MECHANICS. 89 



Piercing instruments are all reducible to wedges of 

 this kind, Nails, Bayonets, Stakes, Piles, &c. The 

 resistance in some of these increases with the sur- 

 face to which it is applied. Though a wedge be of 

 a conical form, it is comprehended under the pre- 

 ceding theorem. 



The common wedge is generally employed for clea- 

 ving, and otherwise overcoming the force of cohe- 

 sion in bodies : it may sometimes be used with 

 advantage for raising great weights to a small 

 height. 



All cutting instruments may be referred to it, Knives, 

 Chisels, Scissors, Files, the Teeth of Animals, &c. 

 A Saw is a series of wedges, on which the motion 

 impressed is oblique to the resistance. A Wimble is 

 a wedge to which the circular motion is given. 



The power applied to the wedge, when great resist- 

 ance is to be overcome, is usually percussion ; and 

 almost the only instance in which the wedge is used 

 for the purpose of equilibrium, is in the construc- 

 tion of arches, which are built of truncated wedges. 

 As the consideration of this equilibrium would lead 

 to too long a digression in this place, it will be treat- 

 ed of in an Appendix. 



Of the Screw. 



150. The screw is a spiral groove or thread, 

 winding round a cylinder, so as to cut all the lines 

 drawn on its surface parallel to the axis, at the 



same 



