44 



MECHANICS. 



as the slightest percussion is far more 

 effective. ... 



The only general theoretical principle 

 respecting the wedge, which obtains 

 always in practice is, that its power is 

 increased by diminishing the angle. 



All cutting instruments, as knives, 

 swords, hatchets, chisels, planes, &c. 

 are wedges. In these cases, the harder, 

 in general, the substance to be divided 

 is, the greater wiU be the angle of the 

 wedge. Thus, chisels for cutting soft 

 woods are sharper than those used for 

 the harder species, and these, again, 

 are sharper than chisels used for cutting 

 metals. 



In the preceding observations on the 

 wedge, we have not attempted to explain 

 its theory, conceiving, that a theory, 

 which is utterly inapplicable in practice, 

 is better omitted in the " Library of 

 Useful Knowledge." 



CHAPTER XI. On theScrew Hunter's 

 Screw Perpetual Screw Micro- 

 meter Screw. 



(99.) THE screw is a machine of great 

 mechanical power, and is applied to 

 various purposes ; but is most generally 

 used in cases where an intense pressure 

 is to be exerted. This machine is a 

 modification of the inclined plane. Let 

 an inclined plane C/^.80.) be placed with 



Fig. 80 



By this process the body is forced up 

 the inclined plane, which is wrapped 

 upon the cylinder, and the power being 



Fig. 81. 



its altitude B C parallel to the axis of a 

 cylinder, and placing the altitude B C 

 on the side of the cylinder, let the plane, 

 supposed flexible, be rolled round it. 

 The length A B of the plane will trace 

 upon the cylinder a spiral thread, which 

 is called the thread of the screw. 



A perspective view of this machine is 

 given in Jig. 81 . If any body be placed 

 between two threads, and the screw be 

 turned once round, the body so placed 

 not being permitted to turn with it, it 

 is plain that this body will be moved 

 from its first position to a similar place 

 on the thread next above that on which 

 it was first placed. In fact, it will be ele- 

 vated through an height equal to the dis- 

 tance between two contiguous threads. 



supposed to be applied at the circum- 

 ference of the screw acts parallel to the 

 base of the plane. 



In this case, the proportion of the 

 power to the weight is that of the height 

 of the plane to its base (93). But in 

 one revolution of the screw, the body, 

 which is placed between the threads, is 

 moved up an inclined plane, whose 

 height is the interval between two con- 

 tiguous threads, and whose base is the 

 circumference of the screw. Hence, it 

 follows that the power is to the weight 

 or resistance as the interval between 

 the threads is to the circumference of 

 the cylinder on which the thread is 

 raised. 



The energy of the power on the screw 

 is transmitted by means of a concave 

 cylindrical screw, on the inner surface of 

 which a spiral channel is cut, corre- 

 sponding exactly to the thread raised 

 upon the cylinder, so that by turning the 

 one within the other the convex screw 

 will pass through the concave screw, and 

 will advance every revolution through a 

 space equal to the distance between two 

 contiguous threads. 



A section of the convex and concave 

 screw, by a plane through the axis, is 

 represented in Jig. 82. If the concave 

 screw be fixed, so as to be incapable 

 either of revolving or moving in the di- 

 rection of its length, it is evident that 

 the convex screw will gradually pene- 

 trate it, advancing through the space 

 between two contiguous threads every 

 revolution. If, on the other hand, the 

 convex screw be incapable of moving 

 in the direction of its length, it will, by 

 its rotatory motion, force the concave 



