172 CONSTRUCTION OF APPARATUS 



viously ; this may be done with the point of the centre-punch or 

 one of the corners of the chisel. The edge of the latter should 

 be firmly placed upon the line before each blow is delivered; if 

 this precaution is neglected, the edge is liable to break off. Since 

 the chisei is apt to get out of the depression produced by the first 

 blow, it must be firmly placed back again into it, before the next 

 blow is given. 



In sharpening a chisel which has become blunt, the chief point 

 to attend to is to grind the surfaces flat and not rounded ; for this 

 purpose care must be taken to hold the tool in the same position 

 while it is moved to and fro across the breadth of the revolving 

 stone ; if the chisel is kept on one part of the circumference of the 



FIG. 124. 



grindstone, the latter will be spoiled by grooves, and the corners of 

 the chisel will become rounded instead of being sharp. The rough- 

 ness of the edge, which remains after the grinding, must be removed 

 upon the hone. If a piece has been broken out, the edge of the 

 chisel may be restored by softening the metal, producing a new edge 

 by the file, and tempering again, in the manner previously explained. 1 

 The two nuts should be bored quite straight through at the centre 

 and the threads cut in them before the screw is prepared ; one oi 

 them is filed accurately square, the other circular. The latter is 

 screwed down until its distance from the leaden weight is l cm '5, and 

 soldered on its lower side to the spindle ; no solder must reach the 



