THE MACHINES OF THE ANCIENT WORLD 19 



the wood and while pounding it out split the block. 

 This use of a wedge for prying objects apart is so old as 

 to be almost instinctive. Where a wedge is driven in 

 by pounding, friction is very desirable, for if there were 

 no friction it would drive hi somewhat more easily but 

 it would slip back and out following each blow. 



If one cuts a long right triangle of stiff paper and 

 then winds it about a pencil, he will see how a screw 

 is but a special form of an inclined plane. Resting 

 a finger nail lightly on the " thread" of the screw he 

 may observe how turning the screw slides the finger 

 up the spiraled incline. In practical applications of 

 this principle the screw turns in a nut. If the nut is 

 prevented from turning while the screw is turned the 

 rotation of the screw will result hi its translation rel- 

 ative to the nut. The screw offers a large mechanical 

 advantage, to which friction, however, imposes a 

 practical limit. 



The pulley followed in the development of man's 

 mechanical ability his invention of rope. For rope 

 he may first have used long vines, and later, using two 

 or more together to obtain greater strength, learned 

 to twist rope. Considerable invention, however, was 

 required to produce a pulley, although the idea of 

 changing the direction hi which a force is exerted by 

 passing the rope over the limb of a tree was perhaps 

 obtained by the easy accident of pulling at some vines 

 which grew over a tree. Between such an observation 

 as this and the idea of reducing the friction by allowing 

 the bar over which the rope passed to roll, there is a 

 large step. The idea that round objects roll more 

 easily than they slide may have come from noticing 



