ON MODES OF CHANGING THE FORMS OF BODIES. 175 



a screw, a screw of any diameter 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, 

 communicated 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 con- 

 stitutes the cannon, assists in denning 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, and during the operation the outside is also 

 turned into its intended shape by the application of proper instruments. 

 Cylinders for steam engines are cast hollow, and afterwards bored ; but in 

 this case the borer revolves, and the cylinder remains at rest. 



Ploughs, spades, pickaxes, mattocks, harrows, and other agricultural 

 instruments, resemble in their operation the chisel and the wedge : the 

 numerous diversities in their form and the complications of their structure, 

 are determined more by the various modifications of their action, required 

 for particular purposes, than by any material difference in the mode of 

 application of the principles on which they depend. (Plate XVIII. 

 Fig. 236.) 



The process of mining is a combination of boring and digging. Shafts 

 are sunk, levels are driven, and drains are carried off, by the help of picks 

 or pickaxes, wedges, and hammers, the rocks being also sometimes loosened 

 by blasting with gunpowder. In searching for coal, a shaft is sunk 

 through the uppermost soft strata, and the rock is then bored by striking it 

 continually with an iron borer terminating in an edge of steel, which is in 

 the mean time turned partly round ; and at proper intervals a scoop is let 

 down to draw up the loose fragments. In this manner a perforation 

 is sometimes made for more than a hundred fathoms, the borer being 

 lengthened by pieces screwed on to it ; it is then partly supported by a 

 counterpoise, and is worked by machinery ; if it happens to break, the 

 piece is raised by a rod furnished with a hollow cone, like an extinguisher, 

 which is driven down on it. Sometimes the borer is furnished with knives, 

 which are made to act on any part at pleasure, and to scrape off a portion 

 of the surrounding substance, which is collected in a proper receptacle. 



For sawing wood on a large scale, sawing mills are very advantageously 

 employed, being usually driven by water. Several saws are generally 

 fixed in a frame, parallel to each other ; they are worked up and down by 

 a cfank, and at every alternation, a wheel is drawn round a little by a 

 catch, or click, and moves forwards the frame which supports the timber. 

 When the machine is employed for cutting the fellies which form the cir- 



