ON MODES OF CHANGING THE FORMS OF BODIES. 227 



the bowstring acts on it, as the greatest force applied in drawing the bow is 

 greater thantwice the weight to be moved. 



The action of a whip, either on the air, or on a solid body, depends on 

 the int;rease of velocity, occasioned by the successive transmission of the mo- 

 tion from a thicker to a thinner portion of its flexible substance, so that at 

 last, the energy of the lash, and of its knots, gives it a sufficient capability of 

 exciting sound, or of inflicting pain. 



The instruments generally employed for the division of solid bodies, are 

 wedges, chisels, knives, and scissors; they sometimes act by pressure only, 

 but they are more powerful when impulse is added to it. Hatchets, planes, 

 saws, and files, always act with some rapidity. Cutting instruments are in 

 general very thin wedges, but the edge itself is usually much more obtuse; 

 Mr. Nicholson has estimated the angle, formed ultimately by the surfaces 

 constituting the finest edge, at about 56 degrees. Knives are sometimes fixed~- 

 on wheels, so as to revolve in a direction oblique to their edges, as in some 

 machines for cutting chaff, where the straw is also drawn forwards, through 

 a space variable at pleasure, during each revolution of the knife. An instru- 

 ment of a similar nature has also been invented for the purpose of cutting 

 weeds under water. 



For the edges of all cutting instruments, steel is principally employed. 

 After being hardened, by plunging it when red hot into cold water, it is 

 tempered, by laying it on a heated iron, or more accurately, by Mr. Stodart's 

 method, of immersing it in a metallic composition in the state of fusion. 

 When its surface has acquired a yellow tinge, it is fit for edge tools, and the 

 degree of heat proper for watch springs is indicated by a blue colour. The 

 backs of knives are often made of iron, which is less brittle than steel: these 

 substances are generally welded together, by hammering them when red hot; 

 but sometimes, in large instruments, a back of iron is only rivetted on. 



The iron employed for making nails, and other small articles, is first rolled 

 into flat bars, and then cut into narrow rods, by causing it to pass between 

 the cylinders of the slitting mill, the surfaces of which are formed into rect"- 



