Popular Science Monthly/Volume 51/June 1897/Evolution of the Modern Heavy Gun
|EVOLUTION OF THE MODERN HEAVY GUN.|
PROFESSOR OF PHYSICS IN RENSSELAER POLYTECHNIC INSTITUTE.
DURING the last half of the nineteenth century, a period of extraordinary fertility in the industrial application of all departments of physical science, it would be remarkable if great progress were not made in the development of the materials of warfare, both offensive and defensive. It is true there have been few great wars during the half century just closing, fewer than during the corresponding previous period, when Napoleon made all Europe his chronic battle ground. But with progress in the arts of peace there comes progress in machinery of all kinds. Guns are machines which happily we are not often called upon to use in deadly earnest. The degree of perfection with which a machine does deadly work serves as a powerful argument to induce caution before bringing it into use. If the civilized world ever attains the millennium of freedom from warfare, it will not be because the philosophy of good will to men has triumphed, but because war is too terrible and costly for any nation to risk the sure and swift destruction it brings upon the vanquished. Patriotism will not be extinguished, but it will be tempered with the spirit of rational compromise. During the thirteen years of Napoleon's leadership his wars cost France one billion dollars. During the four years of civil war in America the cost to the Government of the United States was about four billion dollars, apart from treasures expended in vain by the Confederate States. The American civil war was thus at least a dozen times more expensive per year than war was during the time of Napoleon. With the construction and use of the materials employed in modern warfare none but the professional military engineer can be reasonably expected to attain much familiarity. But all have an interest in national preparation for contingencies, and even to the nonprofessional it may be an engaging study to trace in outline the evolution of the cannon as now made at great armories like that at Watervliet, near Troy, New York.
It would be only repeating an oft-told tale to show that our remote human ancestors were all savages, and that the normal condition of society among them was that of warfare. What were the earliest weapons employed we can only conjecture. If we disregard the long and for the most part unknown period that preceded the beginning of definite human records, we find that when these records began man was already acquainted with the ruder processes of metallurgy. But there are no indications that during the age of universal savagery metal was used to any great extent for projectile purposes. Arrows and javelins were early and abundantly employed, and the use of the sling was undoubtedly common among the Israelites long before the dramatic duel between David and Goliath. The Romans in conducting their sieges employed the catapult and ballista for the projection of large arrows and stones; but from the vague description of these instruments we can glean little more than that they were probably immense crossbows. They were unwieldy, but powerful enough to project stones, each as heavy as an ordinary man, over a distance of a hundred yards. During the first dozen centuries of the Christian era there was but little improvement over Roman methods of warfare.
That the elastic force of hot gas suddenly evolved should be substituted for that of a stout cord under great tension could not have been possible without the previous discovery of the means by which such gas could be appropriately generated. There is no probability that we will ever learn definitely the true history of the invention of gunpowder. Quite probably it was independently invented by different persons at different times. There can be little doubt that the knowledge of its composition existed at a very early date among some of the inhabitants of India, where the rich soil under a tropical sun has during many centuries been leached for the purpose of procuring niter. Assuming the presence of this salt in abundance, it would hardly be possible for one who handles it to remain long ignorant of its capacity to explode when sufficiently heated in contact with charcoal, sulphur, or any other kind of fuel. It is not surprising that some of the earlier alchemists should be credited with the preparation of gunpowder. It has been common to attribute its invention to Roger Bacon, whose life lasted through the greater part of the thirteenth century. But his language is characteristically vague; for, in regard to the mixing of saltpeter with sulphur and another Roger Bacon. Born near Ilchester, about 1214; died probably at Oxford in 1292. undefined substance, he merely says, "You will thus make thunder and lightning if you know the method of mixing them." Another claimant to the invention of gunpowder was the German monk, Berthold Schwartz, who is said to have ground together in a mortar a mixture of niter, charcoal, and sulphur. Accidentally allowing fire to come into contact with the mixture, an explosion ensued. The pestle was projected from the mortar and from the hand of the surprised alchemist. This suggested the use of the uncanny substance for military purposes, and the mortar was subsequently made on a larger scale for the special purpose of propelling projectiles.
