MEMOIRS

OF THE

AMERICAN ACADEMY.

I.

On the Practicability of Constructing Cannon of Great Caliber, capable of enduring longcontinued Use under full Charges.

BY DANIEL TREADWELL,

VICE PRESIDENT OF THE ACADEMY, AND LATE RUMFORD PROFESSOR IN HARVARD UNIVERSITY.

Communicated February, 1856.

THE importance of constructing cannon of a size larger than any now in use, to every nation that may be called upon to encounter the trials of war, is one of those facts acknowledged alike by the soldier and the civilian; and to obtain such instruments, capable of throwing projectiles larger and heavier, and to greater distances than have hitherto been attained, is now occupying the attention of the scientific engineers and projectors of Europe more than any other question open to them. The present age has witnessed a remarkable increase in the size of all the great instruments of human industry. Ships within twenty years have been doubled in their dimensions, and steam-engines are now constructed which compare with those of the last age as giants compare with common men. But although the want is fully acknowledged, and attempts have been made in hundreds of forms, no one has succeeded in producing a cannon essentially more powerful than those used in the days of Napoleon and Wellington.

I propose, in this paper, to search for the causes of these failures, to examine the action of the forces, both active and passive, which are called into operation in throwing shot and shells by gunpowder, and, at last, shall endeavor to show that our present cannon do not approach the size and power of those that may be constructed. 1

VOL. VI. NEW SERIES.

I have said that no essential improvement has been made during the present age in the size of cannon. It is true that they have been increased in caliber from seven, up to eight and ten inches, and a few bomb-cannon have been made of twelve inches. But in the use of these the charges are so diminished, to be brought within the limits of safety, that the initial velocities, as inferred from their short ranges, are not so great as those of the old forty-two pounders; while with mortars, those of thirteen inches were used in the time of Vauban, and this remains, stereotyped, as the limit at the present day.

But to my examination. The properties or qualities of hardness and of tenacity or strength are the qualities indispensable to all cannon, and the superiority of one cannon over another is measured by the excess in which it possesses them. Inertia * is likewise required, in a certain amount, to prevent excessive recoil. Now these properties of strength and hardness are possessed in an eminent degree by bronze and cast-iron, and these bodies alone constitute, in practice, the materials for cannon; for although various attempts have been made to introduce steel and wroughtiron, it is enough for my present purpose to say, that there are not twenty cannon in use in the world, that are not made of bronze or cast-iron. For strength, bronze is generally taken at 30,000 pounds to the square inch; that is, that it will require a weight of 30,000 pounds to tear asunder a bar of good gun-metal bronze of one inch area. Following the mean of many experiments, cast-iron has generally been taken at 20,000 pounds. But that I may be sure not to under-estimate the strength of this material, and as it has been considerably improved by gun-makers within a few years, I shall estimate it at 30,000 pounds, or as equal to bronze, although it is not to be relied upon as so constant in its strength as the latter material. For hardness cast-iron greatly exceeds bronze. This renders it more suitable for very large guns, and it has, in truth, become so exclusively the material for everything above the size of field-pieces, that I shall deal with it alone in the examination proposed in this paper.

Before examining the force of gunpowder it may be well enough to say a word upon the time of its explosion. Is the firing of gunpowder instantaneous? This question has been discussed, and experiments made upon it, by Mr. Robins, Dr. Hutton, Count Rumford, and many others, besides a special committee of the Royal

*This word is used throughout this paper in its strictly technical sense, as the force, or power of resisting all change of state, whether it be from rest to motion or from motion to rest; and I use, without a doubt of its accuracy, the square of the velocity by the mass, as the measure of this force.

Society. If it be instantaneous, then it must be evident that no other substance can be fired with a greater rapidity. For instantaneousness, bearing the same relation to time that a point does to space, can admit of no degrees. Both are existences without extension, and we cannot say of any two events that one is more instantaneous than the other, without implying duration to one at least, which also implies that it is not instantaneous. Now many of the fulminating powders, and even guncotton, are, as is well known, fired much more rapidly than gunpowder. The firing of this last cannot, therefore, be instantaneous, and we might rest with this logical solution of the question; but, like many other logical solutions, it adds but little to our wisdom, and the amazing rapidity with which a large mass of powder is inflamed, when in a close cavity, awakens our attention to the course of the events causing, or at least accompanying, this inflammation, and I shall notice two experimental results which seem to me to indicate the state of things during that whole course.

