At one time, when cast iron was employed for boilers, shafting, large ordnance, and bridges, its strength was of great consequence. It has now become usual to employ wrought iron or steel for the applications just named, and, indeed, wherever great absolute strength is required. Even engine beams, since the lamentable failure at Hartley, are being made of wrought iron. So the importance of great strength in castings has no doubt been lessened; and for most purposes it has been found cheaper to employ a somewhat larger quantity of ordinary iron than to pay a higher price and incur the delay often attending the search for a superior quality. For many purposes, indeed, as in engineer's tools, a liberal allowance of metal is requisite to secure stiffness,-a kind of stiffness better provided in such cases by inertia, or mere dead weight, than by the absolute resistance of the metal per square inch. Yet there are still purposes, as in the case of railway chairs, water pipes, columns, &c., without mentioning hydraulic press cylinders and steam cylinders, where great strength in cast iron is of much importance; and cast iron is still the material principally employed in America for cannon of 13-inch, 15-inch, and in recent instances even 20-inch calibre. This is not the cast iron, however, of which guns have long been made in England, and were it not indeed greatly superior to our own, it would never withstand the proof and service charges which the heavy ordnance in question is known to bear. The report of the chief of ordnance in the United States Navy gives the service of one of the 15-inch cast iron guns as follows: It was fired 900 rounds with a 440-pound solid shot. The charge of powder, at first 35 pounds, was successively increased to 50 pounds, 60 pounds, and 70 pounds. With 60-pound charges 220 rounds were fired, and the gun only burst with a 70-pound charge and 440-pound shot at the end of 900 rounds. It is doubtful if even as good results have been, or will be, attained by the most carefully wrought iron guns of the same calibre. Upwards of 100 of these 15-inch guns are now in service. Before going on with the consideration of how such great strength in cast iron is attained, it may be as well to give the following notes of the 20-inch cast iron guns, of which a number have already been made. They weigh 51 tons each, and the first of these guns was 13 days in cooling. They are 20 feet 3 inches long over all, and 17 feet 6 inches long in the bore. Their greatest diameter is 5 feet 4 inches. They are fired with 100-pound charges of powder, and a solid shot weighing 1000 pounds.

I shall say nothing of the selection of particular brands of iron, nor of the great importance of proper mixing in the cupola, for I could only say, what every qualified founder well knows, that upon these a great deal depends. I could give no direction better than those upon which founders now act, each having to choose and mix the irons which he has found best for his own purposes in his own district, for it is always important to him not to send further than is necessary for his pigs. But there are modes of increasing the strength of a large number, if not all, of the irons known to commerce, and although there is still much doubt as to the relations between the chemical constitu

tion and the strength of iron, it is certain that all the known modes of strengthening cast iron are modes whereby its proportion of uncombined carbon and of silicium is known to be diminished. puddle cast iron up to a certain extent, and stop at the right point, we have steel of very great strength, and if we carry the puddling far enough we have wrought iron. So if we melt cast iron with wrought iron, as in making what is called Stirling's toughened metal, we lessen the relative proportions of the impurities to the iron as contained in the pig, and if we do not get a remarkably tough metal, we, at any rate, produce one of great hardness, and some of our locomotive makers employ such a mixture purposely to obtain hardness in their steam cylinders. So also, by oxidizing cast iron at a high heat, as in the treatment for malleable iron castings, we gain undoubted strength and toughness. Here, too, the carbon and silicium of the iron are lessened in quantity, and so it may be apprehended that they are by the American practice of remelting all the iron employed for cannon and keeping it for some time in fusion. This practice at one time went so far as three and even four remeltings, the iron being kept in the fluid state for three hours at each melting. In this way the tensile strength of iron, ranging from 5 tons to 61⁄2 tons in the pig, has become 9 tons at the first casting, and after remaining in the melted state for two hours, 13 tons at the second casting, and 15 tons per square inch at the third casting, the period of fusion at each melting being from one to three hours. The final strengths thus reached are very great, in one case reported by Major Wade, of the United States Ordnance Board, a tensile strength of 20 tons per square inch of cast iron having been obtained. The American ton is generally 2000 lbs., but the strengths I have quoted are in tons of 2240 lbs. These great tensile strengths do not appear, however, to give a tough metal, using the term tough to express the product of the cohesion and extensibility of the iron. It was found that, in employing iron having an average tensile strength of 38,000 lbs., or 17 tons per square inch for 8-inch guns, they burst at the seventieth or eightieth fire, while 10-inch guns, made from iron having a strength of 37,000 lbs. per square inch, burst at the twentieth round. This was known in 1851, and in the following year, at the Tredegar Iron Works at Richmond, Virginia, where I was then engaged, and which was one of the leading foundries for supplying cannon to the United States government, a return was made to iron of a strength of 30,000 lbs., which, having more elasticity, as it was then thought, gave a really stronger gun. It has since been ascertained that the real fault with the stronger iron was that it contracted more in cooling, and as insufficient provision was made for equal contraction throughout the casting, the guns of strong iron were thus under great initial strain from their own shrinkage. This very strong gun iron contracts generally ths of an inch per foot in casting. The driving-wheels of American locomotives are of cast iron, and when, in 1851, to secure greater strength against breakage, gun iron of a strong quality was experimentally used, it was found that the wheels broke worse than ever, as they were strained to a great

