94 MECHANICS. 



blows of a heavy body, the momentum of which im- 

 parts to it great force. 



All cutting and piercing instruments, as knives, scis- 

 sors, chisels, pins, needles, and awls, are wedges. The 

 degree of acuteness must be varied according to cir- 

 cumstances ; knives, for instance, which act merely by 

 pressure, may be made with a much sharper angle 

 than axes, which strike a severe blow. For cutting 

 very hard substances, as iron, the edge must be form- 

 ed with a still more obtuse angle. 



The utility of the wedge depends on the friction of 

 its surfaces. In driving an iron wedge into a frozen 

 or icy stick of wood, as every chopper has observed, the 

 want of sufficient friction causes it immediately to re- 

 coil, unless it be previously heated in the fire. The 

 efficacy of naUs depends entirely on the friction against 

 their wedge-like faces. 



THE SCREW. 



The screw may be regarded as nothing more than 

 Fig- '74. an inclined plane winding round the surface 

 ^^g^r^ of a cylinder {Fig: 74). This may be easily 

 understood by cutting a piece of paper in 

 such a form rig. 75. 



that its edge, 

 a b {Fig. 75), 

 may represent 

 the inclined plane ; then, ''^- 

 beginning at the wider end, and wrapping it about the 

 cylindrical piece of wood, c, the upper edge of the pa- 

 per will represent the thread of the screw. 



Although the friction attending the use of the screw 



