cavity is again filled with charcoal. It is there again fused, and so on a third time, all these processes being accomplished in three or four hours. The iron, thus again solid, is taken out, and very slightly hammered, to free it from the attached scoria; after this it is returned to the furnace, in a corner whereof it is stacked, out of the action of the blast, and well covered with charcoal, where it remains gradually to cool until sufficiently compact to bear the tilt, or trip hammer, which is moved by machinery, and whose weight is from 600 to 1200 lbs. Thus it is beaten till the scoriæ are forced out, when it is divided into several portions, which, by repeated heating and hammering, are drawn into bars, in which state it is ready for sale. 1758. There are various methods of procuring the blast, which we think it unnecessary here to detail: the first, and most ancient, are by means of bellows; the latest, which has been found in the mining districts to be a contrivance of great importance, is the placing a series of vanes attached to an axis, which are made to revolve in a box with great rapidity. A pipe passing from the outside of the box to the furnace conveys the air to it as the vanes revolve, a new portion continually entering by a hole at the axis. 1759. The proportion of pig or cast iron from a given quantity of ore varies as the dif ference in the metallic contents of different parcels of ore and other circumstances, but the quantity of bar obtained from pig iron is not valued at more than 20 per cent. 1760. The other process for manufacturing bar iron, which is that chiefly employed in this country, is conducted in reverberatory furnaces, usually called puddling furnaces. The operation begins with the fusion of the cast iron in refinery furnaces, like the one above described. When the iron is fully melted, a tap-hole is opened in the crucible, and the metal and slag flow out together into a fosse covered with clay well mixed with water, by which a coating is formed that prevents the metal from sticking to the ground. The finer metal forms a slab about ten feet long, three feet broad, and from two to two and a half inches in thickness. For the purpose of slightly oxidizing it, and to make it brittle, it is much sprinkled over with cold water. In this part of the process it loses in weight from 12 to 17 per cent. After this, it is broken up into pieces, and placed on the hearth of a reverberatory furnace, in portions heaped up to its sides in piles which rise nearly to the roof, leaving a space open in the middle to give room for puddling the metal as it flows down in streams. When the heat of the furnace has brought it to a pasty state, the temperature is reduced, a little water being sometimes thrown on the melted mass. The semiliquid metal is stirred up by the workman with his pudelle, during which it swells, and parts with a large quantity of oxide of iron, which burns with a blue flame, so that the mass appears ignited. As it refines, the metal becomes less fusible, or, as the workmen say, it begins to dry. The puddling goes on until the whole charge assumes the form of an incoherent sand, when the temperature is gradually increased to give it a red white heat, at which period the particles begin to agglutinate, and the charge, in technical language, works heavy. The refining is now considered finished, and the metal has only to be formed into balls, and condensed under the rolling cylinder. From this state it is brought into mill bar iron. After this last operation, several pieces are welded together, from which it acquires ductility, uniformity, and cohesion. A lateral welding of four pieces together now follows, and the mass passes through a series of cylinders as in the first case, and becomes English bar-iron. 1761. The lamination of iron into sheets is by a refinery furnace, with a charcoal instead of a coke fire. 1762. Malleable iron is often obtained from the ores directly, by one fusion, if the metallic oxide be not too much mixed with foreign substances. It is a mode of working much more economical than that above described, and from the circumstance of its having been long known and used in Catalonia, it is known by the name of the method of the Catalonian forge. The furnace employed is similar to the refiner's forge already described. The crucible is a kind of semicircular or oblong basin, eighteen inches in diameter, and eight or ten in depth, excavated in an area, or small elevation of masonry, eight or ten feet long, by five or six broad, and covered in with a chimney. The tuyere is placed five or six inches above the basin, inclining a little downwards, and the blast is received from a water blowing machine. The first step consists in expelling the water combined with the oxide, as well as the sulphur and arsenic when these are present. This, as usual, is done by roasting in the open air, after which it is reduced to a tolerably fine powder, and thrown at intervals by shovels-full upon the charcoal fire of the forge hearth, the sides and bottom of the basin being previously lined with brasques (coats of pounded charcoal). It gradually softens and unites into lumps more or less coherent, which finally melt and accumulate in the bottom of the crucible or basin. A thin slag is occasionally let off from the upper surface of the melted metal in the basin through holes which can be closed and opened at the discretion of the workman. The melted iron preserves a pasty condition owing to the heat communicated from above. When a mass sufficiently great has accumulated, it is removed, put under the hammer, and forged at once. A lump, or bloom, of malleable iron is thus produced in the space of three or four hours. Four workmen are employed at one forge, and by being relieved every six hours, they are enabled to make 86 cwt. of iron per week. In the Catalonian forge, 100 lbs. of iron are obtained from 300 lbs. of ore (a mixture of sparry iron, or carbonate and hemalite), and 310 lbs. of charcoal, being a produce of 33 per cent. 1763. A visit to some of the iron districts is necessary fully to understand the processes we have above shortly described; but the founding of iron may be well enough observed in the metropolis, though not on so large a scale as in some of the provinces. 