extent by their own shrinkage before they came out of the foundry. This gun iron is simply the better classes of iron mixed and melted in an air furnace, the cupola never being used for guns, as indeed it never ought to be used for any castings intended to have great strength, on account of the over-heating of portions of the metal and the direct action of whatever sulphur may be contained in the coke. In the Bessemer process, where the exclusion of sulphur is so important, the pig metal is for this reason melted in a reverbatory furnace, or air furnace, as it is sometimes called.

Now, as all the processes whereby cast iron is strengthened are processes whereby its proportion of contained carbon and silicium is diminished, some quicker and much cheaper mode of effecting this object is required than that by remelting or by partial puddling. This quicker and cheaper mode would be had by a partial application of Mr. Bessemer's treatment, that is, by blowing air through the iron for perhaps three or four or five minutes, instead of twenty. But, it will be asked, if you are to have the Bessemer apparatus at all, why not convert the iron at once into steel? There are several reasons why we should not. To make steel, a much higher quality of iron, and generally the addition of spiegeleisen, is necessary. As steel, the metal cannot be run into goods, but only into an ingot, which requires very heavy hammers to forge it, as well as machine tools of unusual strength to finish it after forging; the wear of the converter and other plant would be much greater for steel than for toughened iron. The waste of metal before the finished article, whatever it might be, could be produced, would be greater for steel than for cast iron. I have recommended this partial application of the Bessemer process, and I believe that when more attention comes to be given to strength in castings, this treatment will be adopted. The apparatus for carrying it out would be exceedingly simple, and would be worked with but little trouble, a blast being derived from the rotary blower already described.

But absolute strength in the iron of large castings is of little consequence unless they cool, after pouring, in such a manner as not to leave them subject to considerable internal strains. We know that the late Professor Hodgkinson found that with the iron he experimented upon the compressive strength was six times that in tension, and hence that the bottom flange of a cast iron girder should have six times the sectional area of the top flange. But very few, if any, engineers adopt such a proportion, as the casting would, in all probability, crack in cooling. Most of my audience have seen the cast iron bridge over which the London and North-Western Railway crosses the Regent's canal. The first girders for this bridge were cast at the Tinsley Park Works. The iron made there was very hard; and I have been told by my friend, Mr. Shanks, who was engaged there at the time, that it would chill to a depth of two inches. It was used, among other things, for making rollers to roll steel. The Regent's canal bridge drawing was sent down there, and they made the patterns and cast the girders. They broke through and through in cooling. Then

Other

they altered the patterns; and by pulling off the sand from the thicker portions of the castings, so as to equalize the cooling, a number were cast with the loss of one out of every six. At last, six were sent up to London; and of these every one broke in a thunder storm. girders were then cast of different form. Castings, over-strained in cooling, are apt to break even under a moderate degree of vibration; and the late Mr. Rastrick, once of the Bridgenorth foundry, and afterwards engineer-in-chief of the London and Brighton Railway, once stated in evidence how a number of cast iron boilers he had made cracked open after a peal of thunder.

