The process of casting which has been recently considered under so great a variety of forms, is one of the most valuable courses of preparation to which the metallic materials are submitted. In the foundry, the metals are made to assume an infinitude of the most arbitrary shapes, but which are in general more or less thick or massive. It is now proposed to consider, a few of the methods and principles of another very extensive and serviceable employment of the malleable metals and alloys, which (excepting iron,) are cast into thick slabs or plates, and then laminated into thin sheets between cylindrical rollers.

Rollers have been used for a considerable period in the manufacture of sheets of malleable iron, steel and copper, when in the red-hot state, but most others of the metals and alloys are rolled whilst cold; and which economic application of power often nearly supersedes the use of the hammer, as it performs its functions in a more uniform and gradual manner; and at the same time increases to the utmost, the hardness, tenacity, elasticity, and ductility of such of the metals and alloys, as are submitted to this and similar courses of preparation for the arts. As stated at the beginning of the twelfth chapter, these processes of condensation cannot be carried to the extreme, without frequent recurrence at proper intervals to the process of annealing; and in rolling the thinnest sheets of metal, several are frequently sent through the rollers at the same time; but as in the instances of tin-foil, gold and silver leaf, and some others, the hammer is again resorted to after the metals have been rolled as thin as they will economically admit of, in this process of part-manufacture.

None of these preparations of the metals can go on without a material internal change of their substance, to which the celebrated Dr. Dalton thus refers: "Notwithstanding the hardness of solid bodies, or the difficulty of moving the particles one amongst the other, there are several that admit of such motion without fracture, by the application of propel force, especially if assisted by heat. The ductility and malleability of the metals, need only be mentioned. It should seem the particles glide along each other's surface, somewhat like a piece of polished iron at the end of a magnet, without being at all weakened in their cohesion."*

This gliding amongst the particles of metals is exemplified by the action of thinning them by blows of the hammer; likewise by the actions of laminating rollers and the draw-bench, in which cases the external layers of the metals are retarded or kept back as it were in a wave, whilst the central stream or substance, continues its course at a somewhat quicker rate. The necessity for annealing occurs, when the compression and sliding have arrived at the limit of cohesion; beyond this the parts would tear asunder, and produce such of the internal cracks and searns met with in sheet-metal and wire, as are not due to original flaws and air-bubbles, which have become proportionally elongated in the course of the manufacture of these materials.

A sliding or gliding of a very similar nature occurs also in every case in which the metals are bent; and this differs only in degree, whether we consider it in reference to a massive beam, a permanently flexible spring, a piece of thin sheet metal, or a film of gold leaf. For instance, the curvature of a cast-iron beam originally straight, is produced by the stretching or extension of the lower edge, and the shortening or compression of the upper edge, the central line or the neutral axis, remaining unaltered during the process. In like manner a spring derives its elasticity from the extension and compression of its opposite surfaces at every flexure; and the spring remains permanent, or endures its work without alteration of form, when the bending is not carried beyond its limit of elasticity: but when it is bent beyond a certain point, the spring either retains a permanent set or distortion, or it will break. In the same manner the beam when only bent to the limit of its elasticity, returns to its original form when the load is relieved, and the constant study of the engineer is so to proportion the beam, that it may never be required to exceed, nor even to arrive at the limit of its elastic force. For those parts of mechanism exposed to sudden shocks and strains, he will employ wrought-iron, the cohesive strength of which is considerably greater than that of cast-iron, although less than that of steel, which is the strongest and most permanently elastic of all metallic substances.

* Dr. Delton's New System of Chemical Philosophy, 1808, p. 209.

The thin metals also possess some elasticity, but this dies away before they reach the tenuity of leaf gold, in which however, the bending cannot be accomplished without a similar change in the arrangement of its opposite sides, although the difference is beyond the reach of our physical senses.

If we desire to wrap a piece of gold leaf around a cylinder of half an inch diameter, so small is the resistance that the least puff of breath suffices; a piece of thin tin-foil offers no more resistance than writing-paper; thin latten-brass,or China tea-lead, is bent more easily than a card; brass and iron the thirtieth or fortieth of an inch thick, could be readily bent with a wooden mallet; but metal of one-eighth of an inch thick would call for smart blows of a hammer, and in iron and steel the further assistance of heat would be likewise required, because in the last case a very considerable amount of the sliding motion of the metal would be called into play.

For example, the piece of metal 1/8 of an inch thick, was originally flat and of the same size on its opposite surfaces; whereas now, neglecting any alteration of thickness, the inner part would equal the circumference of a circle 1/2 an inch diameter, and the outer that of a circle of 3/4 inch diameter; or it would become 11 /2 and 1 1/4 inches long respectively on its opposite surfaces. To produce this change of dimensions, would necessarily require far greater force than the bending of the gold leaf, the internal and external measures of which, viewed as a cylinder, could be ascertained alone by calculation, and not by ordinary means. On the other hand, the sliding of the thick sheet of metal would be illustrated most distinctly, if several pieces of writing-paper, equal to the original metal individually in surface and collectively in thickness, were wrapped around the same cylinder. The inner paper would exactly meet, the outer would present an open seam 3/4 inch wide. The metals possessed of the malleable property, undergo a nearly equal change in their arrangement; but the unmalleable or brittle metals break.

Several of the processes of working the sheet metals are closely analogous to those employed in forging ordinary works in iron and steel; the differences being mainly such as arise from the thin and thick states of the respective materials, and their relative degrees of rigidity, or resistance; the illustrations will be selected indiscriminately from various trades in which the sheet metals are employed. It appears desirable however, to separate the subject into two principal parts: namely, the formation of objects some lines of which are straight; and the formation of objects no lines of which arc straight.

The first division comprehends all objects with plane,cylindrical or conical surfaces, such as may be produced in pasteboard, by cutting out the respective sides, either separately or in clusters, and combining them in part by bending, and in part by cement. Similar works in metal are often produced by the precisely analogous means, of cutting, bending, and uniting, and which call for increase of strength in the methods, proportioned to the rigidity of the materials.

The second division comprehends all objects with surfaces of double curvature, including spherical, elliptical, parabolical, and arbitrary surfaces; as in reflectors, vases, and a thousand other things: none of which forms can be produced in stiff pasteboard, because this material is incapable of being extended or contracted in different parts, in the manner of sheet-metal; this is easily shown, by the following case amongst others.

Terrestrial globes are covered with thin paper, upon which the delineation of the surface of the earth has been printed; the paper may be cut into twelve gores, or fish-shaped pieces, all including thirty degrees from pole to pole.* But the same gores cut out of pasteboard could not be applied to the surface of a globe, as pasteboard docs not admit of that degree of gradual extension and contraction, required for the production of spherical and similar raised forms, from pieces originally flat, but will become abruptly bent and torn in the attempt.

On the contrary, a round disk of metal may be beaten into a hemisphere, or nearly into a sphere; but even thin paper is only possessed of this quality in a very limited degree, for the globe could not be smoothly covered with so few as two, three, or four pieces of the thinnest paper without its puckering up, showing that some parts of the material are in excess. The gliding property, or that of malleability and ductility, possessed by the metals, is indispensable to adapt the flat plate to the sphere, by stretching the central portion and gathering up the marginal part, an action that admits of some comparison to the extension or compression of the slides of a telescope, except that the metal becomes thicker or thinner instead of being duplicated on itself.

* A globe is usually covered with 26 pieces of paper, namely 2 pole papers, or circles including 30° around each pole; and 24 gores meeting at the equator. Sometimes the gores extend from the polo to the equator; every gore has then a narrow curved central notch extending 80° from the equator.