f24 ' LECTURB XIX. 



mon operation of the Bmith's hammer. In forges, the hammers are raised by 

 macliinery, and thrown forcibly against a spring, so as to recoil with great 

 velocity. With the help of this spring, the hammer sometimes makes 500 

 strokes in a minute, its force being many times greater than the weight of 

 the hLimmer, Such forges are used in making malleable iron, in forming- 

 copper plates, a:id in manufacturing steel. (Plate XVIII. Fig. 233.) 



Gold is beaten between the intestines of animals, on a marble anvil; for 

 tliis purpose it is alloyed with copper or silver. It is reduced to the thick- 

 ness of little more than the three hundred thousandth of an inch. Silver 

 leaf is about the hundred and sixty thousandths it is made of silver without 

 alloy. 



The operation of coining depends also principally on an extension of the 

 metal into the recesses of the die ; it is performed by a strong pressure, united 

 with a considerable impulse, communicated by a screw like that of a printing 

 press; and sometimes the impression is formed by the repeated blows of a 

 hammer only. 



Thin plates of silvered copper are moulded into any figure that may be re- 

 quired, by being placed between two corresponding stamps, of which the 

 one is fixed, and the other attached to the bottom of a heavy hammer. The 

 hammer is raised and suffered to fall in a right line, by means of pincers, 

 which open when they have acquired a certain height. Sometimes the con- 

 tact, produced by the forcible impulse of a die, is sufficiently intimate to cause 

 a thin plate of silver to cohere permanently with a surface of iron ; and this 

 anode of uniting metals is actually employed in some manufactures. 



The operations of perforating, cutting, turning, boring, digging, sawing, 

 grinding, and polishing, resemble each other, in great measure, with respect 

 to the minute actions of the particles of bodies which they have to overcome. 

 Penetration is generally performed in the first instance by the effect which we 

 have called detrusion, where the magnitude of the penetrating substance is 

 considerable: but when a fine point or edge is employed, it probably first 

 tears the surface where it is most depressed, and then acts like a wedge on 

 the portions of the substance left on each side, with a force so much the 



