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gun at the place where the fracture usually branches off to either side, thus delaying the longitudinal fracture until the expansion lengthwise of the inner metal is great enough to cause the cross fracture. It may be said

then, in brief, that the fractures at right angles to the plane of the bore are caused by the lengthening of the inner metal about the bore by heat, while the outer metal remains the same length, or with less expansion of length, until ruptured, and that longitudinal fractures are due principally to the enlargement of the inner metal by heat in the direction of the diameter, or radially. If the gun be parallel all the way to the muzzle or banded, as by Parrott, the cross fractures will occur more frequently along the re-enforce, because in that part it is exposed to the highest temperature, and, consequently, the greatest expansion of length, while the band restrains radial expansion to some extent.

70. It is a corroboration of this theory that the guns of the Dahlgren model, with more than double the thickness of metal behind the chamber, though made of the strongest steel, should break in the same direction, forward of the trunnions, but sometimes exhibit only cross fractures (when made of cast-iron) to the rear of the trunnions. It is evident that the model is best in which the direction of the fracture is least uniform, but a properly constructed gun should not burst at all.

Fig.o

71. The gun without bands, however, is usually broken through the breech-the strongest part of the gun-and beyond the range of the pressure, which is, of course, limited to the bottom of the bore or chamber. The diagram in Captain Rodman's book, page 43, exhibiting the various kinds of strain

to which a gun is subjected at each discharge, considers the gun as if made up of staves, and really exhibits only the strain from the expansive force, or direct

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pressure of the powder, bending the staves outward; and page 47 of the same book, by diagram, the direction of fracture due to such strain, not through the breech, but running at an angle to the plane of the bore. 72. To show that it is improbable that the direct pressure of the powder should be the cause of fracture, as exhibited by the gun actually broken by firing, prepare three plates of metal, say 4 inches thick, 12 inches wide, and 60

Fig 9.

inches long, with plane surfaces; the middle one, on being heated to 1,600°, will be found expanded one-sixtieth part of its length, or will be 61 inches long. On placing it between the other two, a part of its heat is immediately communicated to their contiguous surfaces only. The expansion of one surface of the outside plates, while the other surfaces remain cold, warps the latter to the form of a segment of a circle. Now, supposing them placed upon

Fig. 10.

the diagram of a burst, gun, the metal of which has been heated by the combustion of powder, it is evident that the fracture in the particular direction exhibited must have resulted from the unequal expansion of the gun by heat. A narrow band

adjusted upon the middle of the length of the re-enforce thus curved by heat would not much hinder the fracture; if the band were wider, it would only cause the cross fracture more frequently, as in the Parrott gun. A diagram exhibiting

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these curves, the result of this expansion, will be exactly the opposite of the curves on the diagram by Rodman, and will account for the breaking of the gun through the

breech, beyond the range of the pressure made by the powder.

73. That the fracture almost always intersects the vent has been heretofore referred to the weakness resulting from drilling away part of the metal; but on page 355, Major Wade's Reports on Metals for Guns, we find that after a gun had been put to extreme proof, and exhibited signs of fracture, a hole was drilled one inch forward of the base ring, and four from the line of the vent, to a depth of four inches, and of the diameter of one and a quarter inch. The gun was then fired with double charges of powder, and with a bore full of balls and wads, eleven times, to bursting. Although the piece burst into more than twelve fragments, one of the fractures intersecting the vent, it did not split through the large hole, showing that the gun had strength to resist the pressure of the powder, but burst, notwithstanding the drilling away of so large a part of the metal, from the communication of heat. The true cause, probably, of the intersection of the vent by the fracture was the communication of heat to the surface of the vent, thereby expanding a column of metal about it; for it should be recollected that the passage of a large quantity of gases through the vent would communicate more heat to its surface than would be communicated if there was no current, but the capacity of the vent only filled. In that case not much heat would be supplied to the surface, because the quantity contained within the vent would be small.

