Mechanic Powers. 239 



whole weight is sustained, the more powerful 

 will it be. Or if it is a wheel, the more its dia- 

 meter exceeds the diameter of the axle, the 

 greater will he its power. Thus, if the diameter 

 of the wheel is eight times as great as that of the 

 axle, it will have eight times the power ; and a 

 man who by his natural strength could only lift 

 a hundred weight, by this machine will be en- 

 abled to lift eight hundred. 



Of this kind are the machines called cranes, 

 which you see employed at the water-side, for 

 winding up bales of goods out of ships. The 

 large circular crane, in which a man or horse 

 walks and turns it horizontally, is also a machine 

 of this nature; and the capstan^ which draws up 

 the cables of ships, and is turned by hand-spikes 

 inserted in holes at the end of the roller or cap- 

 stan. The windlass, also used in warehouses 

 for raising goods, is the wheel and axle ; and, 

 indeed, many more complex machines may be 

 resolved into this principle. 



The spokes of the wheel, or the winch which 

 turns the axle, may be considered as levers, and 

 therefore by some the wheel and axle are referred 

 to the same principle. 



III. The pulley is usually considered as the 

 third mechanic power, though, in truth, the 

 single pulley AA (fig. 99.) gives no mechanical 

 advantage, and only enables us to change the 

 direction. This is evident from the figure, where 

 the two equal weights W and P balance each 



