lecture xrx.

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nion operation of the smith's hammer. In forges, the hammers are raised bymachinery, and thrown forcibly against a spring, so as to recoil with greatvelocity'. With the help of this spring, the hammer sometimes makes 500strokes in a minute, its force being many times greater than the weight ofthe hammer. Such forges are used in making malleable iron, in formingcopperplates, and in manufacturing steel. (Plate XVIII. Pig. 233.)

Gold is beaten between the intestines of animals, on a marble anvil; forthis 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. Silverleaf is about the hundred and sixty thousandth; it is made of silver withoutalloy.

The operation of coining depends also principally on an extension of themetal into the recesses of the die ; it is performed by a strong pressure, unitedwith a considerable impulse, communicated by a screw like that of a printingpress; and sometimes the impression is formed by the repeated blows of ahammer 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 theone is fixed, and the other attached to the bottom of a heavy hammer. Thehammer is raised and suffered to fall in a right line, by mean* 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 causea thin plate of silver to cohere permanently with a surface of iron; and thismode of uniting metals is actually employed in some manufactures.

The operations of perforating, cutting, turning, boring, digging, sawinggrinding, and polishing, resemble each other, in great measure, with respe ctto the minute actions of the particles of bodies which they have to overcomePenetration is generally 7 performed in the first instance by the effect whichhave called detrusion, where the magnitude of the penetrating substance ,sconsiderable; but when a fine point or edge is employed, it probably h jStears the surface where it is most depressed, and then acts like a wedge 011the portions of the substance left on each side, with a force so much fh c