ON MODES or CHANeiNG THE FORMS OF BODIES. 223 



quired shape, in consequence 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 corresponding elevations. (Plate XVIIL Fig. 232.) , 



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

 tension, 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 mo- 

 mentum 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 

 ship's bottoms. Silver wire, thinly covered with gold, is rendered extremely 

 fine, and then flattened, in order to be fit for making gold thread : the thick- 

 ness 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 pot- 

 ter's wheel consists in great measure of compression and extension, performed 

 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 immediately 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 com- 



