174 LECTURE XIX. 



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

 the increase of velocity occasioned by the successive transmission of the 

 motion 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 capa- 

 bility 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 instrument 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. Stod- 

 art'st 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 rectangular grooves, and which are placed close to each other, so that 

 the prominent parts of the one are opposed to the depressions of the other, 

 and the bars are divided by the pressure of the opposite forces acting trans- 

 versely at the same points, so as to separate them by the effect which we 

 have already considered under the name detrusion. The same machinery 

 also generally works a pair of large shears for cutting bars of any kind. 

 (Plate XVIII. Fig. 235.) 



The lathe is an elegant instrument, in which a considerable relative 

 velocity is produced between the tool and the substance to be cut, by the 

 revolution of this substance on an axis, while the tool is supported by a 

 rest. Ornamental lathes admit of a great variety of mechanical contriv- 

 ance, but they are of little practical use, except for amusement. Picture 

 frames are, however, sometimes turned in oval lathes ; and in the manufac- 

 ture of buttons, machines of a similar nature are occasionally employed. 

 The effect of every lathe of a complicated construction depends on a certain 

 degree of motion of which its axis is capable : if this motion be governed by 



* Nich. Jour. 8vo, i. 47, 210. 



f Nich. Jour. 4to, iv. 127. See also i. 380, 468, 575 ; ii. 64, 102. 



