Mr. Chairman, and Gentlemen: Before touching upon the changes-which have been effected in our own day in the burning qualities of gun-powder, I have thought it might not be uninteresting to present a short outline of the experiments of the early investigators. This may also aid us in appreciating the line of thought adopted in later years, as progress in science has furnished additional tools to work with.
The problem forced itself into prominence with the earliest improvements in artillery, but there are no records of definite experiment before those of La Hire, in 1702. La Hire's conclusions were quite defective, but possess a certain interest regarded in the light which has been thrown upon the subject since his time. He ascribes no role whatever to the products of combustion, but supposed the energy was due entirely to the expansion of the air contained in and among the grains of powder, the latter merely furnishing the heat for expansion.
In 1742 Robins took up the subject, and, as the result of experiment, deduced that the volume of the products of combustion was two hundred and forty times greater than that of the original solid, and that the maximum pressure of powder fired in a confined space was one thousand atmospheres.
Robins further showed the fallacy of La Hire's conclusions, proving that the expansion of the air in the powder space could not effect a pressure exceeding five atmospheres.
In 1778 Dr. Hutton, of Newcastle on Tyne, gave as the result of experiment the maximum pressure as double that of Robins, or two thousand atmospheres.
In 1797 we find Count Rumford attacking the problem in his characteristic, practical way, and his valuable contributions to science in other fields entitle his conclusions to great weight. As Captain Noble and Professor Abel remark in the admirable sketch of their own experiments, from which, I may state here, the historical and statistical portions of these notes are mainly drawn, Rumford's trials remain to this day the standard, if not the only series of experiments, in which the pressure has been determined by direct observation. His results have, however, been questioned on theoretical grounds by a number of writers on the subject.
Rumford conducted his experiments on very much the plan which has been used in later years in the proof of gunpowder. The firing was effected in a small, strong eprouvette-shaped vessel of about 0.08 cubic inch internal capacity. The muzzle was closed by a metallic hemisphere upon which weights were placed until the elastic force of the expanding gases was exactly counterbalanced. A simple calculation then gave him the pressure per unit of area. After an exhaustive series of trials, in describing which he makes numerous deductions which have been completely verified by later experiments, he gave as the pressure of exploding gunpowder confined in its own volume one hundred and one thousand and twenty-one atmospheres.
Considering the great discrepancy between this result and that obtained by other investigators, it is interesting to note the characteristics of the powder used in Rumford's experiments, and we find it to have been fine grained sporting, containing but sixty-seven per cent, of nitre, of a specific gravity of 1.868, and gravimetric density, 1.08.
In 1823, Gay-Lussac found, by an entirely different process, (by dropping grains of powder into a red hot tube provided with an arrangement to receive the gases), the volume of the products to be four hundred and eighty times that of the original solid. Piobert criticized this result and appears to have proven that, considering Lussac's own statement, the results were by error nearly doubled, and that the true increase of volume was two hundred and fifty, which agrees very nearly with other writers.
The results of the experiments of General Piobert in 1830-31, form an important page in powder literature. This investigator first developed the idea that the point of ignition had great influence on pressure and velocity. He also details the effect of the presence of moisture, regarding which there has been considerable controversy.
The modern science of chemistry was at this time in its infancy, but was applied with considerable success in the estimation of products of combustion. Piobert makes one capital error in his deduction that the high temperatures and great tension of the gases have no sensible effect in increasing the rapidity of combustion of the individual grains.
The results of his experiments give the increase of volume to be about three hundred and the maximum pressure as twenty-three thousand atmospheres, which agrees quite well with Rumford's first series of trials, of which I have quoted but the second.
In 1842, General Cavalli proposed and experimented with a piece of artillery, in which, at various distances along the length, tubes of wrought iron were inserted, in each of which was placed a spherical bullet, to be projected by the charge of the piece, in driving out its own projectile. The tension at the several points was estimated from the velocities imparted to the bullets.
Cavalli also deduced the theoretical strength of metal needed at various distances from data procured in this manner. The maximum pressure in the bore of a gun, obtained by Cavalli, was placed at the extremely high figures of from four thousand to twenty-four thousand atmospheres, varying with the different qualities of powder.
In 1854, a Prussian Commission experimented with an arrangement somewhat resembling Cavalli's, in which a small cylinder was projected instead of a spherical bullet. Their conclusions differed materially, however, the pressure for the 6 pounder field-piece being estimated at only eleven hundred atmospheres, and for the 12 pounder, thirteen hundred atmospheres.