The determination of the proper percentages of niter, carbon, and sulphur in gunpowder implies a knowledge of the quantitative laws of chemistry. It is not to be supposed, therefore, that the earlier users of this explosive were able to make powder equal in quality to that of modern times, or that they knew how to adjust its granulation to the special purposes intended under varying circumstances. The Saracens seem to have introduced it into Spain for pyrotechnic purposes about the same time that Schwartz made his suggestion regarding its most important practical application. Its first definitely known use was for cannon. These were called "bombards," on account of the noise occasioned by firing.
The primitive cannon was a rude tube made up of iron bars hooped together, edge to edge, like' the staves of a cask. It was by no means readily portable, and was not provided with any wheeled carriage. As an offensive weapon its natural place was on shipboard; as a defensive weapon, upon the wall of a besieged town. This iron barrel was firmly fastened down upon a horizontal bed or to a fixed framework of timber. The balls shot from it were of stone. Since there was no provision for aiming, it can be readily conceived that the enemy might be equally safe or unsafe at a variety of points in front of such an ostensible engine of destruction.
|"Mons Meg" Cannon at Edinburgh.|
|Caliber, twenty inches. Made in 1486 at Mons, Brittany. The arrangement of hoops around staves is shown at the part injured by its bursting in 1682.|
A gun somewhat similar in construction to that in Ghent was dug up about forty years ago from the bed of a river in Bengal, and now stands on exhibition in the city of Moorshedabad. It was made of wrought iron, was more than twelve feet in length, and about seventeen inches in caliber. That the forging of iron on so large a scale was accomplished at such a time and in such a place indicates a marked degree of progress in metallurgy in the far East, and adds force to the thought that cannon may have been in use in Asia long before they were ever employed in Europe.
During the siege of Constantinople, in the fifteenth century, according to Gibbon, the Turks employed cannon with which stone balls, each six hundred pounds in weight, were projected, and the walls of the city were thus breached. Von Moltke mentions such a gun at the same place, twenty-eight inches in diameter at the muzzle, with which a ball more than fifteen hundred pounds in weight was projected by a charge of one hundred pounds of powder. For some of these ancient Turkish cannon the diameter of the stone shot was as much as a yard, while the length of the gun was only five yards.
It is not therefore so much in the size of heavy ordnance as in its efficiency that we of to-day are warranted in claiming much superiority over our ancestors. The plan of hooping iron staves together gradually gave place to that of molding guns, sometimes in cast iron, sometimes in bronze. Wrought iron also came extensively into use for the purpose of gun construction. The gun was made up of a succession of short forged tubes jointed together. Over each joint a ring was shrunk on while hot, for the sake of strengthening the whole. Many guns made in this way during the sixteenth century are still to be seen in European museums.
The use of breech-loading cannon is of considerable antiquity, despite the great difficulty that has been experienced in securing safety in their use. Among the earliest breech-loading devices was that of a short movable tube or chamber, closed at one end. This was loaded to its muzzle and then inserted into the breech of the large tube. It was propped behind with a heavy block of wood or iron, and firmly wedged into position before firing. It is readily seen that with such loose fittings much of the force of the powder was wasted. None of these guns were provided with any facilities for adjustment in aiming. The stone projectile was but poorly fitted to the size of the bore. Not only did much of the expanding gas escape without doing useful work, but the strength of the gun was never sufficient to warrant a charge of powder large enough to send the projectile more than a few hundred yards.
