CONSTRUCTION OF APPARATUS. 171 



with hooks in order to hold the blade ; the piece near the handle is 

 fixed, but the opposite piece is moveable by a screw and winged 

 nut fer the purpose of stretching the blade, and thus giving to it the 

 required tension. A blade for sawing metal is thin, and the teeth 

 are not set as is the case with a saw for wood, in which every 

 alternate tooth is bent a little on one side and the intermediate 

 teeth, to an equal extent, on the other side. The motion of a saw 

 for metal is necessarily slow, and the ' kerf or cut made by it is not 

 wider than the thickness of the blade. The saw must be lubricated 

 with oil or tallow-grease, when used. 



Pieces may also be cut out of thick sheets of metal with the 

 chisel, which is preferable on account of the facility with which this 

 tool may be sharpened when it has become blunt, while it requires 

 considerable trouble and time to sharpen a saw, as the blade has to 

 be fixed in the vice and each tooth made sharp with a triangular file. 



FIG. 123 (an~proj. real size). 



The use of the chisel necessitates a very heavy support for the metal 

 which is to be cut ; if the vice weighs less than 10 or 12 k s r , a large 

 flat surface of stone such as the sandstone steps of a house, or an 

 invil, should be made use of. The sheet of metal is fixed in the 

 rice, so that the line along which the cut is required is just 

 ibove the edge of the cheeks ; the chisel is placed horizontal upon 

 jhe line and held with the left hand, while vigorous blows with the 

 lammer upon the top of the chisel are given by the right hand. If a 

 i;tone or anvil be used, the metal is cut through to about two-thirds 

 >f its thickness ; the part to be broken off is then clamped in the 

 ice, and the remainder of the sheet bent to and fro until the two 

 )ortions are separated. On a support of wrought iron or lead the 



ut may even be carried through the whole thickness without 



endangering the chisel. The usual form of chisel is that in fig. 



13 J., although for various purposes a smaller edge is preferable, 



s in the form shown in fig. 123 B. 

 The line along which a cut is to be made should be marked pre- 



