Cassini, Marriotte, Hawksbee, Robins, and Hutton, while many of the most important practical details applied to the military art have also come from men such as Forsythe, a country clergyman, the inventor of the percussion lock, to which we may add the following names of Americans: Sharpe, Colt, Treadwell, Dickinson, Hotchkiss, Doremus, Professor Barnard, James, &c.

11. The first efforts at heavy guns were in wrought iron, and they have, from a very early period, been tried in every conceivable shape, but always burst. James II, of Scotland, was killed by one of a pair made by an artist of his day, and the other of the pair burst at a later time. A large wrought-iron gun burst on board the Princeton, killing Mr. Upshur, a member of the cabinet; in fact, they have in every instance failed.

12. After them came a series of bronze guns; then cast-iron was tried for a while, after which wrought-iron built up guns, fabricated with more skill than had previously been bestowed upon them; yet they failed, for 1,476 Armstrong guns, or parts, were returned for repairs up to July, 1862. Then cast-iron again cast solid, the same as the Dahlgren system, which was in turn abandoned for cast-iron banded guns, (Parrott's system.) The English government burst or rejected 356 of this kind, and they were abandoned as old iron in 1862, following after or coeval with which came guns cast hollow as by Colonel Rodman. Not one of these systems has stood the test of battle, and most of them not that of proof.

13. The Dahlgren gun has exhibited better endurance in proof than any other large gun, yet Dahlgren only allowed limited charges of powder fired from them, and that behind spherical shells; they could not stand a charge commensurate with the size of the gun, with a solid shot, especially under rapid firing. Although the gun is so made as to secure a quality of porosity to the interior metal, which can be heated inside without being expanded to sufficient extent to break the dense surrounding metal, under slow firing, as used in proving and ordinary practice: as, for example, the finger may be inserted into a sponge without rupturing the outside, while a needle cannot be inserted into a cake of ice without causing radiating cracks; and the effect on this gun under heavy firing would be similar to the effect of powder in rock blasting. "The rock is somewhat compressible and of brittle texture; the pressure of powder enlarges the diameter of the hole by compressing the material immediately surrounding it; then first, suppose the bore two inches in diameter to be so enlarged as to start two cracks on opposite sides to a depth of two inches, the gases of powder enter these cracks, acting then upon a surface six inches wide. If the pressure in the bore, two inches in diameter, was before sufficient to induce cracks two inches wide each side, when the pressure acts upon six inches the cracks will be increased six inches in addition on each side, making eighteen inches width of surface upon

which the pressure acts to continue the fracture further."
If we make equidistant circular marks on the end
of an India-rubber cylinder, and stretch it, we can see
plainly how much more the inside is strained than the
outside or even the intermediate parts. The spaces
between the marks will become thinner, each space
becoming less thin than that outside of it, and the inner
space much thinner than the others, showing that
when the inside is strained almost to breaking, the
intermediate parts are doing much less work, and
those far removed almost none.

India-rubber cylinder, with equidistant concentric marks.

15. LAW OF STRENGTH OF CYLINDERS.- -"In the first volume of the Transactions' of the Institute of Civil Engineers, p. 133, there is a paper by Professor Peter Barlow, F. R. S., on the Strength of Cylinders. The law he deduces is, that in cylinders of metal the power exerted by different parts varies inversely as the squares of the distance of the parts from the axis. Thus, in a 10-inch gun, when the inside, which is 5 inches from the axis, is fully strained, the metal 2 inches from the inside, or 7 inches from the axis, can only exert a force 25-49, or little more than half as much; 3 inches further, 10 inches from the axis, the force exerted diminishes to 25-100, or but a quarter of that exerted by the inside; and if the gun be 12 inches thick, the outside, which is 17 inches from the axis, can exert but 25-289, or about 1-12 as much power as the inside. Of course, casting the gun still thicker would add but very little to its strength; we cannot, therefore, be astonished that it has been found in practice that cylinders for hydraulic presses, with a thickness equal to about the diameter of the piston, are very nearly as strong as if ten times as thick.

The same cylinder, stretched by internal pressure; the concentric marks show the inferior stretch of the exterior.

16. "In 1855, Dr. Hart, of Trinity college, Dublin, investigated the problem. His calculations (see note W., p. 259 of Mr. R. Mallet's work on the Construction of Artillery) give the greater strength to the inner parts, but still less to the outer, than those of Professor Barlow. Both these gentlemen, as well as General Morin, and Dr. Robinson, the astronomer, who have also studied the question, agree that no possible thickness can enable a cylinder to bear a pressure from within greater on each square inch than the tensile strength of a square inch bar of the material; that is to say, if the tensile strength of cast iron be six tons per inch, a cylinder of that metal, however thick, cannot bear a pressure from within of six tons per inch."