In course of time it became evident that greater efficiency was attainable by the use of smaller cannon and more accurate fitting. The clumsy and unmanageable heavy guns were discarded, and their places supplied by guns many of which were small enough to be carried by a single man. The introduction of the musket was merely one phase in the fluctuation of the waves of custom, a reaction after many unhappy experiences in the use of large cannon which had been inefficient and often more dangerous to the user than to the enemy. The musketeer with his burdensome flintlock became more important than the cannoneer in field work. A variety of forms of small cannon came into use, all of which were, like the muskets, smooth-bored, muzzle-loading arms, made of cast metal of one kind or another. Iron balls were substituted for those of stone, and about the beginning of the present century a weight of eighteen or twenty pounds was deemed best for most artillery purposes. War ships were equipped with armaments sometimes in excess of a hundred small cannon. Custom had fluctuated to the other extreme, but at this stage of evolution guns had become well differentiated into two classes, the musket and pistol being representatives of the one, while the portable cannon was a type of the other. Each was crude in comparison with the war machines of to-day, but efficient enough to make Napoleon the terror of Europe. This warrior's celebrated remark that "God is on the side of the heaviest artillery" was an indication of his view that the limit had not been reached, and that the art of cannon construction was enough developed to warrant the making of yet larger guns.
In the War of 1812 an American officer, Colonel Bomford, introduced a large cast-iron gun, intended specially for seacoast defense by firing bombshells at long range. Up to this time cannon had been made with little or no provision for the variation of stress in different parts of the gun due to the exploding powder. It was known that this stress must be greatest around the seat of the charge, but no experiments had been made to determine even roughly the rate of decrease, although methods were already in use for ascertaining the initial velocity of the projectile shot forth. Bomford bored a hole into the side of a cannon and screwed into this a pistol barrel, with a bullet inserted. A definite charge of powder being exploded in the cannon, the velocity of the pistol bullet gave a measure of the pressure at that point. A series of holes being made in succession from muzzle to breech, the corresponding velocities of the discharged bullets gave an indication of the relative strengths needed to resist explosion and the thickness of metal required. The form of gun was therefore modified to suit the stress, and greater strength in proportion to weight was thus secured. To this improved gun he gave the name of columbiad. This style of gun was soon adopted in Europe, and long continued to be a standard.
But there were inherent weaknesses due to the very fact of employing cast metal. Assume a mass of hot liquid iron poured into a mold to form a solid cylinder, the central part of which is to be afterward bored out. The exterior surface cools first and becomes a rigid solid, while the whole mass has contracted but little. Gradually the interior hardens and crystallizes, but normal contraction is prevented by the rigidity of the exterior shell. The condition of the mass is much like that of a Rupert's drop of glass, which breaks into fragments as soon as the outer shell is broken. The weakest part of the cylinder is the axial region, which is removed by being bored out; but still the weakest parts of the completed gun are its inner surface and breech, the very parts against which the greatest force of the exploding charge is exerted. With such a gun the limit of safety is exceedingly uncertain. The vibration due to discharge weakens the cast iron, and the gun becomes dangerously weak after but little use. Nevertheless, this method of construction did not begin to receive modification of any great importance until about fifty years ago. In 1846 these smooth-bore, cast-iron columbiads varied in caliber from eight inches to twenty inches, and in weight from four tons to fifty-seven tons. The projectiles were spherical iron balls, from sixty-eight to one thousand pounds in weight, the charge of powder never exceeding one sixth of the weight of the ball.
Between 1850 and 1860 Major Rodman, of the United States Army, conducted an epoch-making series of experiments on the improvement of gunpowder and the method of casting iron guns. Dahlgren, about the same time, modified the form of gun, giving it great thickness at the breach and as far as the trunnions, with The Tsar Cannon at Moscow.
Caliber, thirty inches. Seventeenth century. rapidly diminishing diameter thence to the muzzle. This form has often been compared to that of a champagne bottle. The contrast between this and the older forms is well shown by comparing the "Tsar cannon," a thirty-inch gun of the seventeenth century, now in the arsenal at Moscow, with the United States fifteen-inch Columbiads, as improved by Dahlgren. Accepting the proportions thus established, Rodman devised the method of "hollow casting" and cooling from the interior. The melted iron is poured into a vertical mold, the axis of which is occupied by a hollow core. Through a pipe in this cold water is conveyed to the bottom and conducted away at the top after being warmed by the surrounding hot metal. The hardening of this begins thus at the inner surface where the greater strength is needed. The exterior surface of the mold is at first strongly heated from without and this heat gradually diminished, while the flow of water is continued many hours or even days. The cast iron thus goes through a process much like the tempering and annealing of steel. As the metal gradually cools the inner surface becomes strongly compressed, and the outer surface is left in a state of tension. The condition is the exact reverse of that brought about by the older process of solid casting and subsequent boring. The great improvement in strength secured by this process is indicated by Rodman's testing of two columbiads of the same size, material, and form, made at the same time, the one by hollow casting, the other by solid casting. The solid-cast gun burst at the eighty-fifth round, the hollow-cast at the two hundred and fifty-first round. Its endurance was thus three times that of the other.
