ON MODES OF CHANGING THE FORMS OF BODIES, £29 



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 in- 

 struments, resemble in their operation the chisel and the wedge : the numer- 

 ous diversities in their form and the complications of their structure, are de- 

 termined more by the various modifications of their action, required for par- 

 ticular 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, ashaft is sunk through the up- 

 permost 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 ma- 

 chinery; 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 Avood 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 crank, 

 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 circumference of wheels, the 

 frame supporting the timber is made to turn round a centre. A circular saw is 

 used in the construction of blocks and pullies; and in order to make the motion 

 more secure from the effect of accidental irregularities, the wheels are made, 

 to turn each other by contact only, without teeth. The machinery for mak- 