74. But in this example, as in all others, as is well known to ordnance inspectors, the fracture began to exhibit itself on the interior surface of the bore. This would seem to prove that guns burst by pressure rather than by expansion of the inner metal-as if the inner metal were expanded by the communication of heat before the outer metal gave way; a strain of compression resisted by the strength of the outer metal would rest upon the inner metal of the gun, that would prevent inside cracks; and, undoubtedly, if it ever occurred to an ordnance officer to inquire whether the communication of heat to the inner metal of guns was the cause of their failure, the beginning of fracture on the inside would appear to him an argument against the theory. This I consider a critical point, but one directly favoring the theory. It requires a most familiar knowledge of the effects of heat, and careful recollection of time and place of all the phenomena, to comprehend and explain this part of the subject. If the gun were heated continuously from the surface of the bore, the greatest expansion would be immediately there. But we are to recollect that in the most rapid firing the surface of the bore is exposed to this high temperature only about one hundredth part of the time, while during the other ninety-nine hundredths the heat of the surface of the bore is radiating away. If the gun was of six inches diameter of bore, and eight inches thickness of metal about the bore, the range to which the heat would penetrate the metal at the first discharge would be about four inches; for heat enters metal with a velocity depending on the difference in temperature of the source from which it flows and the metal into which it is flow ing The heat is communicated to the small surface of the bore, while it is ra diated from the large outside surface of the gun; from this cause, if from no other, the temperature would be much higher within the mass than on the outside. 75. The penetration from the first discharge being four inches, it might be supposed that the range of the heat from the next discharge would be greater; but heat having been communicated by the first discharge, the range of the

second is less, from the reduced difference of temperature. Although of course the heat flows onward, its motion is very slow. If, then, the first penetration be four inches at the distance of four inches from the surface of the bore, the temperature will be comparatively low-but little higher than that of the metal at four and a half inches from the surface of the bore. The heat, therefore, is conducted from the point of four to that of four and a half inches slowly; more slowly from that of four and a half inches to five, and with a continually reduced and very slow rate of motion to the outside. As the heat is communicated from one inner stratum to the strata surrounding it, for each inch of the increasing distance it travels, the mass of which the temperature has to be raised is greater in circumference also; this is another cause of the retardation to its motion outward. Although for ninety-nine hundredths of the whole time

Fig 13.

the heat is radiating from the surface of the bore, the velocity with which it leaves is much less than the velocity with which it is received, because the difference in the temperature of the gun and the atmosphere occupying the bore is much less than the difference in temperature between the metal of the gun and the gases ejecting the shot by their pressure. The atmosphere occupying the bore receives the heat by radiation in the intervals between firing quickly from the immediate surface, and less quickly a little distance beyond; and so again the heat flows from the metal of the gun with reduced velocity as the distance increases from the bore, leaving the point of highest temperature in the mass of metal, but not far from the surIface of the bore. (See Fig. 13.) Its effect toward causing rupture may be illustrated by taking a cylinder of pine wood a few inches in length, and a cross-section like the diagram, and providing a wedge similar in form to a bayonet, but truly tapered to a point from a cross-section at the head, the same as the lines representing the place and quantity of heat on the diagram, showing its effects by intermittent communication of heat. (Fig. 13.) If the point of this wedge be set upon the end of the wooden cylinder at the point supposed to be the point of great heat, according to the theory above, and by a blow driven into the end-wood, it will. penetrate so as to make an impression like the inner line of the diagram. A second blow, driving it further into the wood, penetrating as if to the second line of the diagram, and expanding the wood, will cause a fracture inward toward the surface of the bore first; a third or fourth blow will split it to the outside. And thus guns burst, the first fracture occurring on the inside, and afterward opening to the outer surface. 76. In Rodman, Plate II, Fig. 2, is shown the interior line of fracture of a 10 inch columbiad. Here a thin bit of metal, indicated by the line marked

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, is shown, which seems nearly to envelop the bore. Nearly one-half the re-enforce was broken off this gun in the same manner as chips break off a stone door-cap when the building is

Fig.15

burning; but in this example the outside of the stone is first heated, while the inside remains colder. The outward pressure of the powder at the time of this fracture would surely have carried away so thin a piece of metal; but it remains standing to show that the pressure had been reduced before the gun broke

a remarkable evidence of the true cause of the bursting of the gun. The diagram exhibiting the place and quantity of heat shows but little heat at any surfaces of the gun. From this, also, we may have been hitherto deceived as to the importance of the study of its effects, and we can only appreciate it by some experiments like the following: a clean rifled musket, the barrel of which weighed about five and a quarter pounds, was fired twelve times with the ordinary charge, at intervals of five minutes between each discharge. The time during which the surface of the musket was radiating away the heat from beginning to end was, therefore, about one hour. At the end of this time its temperature was 2000. The radiation was somewhat hindered by the wood of the stock, which was a non-conductor, partially enveloping the barrel, and the burnished surface of the barrel, which was a non-radiator. The whole amount of powder was less than one ounce, and it communicated this great amount of heat to five and a half pounds of metal. There would be a material difference in the amount of heat communicated in this experiment if the barrel were not clean inside, as the residuum of powder would be a non-conductor, and would prevent its communication to the metal of the barrel. The temperature of the gases in a large gun, say a 100-pounder rifled cannon, would be much greater than in a musket; and its effect towards heating the gun would be greater from the longer term of exposure to the high temperature. The work of the powder in a gun is to overcome the inertia of the shot, and to do this it presses against a certain number of inches of area. If the shot be short the pressure is still exerted against the same area. The projectile in a 100-pounder rifle gun is twelve inches in length, while the projectile from a common rifled musket is less than one inch in length. The resistance from the inertia would be thus about twelve times as great in the large gun as in the small one, and the expansive force or pressure, and consequently the temperature, high in proportion.