Rodman's process was of fundamental importance, because it established experimentally the principle of initial exterior extension Rodman Fifteen-inch Gun. and interior compression. This principle is applied in all gun construction to-day, although the use of cast iron has been wholly discarded. Like many other ideas of great importance in the history of invention, it seems to have been evolved independently by several claimants. The names of Blakely, Whitworth, Armstrong, Longridge, Brooke, Treadwell, and Parrott are at once called to mind. To describe their inventions and discuss conflicting claims would require a volume. The discovery of such an important principle, followed by the outbreak of the American civil war, gave an impetus to the improvement of ordnance which was felt over the entire world.
Hitherto the materials used in gun construction were cast iron, wrought iron, and bronze, this last being an alloy of copper with ten per cent of tin. In tenacity bronze is superior to cast iron, but it is softer, more fusible, and more expensive. Cast iron is moderately fusible, but not fixed in composition, having a variable amount of carbon, silica, and other impurities diffused through its mass. Its properties are correspondingly variable, but it is in general hard, brittle, and more or less crystalline. Wrought iron is the result of oxidizing out all of the carbon by puddling, then squeezing out the silica, and rolling so as to develop a fibrous in place of crystalline structure. It is much more tenacious than cast iron, almost infusible, but capable of ready welding and forging. The admixture of carbon seems to confer the property of fusibility.
Steel is the product of the recombination of pure wrought iron with a very small percentage of carbon and sometimes of manganese or nickel. Like cast iron, it is fusible; like wrought iron, it can be readily forged; and it is superior to each in elasticity and tenacity. The idea long ago suggested itself that steel ought to be the best material for the construction of cannon. But the practical obstacle was the great difficulty of securing large enough forgings of steel, and this of sufficiently good quality. Only since 1860 have the methods of steel manufacture been so improved as to make this metal available on a large scale. So important is the relation between cast iron, wrought iron, and steel that it may be well to illustrate this by the use of a diagram due to Professor Merriman. Assume that short rods of these materials, each of the same length and one square inch in cross section, are subjected to great stretching force by the use of a testing machine. As this force increases up to the elastic limit of six thousand pounds, the cast-iron rod becomes elongated proportionally. It breaks suddenly when the stress reaches twenty thousand pounds. At this limit of tenacity the rod has been increased in length less than one per cent, as shown in the diagram. The wrought iron becomes lengthened at a less rapid rate, reaching its elastic limit for a stress of about twenty-five thousand pounds. In each case, up to the elastic limit, if the stretching force be removed the rod will recover its former length and condition. On further increasing the stress, the wrought iron stretches at a more rapid rate, and bears a stress as great as fifty-eight thousand pounds. If now the force be withdrawn the iron remains in its deformed condition, the lengthening being about twenty-two per Curves showing Tensile Strength of Timber, Cast Iron, Wrought Iron, and Steel. cent. On again applying the stress there is further rapid lengthening up to twenty-five per cent, this yielding causing a decrease of stress till the rod breaks at a limit below fifty-eight thousand pounds. The elastic limit and the breaking limit are thus widely different. In the case of steel the elastic limit is not reached until the stress becomes fifty thousand pounds. Its elastic limit is thus double that of wrought iron. Further increase of stress now causes the steel to increase its rate of stretching, and permanent strain results. Its breaking limit, one hundred thousand pounds, is nearly double that of the wrought iron, and is reached when the yielding attains fifteen per cent. This is not much more than half of the twenty-five per cent of yielding of the wrought iron.
