70 



MECHANICS. 



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

 of acuteness must be varied according to circumstances ; 

 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 formed with a still more obtuse angle. 



The utility of the w^edge 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 recoil, unless 

 it be previously heated in the fire. The efficacy of nails 

 depends entirely on the friction against their wedge-like 

 faces. 



THE SCREW 



The screw may be regarded as nothing more than an 

 inclined plane winding round the surface of a 

 cylinder (fig. 74). This may be easily under- 

 stood by cutting a piece of paper in such a 

 form that its edge, a h (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 paper will 

 represent the thread of the screw. 

 Although the friction attending the use of the screw is 



considerable, and without it it would not retain its place, 



yet the slope of its in- 

 clined thread being so 



gradual, it possesses 



great power. This power 



is multiplied to a still 



greater degree by the 



lever #iich is usually employed to drive it, a (fig. 76). 



If, for)*example, a scrcAV be ten inches in circumference, 



and its thread ]i^lf an inch apart, it exerts a force twenty 



Fie. 75. 



