ing the substance to be divided is, and the less the power required to act upon 

 it, the more acute the wedge may be constructed. 



In many cases, the utility of the wedge depends on that which is entirely 

 omitted in its theory, viz., the friction which arises between its surface and the 

 substance which it divides. This is the case when pins, bolts, or nails, are 

 used for binding the parts of structures together ; in which case, were it not 

 for the friction, they would recoil from their places, and fail to produce the 

 desired effect. Even when the wedge is used as a mechanical engine, the 

 presence of friction is Absolutely indispensable to its practical utility. The 

 power, as has already been stated, generally acts by successive blows, and is 

 therefore subject to constant intermission, and, but for the friction, the wedge 

 would recoil between the intervals of the blows with as much force as it had 

 been driven forward. Thus the object of the labor would be continually frus- 

 trated. The friction, in this case, is of the same use as a ratchet-wheel, but 

 is much more necessary, as the power applied to the wedge is more liable to 

 intermission than in the cases where ratchet-wheels are generally used. 



When a road directly ascends the side of a hill, it is to be considered as an 

 inclined plane ; but it will not lose its mechanical character, if, instead of di- 

 rectly ascending toward the top of the hill, it winds successively round it, 

 and gradually ascends, so as, after several revolutions, to reach the top. In 

 the same manner a path may be conceived to surround a pillar, by which the 

 ascent may be facilitated upon the principle of the inclined plane. Winding 

 stairs constructed in the interior of great columns partake of this character ; 

 for although the ascent be produced by successive steps, yet if a floor could be 

 made sufficiently rough to prevent the feet from slipping, the ascent would be 

 accomplished with equal facility. In such a case, the winding path would be 

 equivalent to an inclined plane, bent into such a form as to accommodate it to 

 the peculiar circumstances in which it would be required tc be used. It will 

 not be difficult to trace the resemblance between such an adaptation of the in- 

 clined plane and the appearances presented by the thread of a screw ; n.nd it 

 may hence be easily understood that a screw is nothing more than an inclined 

 plane constructed upon the surface of a cylinder. 



This will perhaps be more apparent by t'he following contrivance : Let A B, 

 fig. 6, be a common round ruler, and let C D E be a piece of white paper cut 



Fig. 6. 



in the form of an inclined plane, whose height C D is equal to the length of 

 the ruler A B, and let the edge C E of the paper be marked with a broad 

 black line : let the edge C D be applied to the ruler A B, and, being attached 

 '-hereto, let the paper be rolled round the ruler ; the rul-er will then present the 

 appearance of a screw, fig. 7, the thread of the screw being marked by the 

 black line C E, winding continually round the ruler. Let D F, fig. 6, be equal 

 to the circumference of the ruler, and draw F G parallel to D C, and G H 

 parallel to D E,the part C G F D of the paper will exactly surround the ruler 

 once ; the part C G will form one spire of the thread, arud may be considered 

 as the length of one inclined plane surrounding the cylinder, C H being the 

 corresponding height, and G H the base. The power of the screw does noi, 

 as in the ordinary cases of the inclined plane, act parallel to the plane or 