1764. The sand employed in casting is of a soft yellow and clammy nature, over which, in the mould, charcoal is strewed. Upon the sand properly prepared, the wood or metal models of what is intended to be cast are applied to the mould, and pressed so as to leave their impression upon the sand. Canals are provided for the metal, when melted, to run through. After the frame is finished, the patterns are taken out by loosening them all round, that the sand may not give way. The other half of the mould is then worked with the same patterns, in a similar frame, but having pins which, entering into holes that correspond to it in the other, cause the two cavities of the pattern exactly to fall on each other. The frame thus moulded comes now under the care of the melter, who prepares it for the reception of the metal. 1765. All castings should be kept as nearly as possible of the same bulk, in order that the cooling may take place equably. It is of importance to prevent air bubbles in castings, and the more time there is allowed for cooling the better, because when rapidly cooled, the iron does not become so tough as when gradually cooled. 1766. In making patterns for cast iron, an allowance should be made of about one eighth of an inch per foot for the contraction of the metal in cooling. And it may be also requisite that the patterns should be slightly bevelled, that they may be drawn out of the sand without injuring the impression; for this purpose a sixteenth of an inch in six inches is sufficient. 1767. The security afforded, not only for supporting weight, but against fire, has, of late years, very much increased the use of it, and may in many cases entirely supersede the use of timber. Again, it is valuable from its being not liable to sudden decay, nor soon destroyed by wear and tear, and, above all, from its plasticity. 1768. Soft grey cast iron is the best sort; it yields easily to the file when the external crust is removed, and is slightly malleable in a cold state. It is, however, more subject to rust than the white cast iron, which sort is also less soluble in acids. Hence the white sort may be employed where hardness is necessary and brittleness not a defect. Grey cast iron has a granulated fracture with some metallic lustre, and is much softer and tougher than the white cast iron. The white cast iron in a recent fracture has a white and radiated appearance, indicating a crystalline structure. 1769. The most certain test of the goodness of a piece of cast iron is by striking the edge with a hammer, which if it make a slight impression, denoting some degree of malleability, the iron is of a good quality. It is important in any casting to have the metal as uniform as possible, and not of different sorts, for different sorts will shrink differently, and thus will be caused an unequal tension among the parts of the metal, which will impair its strength: and, beyond this, an unevenness is produced by such mixture in the casting, for different sorts can never be perfectly blended together. 1770. It is well known, also, that iron varies in strength, not only in samples from different furnaces, but even from the same furnace and the same melting. This, however, seems owing to some imperfection in the casting, notwithstanding which, it is, when well mixed, capable of resisting the greatest stresses in mill and engine work. 1771. The transverse strength of cast iron, as of any other body, is that power which it exerts in opposing a force acting in a direction perpendicular to its length, as in the case of beams, levers, and the like It is well known that the transverse strengths of beams, &c. are inversely as their lengths, and directly as their breadths and the squares of their depths. If cylinders, that they are as the cubes of their diameters. Thus, if a beam 6 feet long, 2 inches broad, and 4 inches deep, will bear 5000 lbs., another of the same scantling, and double the length, will only bear 2500 lbs., being inversely as the lengths. So, if a beam 6 feet long, 2 inches broad, and 4 inches deep will support a weight of 5000 lbs., another beam of the same material, twice the breadth, that is, 4 inches, will support 10,000 lbs., that is, double, being directly as the breadths; but a beam of the same material 6 feet long, 2 inches broad, and 8 inches deep, will sustain 20,000 lbs., being as the squares of their depths. From a mean of several experiments on cast iron, it may be assumed that the ultimate or breaking strength of a bar of cast iron 1 inch square and 1 foot long, loaded in the middle, was 2580 lbs. ; and taking one third, or 860 lbs., as the weight which will not destroy its elasticity, we thus obtain constants for guiding us in the ordinary computations for the sizes of girders, beams, bressummers, &c. The strongest form of the section of a beam to resist a cross strain is this . We do not however think it here necessary to give much more than the rules for finding their breadths and depths, considered as simple figures. The principles on which the rules subjoined are founded may be seen in Gregory's Me chanics, and Barlow On the Strength of Materials, but divested, certainly, of the refinement of Dr. T. Young's Modulus of Elasticity, and some other matters, which we cannot help thinking unnecessary in a subject where, after exhausting all the niceties of the question, a very large proportion of weight is still considered too much for the constant load to be imposed on the examples. 