77. From the fact that solid-cast guns, of the largest size now in service, have a certain strain upon them within themselves when cast, from the heat leaving the inner metal last, which is relieved by the expansion of the inner metal by the first few discharges, I hold that solid-cast Dahlgren guns, or any columbiads of large sizes, cast solid, may pass the inspection of ten service charges, and be stronger at the tenth discharge than they were at the firstthat number of rounds perhaps being necessary to relieve them of the beforementioned strain, by communicating the proper proportion of heat to place them in the same state in which we find the hollow-cast gun at the first round.

78. The guns in our service having great thickness of metal about the bore should not be relied upon in rapid firing, even when exposed to the hottest rays of the sun on their very large exterior surface-the most favorable circumstances under which a gun can be fired-and should never be fired at all, if a hollow-cast gun, with uniform density throughout the mass, in rain or in cold weather. It may sometimes happen that a hollow-cast gun, after the Rodman plan, would exhibit greater endurance than a solid-cast gun, made from the same metal and at the same time. At the time of the bursting of two large steel 50-pounder navy-guns of my fabrication, each at the ninth round, fired rapidly on a cold day at Staten Island, I suggested to the inspector either that the guns should be fired at longer intervals between the discharges, or that I should be permitted to give elasticity by drilling a series of small holes about the bore having a quincunx position relative to each other, and a Fig 19

[graphic]

proper direction to permit the expansion of the inner metal without any undue strain upon the reinforce, as shown in the two sections above. Captain Rodman's book, page 297, exhibits the impossibility of casting a solid projectile, cavities Fig 20

being formed in the centre of the mass, due to the shrinkage of the inner metal after the outer shell had frozen, so as to prevent any supply of metal to the centre thereafter; and this is related as the cause which led to the hollow mode of casting. These cavities do not occur of necessity in the centre of a casting, but of necessity in the centre mass at equal distances from the cooling surfaces if subjected to an equal rate of cooling; and they are to be found near the centre of the mass in the Rodman gun as well as in any other. Their presence

Fig. 21

cannot be detected at any of the surfaces of the casting. If they were generally dis'ributed between the inner metal of the gun and the reinforce, a sufficient elasticity be ween these parts of the gun might be had, and

a similar result arrived at to that obtained by the drilling of the holes, proposed by me, as shown by diagram. Fig. 19.

79. The 15-inch guns can be shown to be inefficient, for they only give a velocity of 750 feet per second; or they are unreliable (perhaps both) for they will burst as often as the accidental porosity above spoken of is not evenly distributed between the inner metal and the reinforce. And who can say when these condition are all fulfilled, except by a practice inaugurated by the man 'who had the goose that laid the golden eggs," viz., BY CUTTING OPEN. By that practice, THE GUN THAT WE KNOW TO HAVE BEEN A GOOD GUN IS NO

66

LONGER A GUN.

gun

80. It is perhaps pertinent to this subject to consider that the gasses of powder always have the same weight as the powder from which they have been evolved. A musket barrel is burst at the muzzle if the shot is carelessly inserted and not put down against the powder, by the momentum, it is supposed, or vis viva of the gasses, that, having weight and velocity, are projected against the bullet.

momentum, for a change of direction of 1°.

When a long rifled cannon is fired at a high elevation, the gun recoils backward on a plane, representing the deck of a ship, different from the plan of the bore. All bodies in motion resist a

change of direction, in the proportion of 1-90th of their whole If one ivory ball of a pair, suspended to the ceiling by threads is projected against the other at rest, striking it at right angles 90°, the one in motion comes to a state of rest, communicating ts whole momentum to the one before at rest. If the one at rest should be struck at angle of 45°, the ball in motion would have its direction changed 45° and it would give one-half its momentum to the other. Each would be projected the same distance. So, also, if the angle with which they came in contact was one degree, 1-90th of the momentum would be given to the ball at rest. The whole sum of the momentum of a shot projected from a rifled cannon, is very great. At the muzzle of the gun, the resistance to a change of direction is sufficient to overcome the preponderance of the gun. If the bore was crooked, the shot would not be much diverted, but the gun would be moved to conform to the direction of the shot, and many have noticed, when firing guns Mis. Doc. 47- -2

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