The figures just given are only averages. Cast iron has been made with a tenacity in excess of forty thousand pounds, while that of steel may vary in different specimens from sixty thousand to three hundred thousand pounds. This wide range shows that for the construction of a heavy gun, if steel be employed, the utmost care should be exercised to secure that of the highest grade possible, in order to withstand the enormous tension due to explosion. As soon as this tension becomes equal to the limiting measure of elasticity for the steel, the wall must yield, even if the thickness of the gun were infinite. Since the breaking limit, or ultimate tenacity, of cast steel has just been seen to be, on an average, at least five times that of cast iron, it follows that, with the same diameter and thickness of metal and the same weight of projectile, a steel gun warrants the use of a charge of powder of the same quality five times as great.
Professor Treadwell showed in 1856 that, if we assume a gun to be made up of a large number of uniform, cylindrical, concentric layers of metal, then the resistance of each layer to the bursting force of explosion will vary inversely as the square of the diameter. The stress, therefore, decreases at a rate very similar to that of the radiation of heat or light. If the wall of the gun be under no initial stress of any kind, its inner portion must have great resisting power, and very little is gained by thickness of wall much in excess of the diameter of the bore. Treadwell therefore proposed a plan of construction by which a cast-iron tube of only moderate thickness should be re-enforced by a series of layers of encircling wrought-iron hoops. These should be shrunk on while hot, so that, after cooling, the cast iron tube is strongly compressed while the wrought-iron hoop becomes stretched. The force of compression is thus added to the ordinary strength of the cast iron to resist explosion. With various modifications this plan has been carried out by most gun constructors during the last forty years. During the civil war it was applied with great success by R. P. Parrott, of West Point, and by Blakely, Armstrong, and Whitworth in England.
It is perhaps impossible to say what inventor was the first to introduce the use of rifled cannon. They have now entirely superseded smooth-bore guns. The Parrott rifled cannon, made of cast iron according to the Rodman plan and re-enforced around the chamber with a hoop of wrought iron, was the most generally serviceable gun employed during the late war, more than two thousand of them coming thus into use. The largest of these was twelve feet in length, with a bore ten inches in diameter, its weight being about twelve tons. A charge of twenty-five pounds of powder was employed to project a shot weighing two hundred and fifty pounds. The cost of its construction in 1863 was forty-five hundred dollars.
These details are given for the sake of subsequent comparison with the rifled cannon of to-day. For twenty years after the close of the war there was a period of stagnation in America, so far as development in ordnance was concerned. Our coast defenses continued to be provided with, nothing better than the Parrott rifles and smooth-bore Rodman guns which had been in use during the war. Meanwhile there had been great progress in Europe, particularly in France and Germany. In 1885 a commission appointed by Congress reported the necessity for heavy expenditure of money in order that this country be put into a condition of reasonable readiness to repel foreign invasion. During the last ten years appropriations to the amount of twenty million dollars have been made to meet these needs, and the work of rehabilitation is now well started.
The rifled gun of to-day, as finished at the Watervliet Arsenal, is constructed almost wholly of steel. This is of the best quality that can be produced on a large scale in American foundries. It is made by the "open-hearth" process, for the most part at Mid-vale and Bethlehem in Pennsylvania. The forgings, after undergoing thorough official inspection and careful testing, are sent to the great gun shops at Watervliet. Here the various parts composing a gun are worked up, assembled together, and finished. Before assignment for government service each gun is subjected to a searching test, more severe than should reasonably be expected in actual use.
The largest gun thus far designed at Watervliet is a rifle of twelve-inch bore, forty feet in length, and fifty-seven tons in weight. From such a gun an elongated steel-pointed projectile, weighing one thousand pounds, or as much as an ordinary horse, is shot with a charge of five hundred and twenty pounds of powder. It receives an initial velocity of two thousand feet per second, and would penetrate through rather more than two feet of steel armor plate put in front of the muzzle. If shot into the air at the proper elevation it would pass over a range of nearly nine miles. Such a missile, thus fired from the lower end of New York city, would pass over Central Park into the district beyond Harlem River. This range would be covered so quickly that the shot would reach its destination several seconds before the sound of the explosion is heard at the same point. The initial energy of the projectile would be sufficient to lift a weight of twenty-seven thousand tons through a height of one foot. If this weight were that of a spherical mass of gold, the heaviest popularly known metal, its diameter would be nearly forty-six feet, and its value eighteen billion dollars. This is more than a dozen times the value of the total gold production of the world during the last twenty years.