We have now reached the period of Major Rodman's experiments, 1857-8, the importance and value of which are recognized in the practical utilization to-day of most of the ideas he developed. It is well to remember his work, when we read in a recent English scientific journal, that to Germany belongs the credit of the invention of prismatic powder.
You are all acquainted with the Rodman method of measuring pressures, which has been used by the Navy Bureau of Ordnance for the past fifteen years. The Woodbridge modification, recently adopted, does not differ in principle from the original idea. In place of the cutter, a hollow punch, provided with a spiral cut in relief on its surface, is pressed over a copper disc, and the length of the impression measured and compared with a similar indentation made by mechanical appliances.
Rodman's experiments to determine the maximum pressure of confined powder were conducted in a manner which does not admit of direct comparison with the results obtained by other investigators. In noting his figures—from four to thirteen thousand atmospheres—it is necessary to remark that he exploded his charges through a vent in a cast-iron shell (provided with his pressure gauge), thus using an arrangement in which the conditions of explosion in a confined space and those which obtain in the bore of a gun are combined.
In 1856, Messrs. Bunsen and Schischkoff made an exhaustive series of trials to determine the products of combustion of gunpowder, to which Professor Munroe, of the Department of Physics and Chemistry, made reference, from a chemical stand-point, in his interesting paper on the "Causes which promote Explosion," read before the Institute, in 1877.
These experiments were conducted in a manner somewhat resembling Gay-Lussac's,—the powder grains being allowed to pass singly into a heated bulb, the gases being collected in a series of tubes. The results obtained were that the gases form thirty-one per cent, of the weight of the original solid, with a volume two hundred times as great. Their maximum pressure was calculated to be forty-three hundred and seventy atmospheres, and the temperature of explosion in a confined space 3300° C. These writers also estimated the theoretical work of a kilogram of gunpowder to be sixty-seven thousand kilograms.
We will consider Noble's and Abel's experiments with more minuteness, their work being the most exhaustive research that has been made, as well as the most recent.
The powder charge, a kilogram in weight, was exploded in a strong cylindrical-shaped vessel of mild steel, carefully tempered in oil. (Fig 1). The firing plug C is provided with a second interior plug D conical in shape with tissue paper as an insulator between contact surfaces. At E is the arrangement for allowing the gaseous products to escape, either through the outlet H for analysis, or the plug aperture E for measurement of volume. The two wires L L are connected with a short platinum wire passing through a small glass tube filled with mealed powder, thus constituting an arrangement for firing by electricity.
These writers say: "the difficulties met with in using this apparatus are more serious than might at first sight appear. In the first place, the dangerous nature of the experiments rendered the greatest caution necessary, while, as regards the retention of the gaseous products, the application of contrivances of well known efficacy for closing the joints, such as papier-mâché (5 wads between disks of metal, (a method which has been successfully employed with guns) are inadmissible, because the destruction of the closing or cementing material used, by the heat, would obviously affect the composition of the gases. Every operation connected with the preparation of the apparatus for an experiment has to be conducted with the most scrupulous care. Should any of the screws not be perfectly home, the gases, instantly on their generation, either cut a way out for themselves, escaping with the violence of an explosion, or blow out the part improperly secured—in either case destroying the apparatus."
Messrs Noble and Abel further note the difficulty experienced in withdrawing the firing and pressure plugs and to the cementing effect of the powder residue when subjected to the high pressures of explosion, and the interesting fact that the solid residue is in a liquid state immediately following the instant of explosion. This was surmised from the appearance presented on opening the vessel, and afterwards demonstrated by tilting the cylinder 45° and then righting it after a period of two minutes. On opening, the marks of the displaced residue were plainly visible. The pressures were calculated from the chemical data and also measured with the crusher gauge.
The volume of the permanent gases was measured by the apparatus shown in Fig's 2 and 2a which need no description.
To collect the gases for analysis, the outlet H was used, the plug E being only started from its seat. The operation of transferring them from the cylinder to the tubes over mercury occupies from five to fifteen minutes, and the experimenters believe no change in their nature takes place, through contact with the solid residue, in this interval.
Great difficulty was experienced in collecting the solid residue, it being so solidly packed on the bottom and sides of the vessel as to require to be cut out with steel chisels. The tendency of the residue to absorb oxygen from the air is referred to, and a case is cited in which the heat developed was sufficient to burn the paper on which it was placed. The writer has frequently observed this phenomenon in the residue from the one hundred pound charge of the XV-in gun, and noticed also, that other portions from the same discharge would absorb moisture so rapidly as to entirely change their physical qualities.
