110 



THE SCREW. 



FIG. 60. 



and the weight remaining the same, the distance moved is 

 twice as great as when only one wedge is used. 



207. A More Common Use. A more com- 



mon kind of wedge is that of two in- 

 clined planes united at their liases. Such 

 wedges are used in splitting timber, stone, etc. 

 The power is given in repeated blows instead 

 of continued pressure. For a wedge thus used 

 no definite law of any practical value can be 

 given, further than that, with a given thick- 

 ness, the longer the wedge the greater the gain 

 in intensity of power. 



208. What is a Screw? A Screw is a cylin- 

 der, generally of wood 



or metal, ivith a spiral 

 groove or ridge winding 

 about its circumference. 

 The spiral ridge is called 

 the thread of the screw. 

 The thread works in a nut, 

 within which there is a 

 corresponding spiral groove 

 to receive the thread. 



(a.) The power is used to turn the screw within a fixed nut, or to 

 turn the nut about a fixed screw. In cither case, a lever or wheel 

 is generally used to aid the power. Every turn of the screw or nut 

 either pushes forward the screw or draws back the nut by exactly 

 the distance between two turns of the thread, this distance being 

 measured in the direction of the axis of the screw. The weight or 

 resistance at W is moved this distance, while the power at P moves 

 over the circumference of a circle whose radius is PF. The differ- 

 ence between these distances is generally very great. Hence this 

 machine affords great intensity of power with a corresponding loss 

 of velocity. 



FIG. 61. 