First, Count Rumford has proved that the burning of the grains is slow, or that a sensible time is required with each grain before it is wholly converted into the gaseous state; and secondly, various experiments made in England and in Prussia have shown that there is no sensible difference produced in the velocity of the shot by communicating the fire to the centre rather than to one end of the charge, which ought evidently to take place if the fire is communicated from one grain to another in succession, as this communication, being in both directions, when proceeding from the middle, would require but half the time that is required when proceeding from one end, and ought to produce a sensible increase in the velocity of the shot. I think, therefore, that these two facts warrant the following inference as to the course of the action during the production of the force. When the fire reaches the charge from the touchhole, the nearest grains become kindled; the hot fluid evolved is thrown farther into the charge, and the burning succeeds successively until the pressure becomes so great as to condense the air contained between the grains sufficiently to produce the heat required for firing those grains, which are then consumed more or less rapidly, as they are fine or coarse. We have, then, first the burning in succession of a small part of the charge; then the immensely rapid, though not instantaneous, kindling of every grain composing it; and then the consumption of those grains, which is not accomplished without time. It is a task for the conception to grasp these events, following one another in distinct succession; each having its beginning, middle, and end, and all being comprised in the period of 2th of a second (gun 4 feet long, formula t = 2). When we have mastered the imagination of these, we may go further and combine with them the connected and

contemporaneous action of the ball, which passes from rest to motion, and through every gradation of velocity up to 1,600 feet a second, and leaves the gun as our historical period of 20th of a second expires.

The expansive force of gunpowder, which must be resisted by the strength of the cannon, depends almost entirely upon the circumstances under which it is fired. Count Rumford has shown, by his experiments made about sixty years ago, that if the powder be placed in a closed cavity, and the cavity be two thirds filled, the force will exceed 10,000 atmospheres, or 150,000 pounds upon the square inch; and he estimates that if the cavity be entirely filled with the grained powder, and restrained to those dimensions, the force will rise to 50,000 atmospheres. My own experience, made in bursting wrought-iron cannon the strength of which was known to me, leads me to believe that he has not over-estimated its power, although I am aware that it is generally considered as excessive. If, following an opposite course to that herein described, the powder be at liberty to expand upon any side, the force thrown in the other directions is very small. Thus, if a charge be placed loose in a gun, without shot or wad, the force upon the walls of the gun is very trifling; -no more than is produced by the restraint of the inertia of the charge itself, or the fluid formed from it. If we would divest a charge of this property of inertia, and fire it in a constantly maintained vacuum, it would not rend walls made of cartridgepaper, if a single end were left open for its escape. From the preceding statement, it will be seen that gunpowder will take any force, from perhaps 50,000 atmospheres, when confined to a close cavity, down to zero, if it be deprived of inertia and fired in a vacuum constantly maintained.

In artillery practice, the restraining power which causes the powder to act against the walls of the cannon is derived principally from the inertia of the shot. This is so much greater than the inertia of the powder itself, that the latter may be neglected in the considerations that are to follow. Now, bearing in mind what has been already said, let us compare the difference of the force of powder as exerted upon a small and a large gun respectively. It is perfectly well known, that, if we have a pipe or hollow cylinder of say two inches in diameter with walls an inch thick, and if this cylinder will bear a pressure from within of 1,000 pounds per inch, another cylinder, of the same material, of ten inches in diameter, will bear the same number of pounds to the inch if we increase the walls in the same proportion, or make them five inches thick. A cross-section of these cylinders will present an area proportional to the squares of their diameters, and if the pressure be produced by the weight of plungers or pistons, as in the hydrostatic press, the weight required in the pistons will be as the squares of the diameters, or as 4 to 100.