1772. PROBLEM I. To find the ultimate strength of a rectangular beam of cast iron supported at both ends, and loaded in the middle, we have only to multiply the breadth into the square of the depth, and that again by the constant 2580, and the last product divided by the length in feet will be a quotient expressing the weight in pounds averdupois, nearly. Example. What weight will break a cast iron beam 2 inches broad, 6 inches deep, and 15 feet between the supports? If a beam be supported at the middle and loaded at each end, it will bear the same weight as when supported at both ends and loaded in the middle. It may be here observed, that the following rules hold good for inclined as well as horizontal beams, if the horizontal distance between the supports be taken for the bearing. 1773. PROBLEM 2. To find the ultimate strength of a cast iron beam when the weight is placed somewhere between the middle and the end. Multiply twice the length of the longer end by twice the length of the shorter end, which divided by the product will give the effectual length. Using the effectual length thus found as the length in Problem 1., the question may be answered. Example. What is the ultimate strength of a cast iron beam 15 feet between the supports, 2 inches wide, and 6 inches deep, the weight being placed at 5 feet from one end? In the case of any beam fixed at one end and loaded at the other, it is known that it will bear only one fourth of the weight it will bear in the middle when supported at both ends. Thus for Example. What weight will break a cast iron beam 2 inches wide and 6 inches deep, projecting 15 feet from the point of support? 1774. The above rules are equally applicable to beams whose forms are cylindrical, except that in such case the absolute strength of a round bar (for which in that of cast iron the constant is for the breaking weight 2026, one third whereof is 675 for cast iron) is found by multiplying by the cube of the diameter instead of by the breadth and square of the depth. Example. What is the ultimate transverse strength of a cast iron cylinder 15 feet long and 6 inches diameter? In the case of a hollow shaft of cast iron of the same length as in the last example, whose diameter is 9 inches, but whose cross sectional area is the same as a solid cylinder 6 inches diameter, 1775. The following points relative to loads on beams are to be here noted. I. If any beam be fixed at both ends, when loaded in the middle, it is capable of bearing one half more than it will if both ends are loose. II. If loose at both ends, and the weight be applied uniformly along its length, it will support double. III. If it be fixed at both ends, and the weight be applied uniformly along its length, it is capable of bearing triple the weight. 1776. In cases where beams of cast iron are intended to support a permanent weight, the application of the following problem is necessary, in which 860, or one third of the breaking weight, is used. 1777. PROBLEM III. To find the breadth or depth of beams which shall support a given permanent weight. The length between the supports must be multiplied by the weight to be supported in pounds, and the product divided by one third (860) of the ultimate strength of an inch bar multiplied by the square of the depth, and the quotient will be the breadth. If multiplied by the breadth, the quotient will be the square of the depth, both in inches. Example. What should be the breadth of a cast iron beam 15 feet long and 6 inches Example. What depth should be given to a cast iron beam 3.25 inches broad and 15 feet long, to bear a permanent weight of 3 tons in the middle? 6720 × 15 Here 860x3-25=36'06, whose square root is 6 inches. Example. Suppose a cast iron beam 15 feet long and 6 inches deep, made fast at both ends, to be loaded with a permanent weight of 3 tons in the middle, what should be its breadth? A beam when fixed at one end and loaded at the other is known to bear only one fourth of the weight; one quarter of the divisor must therefore be taken, or, which is the same, it may be multiplied by 25. Example. What should be the depth of a beam 3 inches broad, to project 10 feet from a wall, and to be loaded with a weight of 3 tons=6720 lbs. ? When the weight is not placed in the middle of the beam, the effective length must be obtained as in Problem 1. Example. What depth should be assigned to a cast iron beam 15 feet long and $ inches broad, to support a weight of 3 tons =6720 lbs., 5 feet from one end? (2×10) x (2×5) =13·33, effective length of the beam. 15 6720 x 13:33 860 x 3 Here And =347, whose square root 5.9 inches, nearly. The strength of cast iron to wrought iron is as 9 to 14, nearly. N.B. All the above rules may be applied in common practice to find the scantlings of beams by using the following factors instead of that of cast iron, such factors being the ultimate transverse strengths of a bar 1 inch square and 1 foot long of the different sorts of timber to which they are attached. 1778. The greatest variable load on a floor, except in public rooms, seldom exceeds 120 lbs. to the square foot, whence the reader may form a pretty accurate notion of the quantity of strain against which he has to provide. 