The cost of such a gun is about sixty thousand dollars; that of the charge of powder, one hundred and seventy-five dollars; of the armor-piercing projectile, three hundred and fifty dollars. The cost of a single discharge thus exceeds five hundred dollars. But this is not all. So great is the wear and tear of each discharge Sectional Diagram, showing compression of tube and extension of hoops after assemblage of the component parts of a gun. upon the hundred and fifty rounds the gun becomes unfit for further use until it is relined by the insertion of a new rifled tube within the original tube, the old rifling having been removed. The gun will then stand two hundred and fifty more rounds. Assuming six hundred rounds for the entire life of the gun, each round thus costs one hundred dollars in wear and tear, in addition to the five hundred dollars' worth of material used in loading. Such a gun as this is but single small element in the cost of a modern war. Several of them, besides a number of smaller guns, are usually placed on every large armor-clad battle ship. The cost of this with its equipment mounts up into millions of dollars. Nevertheless, it has been necessary to coin into our language the word "jingo," to designate the bragging noncombatant who clamors for war because of the fancied stimulus which it is supposed to give to patriotism and prosperity. On comparing this gun with the largest Parrott rifle of thirty years ago we see that its length is more than three times, its weight nearly five times, and its cost thirteen times as great. For the cast-iron Parrott gun the charge of powder weighed about one tenth as much as the projectile. For the modern steel gun this ratio is raised to one half, with corresponding increase of destructive energy.
Passing now to the construction of the modern gun, a longitudinal section shows an inner tube rifled within and slightly enlarged at the breech end of the bore. Around this is a long tubular jacket extending from the breech two thirds of the length of the gun. Around this jacket is a series of compressing hoops, and around this a second or outer series of the same. Originally the interior diameter of the jacket is a little less than the exterior diameter of the tube. By heating the jacket sufficiently it is made to expand until it can be slipped over the cold tube, which becomes enormously compressed by the subsequent cooling of the jacket. In like manner the first hoop is too small to be slipped over the cold jacket except when heated for this purpose. The same remark applies to the second hoop. The final result, as shown by the cross-sectional diagram on opposite page, is that the diameters of the tube, both internal and external, are permanently diminished by the compression of the jacket, while those of the hoops are permanently increased. Their contractile force is not sufficient to compress the jacket, which is itself resisting the enormous reacting force of the compressed tube within. The hoops therefore serve to re-enforce the jacket by their own tendency to contract from the enlarged condition in which they were applied while hot. They are in a state of permanent tension. The scale of differences exhibited in the diagram is greatly exaggerated to make these perceptible. The longitudinal diagram shows by curves how the expansive force of the exploding powder diminishes from breech to muzzle, how the yet greater elastic
resistance of the steel components, after they are assembled together, is adjusted to resist this expansive force, and how the velocity of the projectile increases from breech to muzzle.
All rifled guns built in America at present, whether for seacoast, siege, or field artillery, are breech-loading. Many futile experiments were made before a successful breech-loading mechanism, was perfected. An explanation of either of the two modern systems would be beyond the scope of the present discussion. It
may be sufficient to say that the system in use in America is substantially that of the French, an interrupted screw which fits into the breech and is provided with an efficient gas check. This is so constructed that the mere fact of explosion tightens the gas check and effectually prevents the escape of hot gas between the threads of the screw.
