346 OUR PHYSICAL WORLD 



drops, it unwinds the rope, causing the wheels to revolve, and so 

 winds up the rope on the large cylinder and raises the weight. 

 Since the number of cogs on the wheels will be in proportion to 

 their size, the mechanical advantage may be found by dividing 

 the number of cogs on the large wheel by the number on the small. 

 If power and weight were interchanged, then the weight would be 

 moved rapidly, but at the expense of power applied. On a hand 

 derrick, which combines the advantage gained from the use of a 

 crank with that of the train of wheels attached to the crank 

 (Fig. 1 86), one man may lift a weight of several tons, but his 

 hand on the crank handle must move through a distance of 

 many feet to raise the weight a few inches. 



FIG. 187. The sprocket wheel and chain on a bicycle 



The sprocket wheel on the bicycle is a familiar illustration 

 of the use of such gears (Fig. 187). The pedal shaft and axle 

 form a windlass which increases the power applied by the pedal 

 to the sprocket wheel. Power is lost as this plays into the small 

 gear wheel on the hind axle with which it is connected by the 

 chain, but speed is gained and this is desired. 



The pulley is another simple machine. In its simplest form 

 it consists of a single wheel over which a rope passes. The weight 

 is on one end of the rope, and the power is applied to the other 

 end. The pulley simply serves to change the direction of the 

 application of the power, but this is often convenient. Thus, 

 in hoisting hay into the barn loft, one can stand on the ground, 

 put his whole weight on to the rope that passes over the pulley 

 fastened above the window, and pull the hay up. In raising 

 a flag on a flagpole, it is much easier to tie it to a rope that runs 