In the summary of results, the following points are of interest, and can be mentioned in a paper of this character.
Taking as a unit the volume of a cubic centimeter:
1. On explosion, the products of combustion consist, by weight, of fifty-seven per cent, of matter which ultimately becomes solid and forty-three per cent, of permanent gases.
2. At the instant of explosion the liquid products occupy 0.6 cubic centimeter and the gaseous products 0.4 cubic centimeter. At 0° C and 760mm pressure, they will occupy 280 centimeters, or 280 times the volume of the original solid.
3. The maximum pressure when powder is fired in its own space is about sixty-four hundred atmospheres.
4. The temperature of explosion is about 2200° C.
The writers further discuss the action of the gases when powder is burned in the bore of a gun, taking data furnished by experiments made by the Committee on Explosives, and deduce as follows:
1. The products of combustion, in so far as the proportions of solid and gaseous products are concerned, are the same as in the case of powder fired in a confined space.
2. The work done on the projectile is due to the elastic force of the permanent gases.
3. The reduction of temperature due to the expansion is compensated for by the heat stored up in the liquid residue.
4. The total theoretic work done, when powder is indefinitely expanded, is about 480 foot-tons per lb of powder.
In enumerating the foregoing results of the various investigators, I have purposely abstained from any mention of the chemical reactions which take place in the formation of the products of combustion whether solid or gaseous. You would find on comparison from Piobert's experiments to those of Noble and Abel a most confusing lack of uniformity. No two series of experiments have produced identical results, and among the gaseous products you would occasionally find combinations in one series which are entirely absent in the others. On this subject Noble and Abel frankly admit, as a result of their work, that the variations of composition in the products, both solid and gaseous, under similar conditions of powder composition, pressure, size of grain, density etc, are so considerable, that no value can be attached to any attempt to give a chemical expression for the reactions.
The gaseous products found are:
Carbonic Anhydride,
Carbonic Oxide,
Nitrogen,
Hydrogen Sulphide,
Marsh gas,
Hydrogen, and
Oxygen.
The solid products:
Potassium Carbonate,
Potassium Sulphate,
Potassium Hyposulphite,
Potassium Monosulphide,
Potassium Sulpho-cyanate,
Potassium Nitrate,
Potassium Oxide,
Ammonium Carbonate,
Sulphur, and
Charcoal.
The straits in which the French people found themselves, during the latter part of the Franco-Prussian war, in regard to material, called forth considerable effort from their scientific men, and produced, with others, a remarkable treatise from Professor Berthelot of the College de France on the force of the expanding gases of gunpowder.
Berthelot claims that the results of Bunsen and Schischkoff regarding the pressure and temperature of explosion are much underestimated, as at this high temperature the gaseous products are in a more elementary condition, or are "dissociated." Their separation, he admits, determines a loss of heat, but this is compensated for at their recombination which is rendered possible by the fall in temperature due to the displacement of the projectile.
Berthelot further discusses the effect, on the pressure exerted, of the change of coefficient of expansion, as under the conditions of explosion the laws of Mariotte and Gay Lussac do not hold; and deduces there from an increase of pressure as compared with that calculated by these laws.
Messrs Noble and Abel take exception to this assumption of the dissociation of the gaseous products, and also to the pressure deduced by Berthelot, (sixty thousand atmospheres), but it does not appear that all of Berthelot's arguments are satisfactorily refuted. In his lectures before the British Association at Edinburgh in 1871 Professor Abel gives Berthelot's results with the supplementary remark that they have not been verified by experiment.
But passing these disputed points, which in reality have but little interest for us, let us consider the more positive results obtained by these experiments.
We find on comparing their tables:
1. Variations in the proportional parts of the mixtures produce uniform effects on velocity and pressure, and in this connection it is interesting to note that, with comparatively slight changes the relative weights of nitre, sulphur and charcoal remain the same from
La Hire’s time to the present day.
2. The size of the grain, as affecting burning surface.
3. Variations in specific gravity and gravimetric density having a uniform effect on pressure and velocity.
4. The effect of moisture in incorporation, an excess decreasing both pressure and velocity: in this connection we note also the effect of moisture absorbed after manufacture, and the influence of density, size of grain, glaze, and quality of coal, (either as regards the wood from which prepared or amount of carbonization), on the absorptive capacity of the grain.