1779. The cohesive strength of cast iron, from some of the latest experiments, was found in horizontal casting to be equal to 18,656 lbs. per square inch, and in vertical casting 19,488 lbs. to the square inch. One third, therefore, of 18,656=6219 may be used as the factor in computations of the permanent cohesive strength of cast iron. In English wrought iron the experiment gives 55,872 lbs. for the cohesive strength per square inch of English wrought iron, and for Swedish, 72,064 lbs. per square inch. If, therefore, it be required to find the ultimate cohesive strength of bars of cast or wrought iron, the area of their section being found, and multiplied by the relative cohesive strengths above mentioned, the product will be the ultimate cohesive strength, nearly. Thus for Example. What is the cohesive power of a bar of cast iron 14 inch square? If the weight to be sustained be given, and the sectional dimensions of the bar be required, we must divide the weight given by one third of the cohesive strength of an inch bar, and the square root of the quotient will be the side of the square. 72064 Example. What dimension must be given to the side of a square bar of Swedish iron to sustain a permanent weight of 18,000 lbs., =24,021 lbs. being, as above mentioned, the permanent load a square inch will sustain. Here 18000 If the section be rectangular, the quotient must be divided by the breadth. Example. If the breadth of an English wrought iron bar which is required to carry 3000 lbs. be 2 inches, required its thickness. The permanent cohesive strength 55872 3 18624. 3000 Here 18624 ·161, and 161÷2·5='064 of an inch in thickness. 1780. The power of the resistance to compression was heretofore very much overrated. It has been well ascertained by experiment, that a force of 93,000 lbs. upon a square inch will crush it; and that it will bear 15,300 upon a square inch without permanent alteration. The weight of cast and bar iron is as follows: 1781. Lead, the heaviest of the metals except gold and quicksilver, is found in most parts of the world. It is of a bluish white when first broken, is less ductile, elastic, and sonorous than any of the other metals, its specific gravity being from 11300 to 11479, and a cubic foot, therefore, weighing about 710 lbs. It is soluble in all acids and alkaline solutions, fusible before ignition, and easily calcined. The ore, which is easily reduced to the metallic state by fusion with charcoal, is found mineralised with sulphur, with a slight mixture of silver and antimony, in diaphanous prismatical crystals, generally hexagonal, white, yellowish, or greenish, in Somersetshire, about the Mendip Hills. About Bristol, and in Cumberland, it takes the form of a white, grey, or yellowish spar, without the least metallic appearance; in some places it is in a state of white powder or native ceruse; and in Monmouthshire it has been found native, or in a metallic state. 1782. Exposure to air and water does not produce much alteration in lead, though it quickly tarnishes and acquires a white rust, by which the internal parts are defended from corrosion. Pure water, however, does not alter it; hence the white crust on the inside of lead pipes through which water flows must probably be owing to some saline particles in the water. Lead will form an union with most other metals: one exception, however, is iron. Next to tin, it is the most fusible of metals. It is run from the furnace into moulds which form what are called pigs, from which it is run into sheets, pipes, &c. 1783. Sheet lead is of two sorts, cast and milled. The thicker sort of the former, or the common cast sheet lead, is manufactured by casting it on a long table (with a rising edge all round it) from 18 to 20 feet in length, which is covered with fine pressed sand beaten and smoothed down with a strike and smoother's plane. The pig lead is melted in a large vessel, near this table, and is ladled into a pan of the shape of a common triangular prism, whose length is equal to the width of a sheet, from which pan it is poured on to the table or mould. Between the surface of the sand and the strike, which rides upon the edges of the table, a space is left which determines the thickness of the sheet. The strike bears away the superfluous liquid lead before it has time to cool, as it moves by hand along the edges of the table before mentioned. When lead is required to be cast thin, a linen cloth is stretched on an appropriate table over a woollen one; in which case the heat of the lead, before spreading it on the cloth, must be less than will fire paper, or the cloth would be burnt. The strike must for the purpose be passed over it with considerable rapidity. 1784. In manufacturing milled lead, it is usual first to cast it into sheets from 8 to 10 feet long according to circumstances, but the width is regulated by the length of the rollers through which it is to be passed in milling; the thickness varies from 2 to 5 inches. By a mechanical action it is made to pass through rollers whose distance from each other is gradually lessened until the sheet is reduced to the required thickness. For a long time a great prejudice prevailed against milled sheet lead; but it is now generally considered that, for the prevention of leakage, milled is far superior to cast lead, wherein pin holes, which have naturally formed themselves in the casting, often induce the most serious consequences. 1785. The thickness of sheet lead varies from 5 to 12 pounds in weight to the superficial foot, and is used in covering large buildings, in flats or slopes, for gutters, the hips, ridges, and valleys of roofs, the lining of cisterns, &c. The subjoined table shows the weight of lead per superficial foot from one sixteenth of an inch to one inch and a half thick: |