17. Hence the necessity of shrinking bands one upon another each with a certain pressure upon what it encloses to get a gun into the state of so-called "initial tension" necessary to its sustaining the greatest pressure from within possible to a given thickness of wall, which as applied to a hydraulic press cylinder is correct, but for a gun it is a fallacy, the effect of the expansion of the interior by the heat of the powder not being taken into the account.

18. These solid or hollow-cast guns are thus useless against armored ships, which is now the means of defence to which our means of offence must be adapted.

19. The Navy Bureau design adopting the Rodman system of casting hollow and cooling from the centre, thereby admitting their want of confidence in the Dahlgren plan. Yet this will increase their weakness against the expansive force of heat engendered in battle firing. For it has been seen that the Dahlgren 11-inch gun is not capable of enduring heavier charges than 15 pounds of powder and a hollow shot, and that only safe under slow firing, principally because of its porosity inside, whereas if it is cast hollow as in the Rodinan plan, and cooled from the centre, although it will stand occasional heavy charges better than the other, and have more enduring surface to the bore, it must burst when fired rapidly, even with small charges, thereby rendering it entirely inefficient for actual service. For the inevitable effect of the system is to produce an initial tension to the verge of rupturing, rendering them certain to explode by the force of the metal expanding, when quickly heated from the interior; in fact one of the Rodman guns burst itself on cooling in the foundry at Pittsburg, before it was taken out of the pit.

20. There are the only three systems of casting guns recognized by the departments, and it has been clearly proven that they are all worthless. Of the

classes of guns that have, up to this date, been fabricated, those of cast iron are of three kinds: the Dahlgren gun, cast solid, the Rodman gun, cast hollow, and cast iron guns banded round the breech with wrought iron.

21. Of wrought iron and steel guns there are three kinds : those made from solid forged masses, as by Wiard in 1861, by Krupp of soft steel, and by Ames of wrought iron. Those that are barrel-banded, as by Armstrong, Blakeley, Whitworth and Treadwell, and staves with bands, as proposed by Mallet, and practiced from the most ancient times. Bronze guns are only used for small calibres. Each kind of material fabricated can be used with success, in making small guns, but for large guns, although made in the same manner, they invariably fail when subjected to the actual conditions of service, and at this time no absolutely safe large gun is to be found in any service in the world.

22. That this failure is due to other causes than inability to withstand the pressure of the gases of the powder, may be found in the fact that a steel gun, with a tensile strength of 120,000 pounds to the inch of section, exhibits less endurance under rapid firing than a cast-iron gun of the same calibre, form, and size, with a tensile strength of 18.000 pounds. Or 8-inch cast-iron guns made from metal having a strength of 38,000 pounds to the inch of section, have less endurance than a gun of like calibre and form of which the iron has a tensile strength of only 27,000 pounds.

23. The following extract from Rodman's work on ordnance, pages 137 and 138, (erasing the repetitions of the words solid-cast whenever they occur,) fully lustrate this principle." It is not deemed out of place here, in order to show the necessity of further investigation into the properties of cast-iron, in its application to the manufacture of cannon, to notice some facts in the history of gun foundering in this country since 1849.

24. The very low endurance of the first pair (8-inch) of experimental guns which were cast in that year, was attributed to the inferior quality of the iron of which they were made. Two years were spent in searching after a better quality of iron, which was undoubtedly found; and in 1851 another pair of 8-inch guns was cast.

25. The iron in this pair of guns has a tenacity of near 38,000 pounds, while that of the iron in the first pair was only between 27,000 and 28,000 pounds. The gun of the first pair burst at the 85th fire, and that of the second pair at the 73d fire; the superior iron giving the inferior gun.

These results did not, however, destroy confidence in strong iron for guns, and the first pair of 10-inch guns was made from the same lot of iron; and with a tenacity of iron of 37,000 pounds the gun burst at the 20th fire. This result weakened confidence in very strong iron, and the tenacity was reduced. In 1857, after guns of good tenacity had failed at Fort Pitt, South Boston, and West Point foundries, four out of seven guns offered for inspection at the last named foundry have burst in the proof.

26. Mr. Parrott, proprietor of the West Point foundry, one of our most experienced gun founders, cast his trial contract guns of iron having a tenacity of 30,000 to 32,000 pounds. One of these guns has endured 1,000 service charges of 14 pounds powder, (800 rounds with shell, and 200 with shot.) The iron selected at that foundry, and from which the last five experimental guns have been made, was of the same quality, and in the same proportion as in the guns` last above referred to,

27. In 1858, after the failure at the 169th fire of the West Point experimental gun made from this iron, Mr. Parrott condemned it as being too high for heavy guns.

