228 LECTURE XIX. 



angular grooves, and which are placed close to each other, so that the pro- 

 minent 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 transversely 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. Orna- 

 mental lathes admit of a great variety of mechanical contrivance, but they are 

 of little practical use, except for amusement. Picture frames are,' however, 

 sometimes turned in oval lathes; and in the manufacture 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 a screw, a screw of any dia- 

 meter may be turned by its assistance; if by a frame producing an elliptic 

 curve, any number of ovals, having the same centre, may be described at 

 once; and if a moveable point connected with the work, be pressed by a 

 strong spring against a pattern of any kind, placed at one end of the axis, a 

 copy, of the same form, may be made at the other end of the axis. 



The process of boring is a combination of penetration and division, and 

 sometimes of attrition. Awls, gimlets, screws, augers, and centrebits, are 

 various forms of borers. The drill has the advantage of a rapid motion, com- 

 municated by the drill bow, which turns it round by means of a little wheel 

 or pulley. In boring cannon, the tool is at rest, while the cannon revolves, 

 and by this arrangement the bore of the cannon is formed with much more 

 accuracy than according to the old method of putting the borer in motion ; 

 perhaps because the inertia of so large a mass of matter, as constitutes the can- 

 non, assists in defining the axis of revolution with more accuracy. The borer 

 is pressed against the cannon by a weight, hung on the arm of a bent spring 



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and during the operation, the outsid^ is also turned into its intended shape by 

 the application of proper instruments. Cylinders for steam engines are cast 



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