The largest and most celebrated gun factory in the world is that of Krupp, at Essen in Germany, near the Belgian border. Besides monopolizing the construction of guns for the German Government, this factory has supplied a great number to most of the leading powers of Europe. It was established in 1818, and from the very outset attention was concentrated upon the making of steel. The first finished piece of artillery in cast steel was made in 1847. This was a small field gun capable of projecting a ball of only three pounds. The manufacture of steel at these works has since been so perfected that Krupp can now be scarcely said to have an acknowledged rival in the world. His magnificent display at the Chicago Exposition was seen and admired by many thousands of visitors. Among these exhibits was a steel rifle forty-two centimetres (16·54 inches) in caliber, and thirty-three calibers (forty-six feet) in length. Its weight is one hundred and twenty tons, or a little more than double that of the twelve-inch rifle at Watervliet. With a charge of nine hundred pounds of powder it gives an initial velocity of two thousand feet per second to a projectile weighing twenty-two hundred pounds, whose initial energy is thus sixty thousand foot tons. When fired at an elevation of about eleven degrees it sends this projectile to a distance of five and a half miles, and it pierces through armor a yard thick at a distance of a mile and a quarter. Another rifle, twenty-eight centimetres (eleven inches) in caliber and forty calibers (thirty-seven feet) in length, when elevated forty degrees sends a seven hundred and sixty pound projectile over twelve and a half miles. This is the distance from the Battery to Fordham in New York city. The shot reaches an extreme height of a trifle over four miles. It could thus be easily made to clear the highest mountain peak in North America.
The power of endurance of a gun diminishes rapidly with increase of projectile power. The life of the American twelve-inch rifle has been given as only five hundred or six hundred rounds, while a field gun of modern make may be fired thousands of times if used with reasonable care. Within the next two years a new rifle of sixteen-inch caliber will be constructed at Watervliet. This is nearly equal in size to the monster Krupp gun at Chicago. Such immense guns can be employed only for seacoast defense. In handling them complex machinery is necessary, not only for moving and adjusting the gun but for loading it. No group of soldiers could without machinery lift and put into place a projectile weighing a ton. It seems doubtful whether any real advantage can be gained by going beyond the limits of size already
reached. The difficulty at present is not confined to that of manipulation, but extends to the quality of the forgings made on so large a scale. Krupp makes his guns entirely of "crucible" steel, such as is employed for cutlery. Made by this method, steel is indeed the most uniform in composition, but nowhere outside of the Krupp works has it been manufactured on a scale large enough for great gun forgings. In France, in England, and in America, the "open-hearth" process is depended upon, which yields a high grade of steel; but in uniformity of composition and elasticity it can scarcely be equal to the more expensive crucible steel. This perhaps may at present be only a matter of opinion. On such a point no definite and final conclusion should be reached without a series of comparisons such as can not be accomplished in a day.
An unfortunate mishap which occurred at Watervliet in 1895 may have some bearing in this connection. In assembling the parts of a forty-caliber twelve-inch rifle, the tube was, as usual, rested vertically upon its breech end, and the heated, jacket was let down over it. The heating had been insufficient to secure all
Weight of gun, seventy-one tons.
the expansion needed, and as a result the cooling jacket gripped the tube before quite reaching the final position intended. An interesting problem was now presented, that of separating the tube and jacket after they had become thoroughly cool, and completing the process which had been so unexpectedly interrupted. The gun was provided with the inlet and outlet tubes such as Rodman employed to secure a continuous flow of water in hollow casting, and the exposed part of the tube below the edge of the jacket was inclosed in a bag of asbestos cloth through which a stream of cold air could be transmitted. The gun with its adherent jacket and these adjuncts was let down into a furnace so as to heat the jacket. Immediately a flow of cold water was started through the tubes, and of cold air through the bag, while the inclosing jacket was soon raised to a temperature estimated to be 1100° F., which was maintained for several hours. The experiment proved unsuccessful. It was subsequently repeated twice with slight modifications, but all in vain. To test the correctness of the theory thus applied, a "dummy" was constructed, its parts assembled together firmly, and the experiment of separating them was rewarded with prompt success. On account of the magnitude of the large gun it had been impossible to heat it with perfect uniformity from without, while no such difficulty was experienced with the much smaller dummy. A series of measurements upon the large gun revealed the fact that during the first experiment it had become warped, and the diameter of the tube had been diminished in varying degrees at different parts.
Whether such results as these would have been brought about had the materials been of the best quality of crucible steel instead of open-hearth steel can not be answered positively. The larger the gun the greater is the danger of such mishaps. It is left to coming experience to determine which is to be the steel of the future for gun construction.