c. Heating ovens of a superior kind are at present constructed on the principle of the reverberatory furnace. In these, the fuel and iron are properly separated, and all contact between them obviated. Fig. 123 represents a vertical section of a heating oven for sheet

iron; a is the hearth, 6 the fire grate, and c the chimney. The height of the furnace is often thirty inches. The object of this is partly to prevent the contact of the flame and iron, but principally to gain room for setting the sheet edgewise; they are thus set on both sides of the furnace; besides, in the middle of the hearth, sufficient room is left for laying a sheet or two flatwise. d is a cast iron plate, forming a sliding door. The chimney has two flues, the one inside, the other outside of the oven. Its draft is weak, and the smoke or flame frequently issues from the mouth, in which case it is carried off by the second or outside flue. Fig. 124 represents a vertical section across the furnace and the flues; and Fig. 125 a ground-plan of the furnace, hearth, and fireplace. The cast iron plate e is here shown more distinctly. Its object is to protect the bricks or stones from the destructive agency of the tongs and iron. Like puddling and reheating furnaces, these ovens are built of fire bricks, inclosed with cast iron plates, and preserved from the effects of expansion and contraction by wrought iron cross binders. A slight variation from the form of the oven we have described, occasioned as well by individual taste as by locality, is sometimes

observed; still, the one we have presented is the one generally

employed for the manufacture of sheet iron. If it is desirable that the surface of the iron should be kept very clean, the fire bridge

and the inside flue may be raised; but, in all such cases, pure fuel is our safest reliance.

IX. Shears, and Turning Machines.

These are of much importance in a rolling mill. The first we shall describe somewhat minutely; but a brief description of the latter must suffice.

a. When rollers are cast, and ready for turning, they are placed upon a strong and heavy turning lathe, and the gudgeons and couplings turned between points. They are then put into cast iron standards, into which brass pans are inserted. In the latter, the gudgeons revolve. At first, the rollers are turned into smooth cylinders. After a set is thus far completed, the grooves are cut in, according to a design previously drawn on a board. Sheet iron or sheet brass patterns are made for each groove in every roller. These should be preserved, in case a roller is injured, or fails to answer its purpose. Rollers for sheet iron are of course smooth cylinders, but it is not necessary that the bottom and top of the roller should be of the same diameter. Those for thin sheet iron should be turned one upon the other, that their surfaces may be perfectly parallel. Unless there is too great a variation in the surfaces, this may be done in the housings. After using the rollers for a time, their surface is apt to become rough. Its smoothness may be restored by cutting it with one edge of a square piece of cast steel, from three to four inches in length. This operation is generally performed in the housings, for the moving of the rollers to the turning lathe is attended with great expense. Good hard rollers are turned with difficulty by common methods. A steady turning machine, of slow motion, excellent cast steel chisels, and patience, are the conditions of success. Hard rollers are required for making thin and polished sheet iron. They are polished by means of emery and leaden pans, which extend almost quite around the roller.

b. The shears required in a mill are the movable hand-shears, for cutting small rod and hoop iron, and force-shears connected with a waterwheel or steam-engine, for cutting common bar, rough bar, and sheet iron. The first are small lever shears, fastened upon a two inch plank, as represented in Fig. 126. The length of the whole is about two feet or thirty inches. The shears are placed at each end of a pile where small bar or hoops are deposited. The boys, who catch behind the rollers, cut off the bad ends, before a rod or hoop is laid down.

c. Fig. 127 represents the common force-shear. It is a powerful cast iron lever, varying, according to locality and purpose, from

seven to twelve feet in length. The excentric a is generally fastened upon the main shaft, or, if such is not accessible, upon any

other strong and well-supported shaft. The foundation must be very firm, and not inferior in solidity to the roller trains. The steel

blades are made of good shear or cast steel, tightly fitted into the cast iron lever and standard, and screwed on with screw bolts. For the cutting of heavy bar and rough bar, the standard block is generally placed very low, about a foot above ground; but for cutting common bar and sheet iron, it is raised from two feet to thirty inches above ground. If sheet iron is principally brought to the shears, an iron frame b, b, as high as the lower cutter, is to be fastened to the standard. Upon this frame, the sheet is moved. In working sheet iron, shears of this construction are attended with some disadvantage. The acute angle at the points, and the obtuse angle close to the fulcrum which they form, in addition to the difficulty of adjusting them accurately, make them somewhat objectionable.

To obviate these disadvantages, various plans have been devised, of which the following, Fig. 128, appears to be the most prac

ticable. These shears are generally used for cutting nail plates, and for trimming sheet iron. The cutters b, b are shown in section, and are frequently from sixteen to twenty inches in length, so as to cut over the entire breadth of a sheet; the same length is required where sheet iron is used for making nails. In case small nail plates are used, shorter cutters can be employed. The lower or fixed cutter is horizontal, but the upper is screwed to the cast iron lever in such a manner as to form an angle with the lower cutter seldom greater than fifteen degrees. The motion of the lever can be produced by an excentric, as in Fig. 127, or by a crank, as in the present case. It frequently happens that shears are wanted where we cannot reach directly to an excentric with a lever, nor in a short way with a crank. In such cases, a crank motion from some shaft is conducted below ground to the desired point by means of an iron connecting rod. This arrangement, which may be modified according to circumstances, is exhibited by Fig. 129. The tail may be turned above or below ground, forward or back; but care should be taken that the connecting rod is always on the pull side, as shown in the drawing, for a long connecting rod is not adapted to push the lever. This arrangement-in which the shears are directly connected with the elementary power-is necessary where heavy bar iron and boiler-plate are to be cut, for these require a strong foundation. Portable shears, with their own independent flywheel, propelled by means of a belt and pulley, are preferable for light iron, sheet iron, or nail plates. Bar iron more than two inches square cannot be conveniently cut by shears; like railroad iron, this is to be trimmed by circular saws.