28. And again," these facts to my mind are conclusive as to the fact that we are at present far from possessing a practical knowledge of the properties of castiron in its application to gun foundering; and it is too much to expect of private enterprise to take up and prosecute so intricate and expensive an inquiry." Thus

we see how recently the author of the system of casting hollow acknowledges his ignorance of casting iron, and I venture to say that neither bureau knows more to-day. Colonel Rodman's plan of hollow casting provides well for the pressure of the powder in a gun, but the gun that is in the best state to resist the pressure, viz. by initial tension, is in the worst state to resist the unequal expansion from the heat of firing; the same holds good in a wrought-iron gun. 29. Built up guns of all kinds are better able to withstand pressure than solid forgings or castings, as has been heretofore and will be hereinafter more fully explained; but such guns must be made of soft ductile iron that will stretch, so thus be disabled and need repairing or they will be burst by unequal heating.

30. The tensile strength of gun metals is usually only tested by simple extension of length, or of one of the dimensions undergoing which extension other dimensions are reduced. It is not known how the tensile strength of a sample would be affected if force was applied to extend all its dimensions at the same time. As for instance, take a hollow cylinder of which the area of cross section is known and subject it to a pressure of liquid on the interior, having only the tendency to enlarge its dimensions radially, while the measured force of the instrument by which its tensile strength was being measured, was applied to extend its length only. Or have the forces acting to enlarge the diameter pulling from the outside instead of pushing from the inside, it would undoubtedly be found that while subjected to such additional forces the extension or rupture would be accomplished with less force. For the attraction of cohesion acts with greater energy as the faces of crystals or atoms are near or distant, in the same ratio as the shadow of a screen upon a wall from a point of light increases or diminishes in superficial area as the screen is moved toward or from the light.

31. Hence the more dense the sample, of cast-iron from remelting, or greater time in fusion, the greater the tensible strength, when the initial tensions within the mass of the sample, from unequal cooling or heating, do not affect the result; the extension of length would require more force, because the faces of crystals are not all perpendicular to any axis. The strength of metals in large masses is much less than in small ones, owing to initial tensions.

32. If we produce a homogenous ball of wrought or cast-iron, and subject it to repeated heating and cooling, it becomes larger in dimensions, and may, by continuing the process, be made distinctly hollow. Upon heating it the outside receives the heat first, enlarging the interior-the dimensions of masses of metals may be enlarged or separated by the application of sufficient force, indefinitely; while the dimensions of a mass cannot be materially reduced no matter what force is applied-at a later time when the interior is heated, the outside is enlarged, or receives what has been termed a "permanent set." Again, in cooling the outside loses its heat first, being unable to compress the inner metal, (for the resistance to first increment of compression is about six times as great as the resistance to first increment of distension to give a permanent set;) it is distended by the inside cooling first, and receives permanent enlargement, and by repeating the process another enlargement occurs, and so on. 33. " One of the illustrations given by Lieutenant Colonel Clerk represents a 12-inch wrought iron cylinder, 4-inch thick and 9-inch deep, after being heated to redness, and cooled by immersing its lower half in cold water-these operations having been repeated 20 times. The upper edge of the cylinder (in the air) did not alter; the lower edge (in the water) contracted 3-inch in the circumference, and at about one inch above the water-line the circumference was reduced 5.5 inch.

11

34. "The general effects noticed in the paper are a maximum contraction of the metal about 1 inch above the water-line; and this is the same whether the metal be immersed one-half or two-thirds its depth, or whether it be 9, 6, or 3 inch deep. With wrought-iron the heatings and coolings could be repeated from 15 to 20 times before the metal showed any signs of separation; but with cast-iron, after the fifth testing, the metal was cracked, and the hollow cylinder separated all round just below the water-line after the second heating. Cast-steel stood 20 heatings, but was very much cracked all over its

and coolings.

35. "As respects the change of form of castiron and steel, the result was similar to that in wrought-iron, but not nearly so large in amount. Tin showed no change of form, there being, apparently, no intermediate state between the melting point and absolute solidity. Brass, gunmetal, and zinc showed the effect slightly; but instead of a contraction just above the water line there was an expansion or bulging."

36. The following extract also from Holley's late work on ordnance and armor, page 352 and 353, illustrates this:

37. "The initial strains of large cylindrical forgings are, to some extent, deranged by a cause that operates so unfavorably in solid cast-iron guns-the cooling of the exterior first, and the consequent stretching of the interior. Mr. Clay acknowledged this difficulty before the defence commissioners, and stated that his new process, hollow forging, overcame it. Such a result actually occurred in the case of the Horsfall gun; a breach-plug or false bottom was placed in the chamber to cover a crack arising from this cause.

38. The masses were forged from puddled slabs of manageable size, "by slabbing up two or more large flat pieces, laying these upon each other, and welding them together into a rude sort of square prison, which was afterwards partially rounded down, at the corners, under the hammer.

39. Mr. Mallet, in the paper before referred to, gives the following facts and illustrations as to the cause of fissure: Two masses, about 23 feet in diameter and 8 feet long, were forged for two 36-inch mortars, which Mr. Mallet was constructing for the British government. They were slightly tapered, and at one end there was a collar projecting 6 inches all round, and about 12 inches wide in the line of the axis, presenting laterally the general form shown.