ON MODES OF CHANGING THE FORMS OF BODIES. 171 



' press, and made so thin as to be afforded at a moderate expense. The 

 glazier's vice is a machine of the same nature for forming window lead : 

 the softness of the lead enables it to assume the required shape, in conse- 

 quence of the pressure of the rollers or wheels ; and the circumference of 

 these wheels is indented, in order to draw the lead along by the correspond- 

 ing elevations. (Plate XVIII. Fig. 232.) 



In drawing wire, the force is originally applied in the direction of the 

 extension, but it produces a much stronger lateral compression, by means of 

 the conical apertures through which the wire is successively drawn. For 

 holding the large wire, pincers are at first used, which embrace it strongly 

 while they pull, and open when they advance to a new position, the inter- 

 ruption being perhaps of use, by enabling the pincers to acquire a certain 

 momentum before they begin to extend the wire ; but afterwards, when the 

 wire is finer, it is simply drawn through the aperture from one wheel or 

 drum to another. During the operation, it requires frequent annealing, 

 which causes a scale to form on its surface ; and this must be removed by 

 rolling it in a barrel with proper materials ; for the application of an acid 

 is said to injure the temper of the metal. Copper is sometimes drawn into 

 wire so large as to serve for the bolts used in shipbuilding, especially for 

 sheathing ships' bottoms. Silver wire, thinly covered with gold, is ren- 

 dered extremely fine, and then flattened, in order to be fit for making gold 

 thread : the thickness of the gold is inconceivably small, much less than 

 the millionth part of an inch, and sometimes only a ten millionth. 



In order to form the handles of vessels of earthenware, the clay is forced 

 through a hole of a proper shape in an iron box. The operation of the 

 potter's wheel consists in great measure of compression and extension, per- 

 formed by the hands ; the vessels are finished, when they are partly dry, 

 in a lathe, or by other instruments ; some kinds of earthenware are formed 

 in a mould only. 



When a thread or a plate of glass is extended in a semifluid state, it has 

 a tendency to preserve an equable thickness throughout : this is derived 

 from the effect of the air in cooling it, the thinnest parts becoming imme- 

 diately a little colder than the rest, and consequently harder, so that they 

 retain their thickness, until the neighbouring parts are brought into a 

 similar state. 



Extension is performed by means of percussion, in forges and in the 

 common operation of the smith's hammer. In forges, the hammers are 

 raised by machinery, 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 hammer. Such forges are used in making malleable 

 iron, in forming copper plates, and in manufacturing steel. (Plate XVIII. 

 Fig. 233.) 



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

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

 iies"s of little more than the three hundred thousandth of an inch. Silver 

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

 alloy. 



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