I have seen, and so, no doubt, have many others, a railway wheel cast in a chill, and which, on being taken from the mould immediately after pouring, broke in two within a quarter of an hour. And, if the experiment were made, there is not the least doubt that a heavy gun, pulled out of the sand as soon as the metal had set, and then finished, would burst at the first round. The outside would cool first, compressing the liquid iron within. This, cooling afterwards, would pull away from the iron already set around it-or, if it did not actually separate, the strain of contraction would be such that the gun would be ready to crack as soon as it was violently disturbed. An unannealed glass tumbler is as good a comparison as any. The old-fashioned playthings, Prince Rupert's drops, illustrate the same effect of internal strain due to unequal cooling, glass being particularly brittle in this respect, in consequence of its low conducting power, and from its having no ductility when cold.

To make a casting of great strength it is necessary that all parts cool alike or nearly so. In the case of guns cast solid, the core bored out is often found honey-combed by retarded cooling; and the metal forming the surface of the bore can be proved to be under considerable initial strain in consequence. Of course, guns were cast hollow many years ago; but not until 1847 was it proposed to cool the core, after casting, by means of water circulating in pipes within it. Captain Rodman, in that year, patented the mode by which all the larger American guns have been cast. Within the core are two water pipes, one inside the other, and like those in Mr. Field's boiler, known to so many in this society. Water flows down the inner pipe, which is open at both ends, and rises through the outer pipe, which is closed at the bottom. A perfect circulation of water is thus secured. In casting one of the 20-inch guns, February 11th, 1864, water was thus run through the core for twenty-six hours, at the rate of thirty American gallons per minute for the first hour, and sixty gallons per minute for each subsequent hour, equal to 341 tons of water in all. The iron was considered of too hard a quality to be further cooled by water, and for the next twelve days air was forced down the bore of the gun at the rate of 2000 cubic feet per minute. During the first hour after casting, the water flowing in at 36°, came out at 92°. During the second hour, with twice the quantity of water flowing through, it came out at 61°. In other cases, in casting 10-inch guns, as much as 700 tons of water have been run through the core, the water-cooling

occupying four days, or nearly 100 hours. In some of these cases, a fire was made at the bottom of the gun-pit, and continued for sixty hours, the outer iron casing of the gun-mould being kept at nearly a red heat for the whole time. It is by these means that all parts of the gun are cooled alike, or nearly so, and, with iron of a tensile strength of, say, 13 tons per square inch, that such great endurance has been attained in firing.

Nearly all the railway wheels in use on the American lines are of cast iron, chilled on the periphery. It is not merely that these wheels are cheap, but they are preferred to the wrought iron wheels as used on English railways. I am not now speaking of the engine driving wheels, but of the carriage and wagon wheels, of from 2 ft. 6 ins. to 3 ft. in diameter, although the size is very seldom greater than 2 ft. 9 ins. The cast iron wheels run until they are worn out, and they wear for a long time; whereas the wrought iron wheels require frequent turning, and, still worse, their flanges soon become worn so thin as to become unsafe, a fact due, perhaps, to the inferior condition of the American lines. It was, however, a long time before the American founders could produce chilled wheels which should be safe under all circumstances; and when it is remembered that they are now employed as the leading wheels of the heaviest express engines working on lines, of which, what we should call the ballast, is sometimes frozen as hard as a rock for two or three months in the year, and in a climate where the mercury is occasionally from 10° to 20° below zero, or 40° to 50° of frost, and when it is added that these wheels do not break oftener than wrought iron wheels on the best English lines, it must be added that they are as safe as anything can be. In this I am speaking from my own knowledge, extending over a period of ten years, during the whole of which time I was closely connected with the leading American locomotive factories and lines of railway. The founders had to obtain not merely strong iron, in respect of tensile strength, but an iron of considerable toughness, and, besides, an iron that would chill well. As a rule such iron is only obtained by careful mixing; and it must be sought by long and costly experiment. I do not doubt that iron for excellent chilled wheels, if they were ever required, might be found in England; but I would not run the risk of saying what mixtures would produce it, although I should say Blænavon cold blast and the Forest of Dean irons would enter into such a mixture, with a little iron like that made at Tinsley Park for hardening. The chief difficulty with the American founders was that presented by the unequal contraction of the wheels in cooling. At first the wheels were made with spokes, but as the rim was quickly cooled in the chill, thus compressing the still fluid iron in the nave, which subsequently contracted away from the rim, it was necessary to divide the nave radially into two or more portions, and to afterwards fill the openings thus made in the nave with lead and antimony, a pair of stout wrought iron rings being shrunk over the ends of the nave, to compress it properly upon the axle. But it happens in the case of spoke wheels cast in a chill, that, from the greater quantity of iron at the ends of the spokes,.