve, either through the 

 weight, or pressure of the water, or by both conjointly. 



Nvhat is a 220. TliG PuLLEY IS a Small wheel fixed in 



Pulley? 



Fig. 79. 



How many 

 kinds of pul- 

 leys ftre there ? 



What is a fix- 

 ed pulley ? 



Describe the 

 working and 

 advantage of 

 the fixed pulley. 



a block, and turning on an axis, by means of 

 a cord, which runs in a groove formed on the edge of the 

 wheel. 



This simple machine is represented in Fig. 79. 



220. Pulleys are of two kinds ; 

 — fixed and movable, 



221. By a fixed pulley we 

 mean one that merely revolves 



on its axis, but does not change its place. 



Figs. 79 and 80 are illustrations of fixed 



puUeys. In Fig. 80, C is a small wheel turning upon its 



axis, around which a cord passes, having at one end the 



power P, and at the other, the resistance, or weight, W. It 



Fig. 80. is evident tliat by pulling the cord at P, the weight, W, must 



ascend as much and as fast as the cord is drawn down 



As, therefore, the power and the weight move with the 



same velocity, it is clear that they balance one anothei; 



and that no mechanical advantage is gained. 



In all the applications of power there are always soma 

 directions in which it may be exerted to greater advai> 

 tage and convenience than others; and in many cases 

 the power is capable of acting in only one particular di- 

 rection. Any arrangement of machinery, therefore, which 

 ^ill enable us to render power more available, by apply- 

 ing it in the most advantageous direction, is as convenient 

 and valuable as one which enables a small power to balance or overcome a 



© © 



