ON MODES OF CHANGING THE FORMS OF .BODIES.

223

quired shape, in consequence of the pressure of the rollers or wheels; and thecircumference of these wheels is indented, in order to draw the lead alongvb y fire corresponding elevations. (Plate XVIII. 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 oftile conical apertures through which the wire is successively drawn. Forholding the large wire, pincers are at first used, which embrace it stronglywhile 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 thewire is finer, it is simply drawn through the aperture from one wheel ordrum to another. During the operation, it requires frequent annealing,Which causes a scale to form on its surface; and this must be removed byrolling it in a barrel with proper materials; for the application of an acid isS; dd to injure the temper of the metal. Copper is sometimes drawn into wire* s ° large as to serve for the bolts used in shipbuilding, especially for sheathingblip’s bottoms. Silver wire, thinly covered with gold, is rendered extremely*me, and then flattened, in order to be lit for making gold thread: the thick-ness of the gold is inconceivably small, much less than the millionth part of:in inch, and sometimes only a ten millionth.

In order to form the handles of vessels of earthenware, the clay is forcedtrough a hole of a proper shape in an iron box. The operation of the pot-wheel consists in great measure of compression and extension, performed

die hands; the vessels are finished, when they are partly dry, in a lathe,

01 by other instruments; some kinds of earthenware are formed in a mouldonly.

When a thread or a plate of glass is extended in a semifluid state, it lias ate ndeu C y to preserve an equable thickness throughout, this is deiived fiomJ !le effect of the air in cooling it, the thinnest parts becoming immediately a! ttlu colder than the rest, and consequently harder, so that they retain their* “ckness, until the neighbouring parts are brought into a similar state.

'^tension is performed by means of percussion, in forges, and in the com-