5. The shape of the grain and the effect of uniformity in this respect in limiting burning surface.
6. The point of ignition, the pressure being much greater when ignited at the rear.
7. The effect of glazing, the lead used in this process collecting on each grain a thin layer of dust, which increases the rapidity of ignition.
8. The size of the charge, an increase in weight augmenting the rapidity of combustion.
With a knowledge of the effects of variation in these points, the powder men are not entirely unprepared as new devices in gas-check rings, and expanding bands, or changes in grooving, are proposed, and we will now consider the more notable modifications which have been suggested.
Rifled guns demanded a less violent explosive, and we can observe the change in the powder curve by comparing the profile of the Dahlgren system (which is not by any means a bad representation of the old powder curve) with the lines of the latest system of rifled artillery. The present curve has a much shorter initial ordinate, and does not fall nearly so abruptly. In the gun we see a relatively smaller diameter at the breech with the strengthening bands extending toward the muzzle. A neglect of the principles of equilibrium between the two and we have the oft repeated accident of the muzzle's blowing off.
The powder experts first increased the size of the grain to diminish burning surface, and we see Rodman's mammoth or pebble grain used all over the world.
The English produce their pellet in regular forms and the Germans adopt Rodman's prismatic grain with its seven perforations, and are imitated by the Russians, Austrians, Spaniards, Italians, Turks and Egyptians.
A few words regarding Prismatic Powder may not be uninteresting.
A Russian officer, present at the original experiments of Major Rodman, at Fortress Monroe, in 1861, was struck with the idea and carried it to Europe. The ingenious mechanical contrivance, at present used in pressing the prisms, is the invention of the Russian Professor Vichnegradski.
The general principle of the increasing burning surface in the prismatic grain, is theoretically very advantageous, ensuring a uniform augmentation in the pressure exerted; but actual practice in service has demonstrated the fact that this cannot always be depended upon. A slight deviation in the continuity of the perforations through the entire charge, or a want of precaution in assuring solidity in the system, and the first rush of gas breaks the grains into fragments which are then consumed with much greater rapidity and consequent unsafe pressures. To this cause may be mainly ascribed the large number of Krupp guns which burst during the Franco-German war, the list of which appeared in the Engineering newspaper a few years since.
The prismatic grain ignites slowly and with safety, being very dense, and having a smooth polished surface which is generally covered with a slight efflorescence of saltpeter. To eliminate the defect of 'breaking up,' the Germans reduced the number of perforations to a single one in the center, but recent reports speak of the merely comparative success of the innovation, and a return to solid grains has been proposed. In view of this fact, it is interesting to note the recent adoption, by the English, of prismatic powder for the 80-ton gun.
The progressive powder of Colonel, now General, de Reffye, burns on the same principle as the prismatic grain, being simply a cartridge built up of a number of disks or cakes having a large central perforation. The same defect of breaking up on firing and in transport led to its abandonment in the French artillery.
In 1872, the Navy Bureau of Ordnance instituted a series of experiments with gunpowder which were conducted with great success by the late Commander Marvin. The qualities of density and burning surface were established for all the Navy calibers, and the hexagonal grain, a device of the present Chief of Bureau, perfected and definitely adopted. The last named powder has the defect of all solid grains, (excepting Fossano) a decreasing burning surface, and, being nearly spherical in shape, the diminution is very rapid, being with the square of the radius, but this loss is more than compensated for by the low density of the interior layers. That portion of the grain which was joined with its neighbors in the same pressing, is very dense, and materially affects the gravimetric density.
In 1872, while attending at Fortress Monroe, some experimental firing for the Navy Bureau, the writer witnessed a series of trials, by an army board, with cakes or disks placed with intervening airspaces in the powder chamber, the cakes being in some cases provided with perforations in others without. The results were quite unsatisfactory however and showed great variation in pressure and velocity. This was due, again, to the half consumed cakes breaking up as in the case of prismatic powder.
In 1875, while stationed as Military Instructor at Amherst College, Lieutenant Totten of the Army devised and carried out a few experiments with a powder to which he gives the name of Compensating.
He proposes to enclose a gun-cotton sphere, 0".5 in diameter, within a gun powder shell 0".25 in thickness, the whole thus forming a grain 1" in diameter. " This grain," says Lieut. Totten, " would be constructed with a scientific regard to the peculiar characteristics of each substance, and would evidently burn on the accelerating principle." Then, criticizing the navy hexagonal, and referring only to the decreasing burning surface and ignoring entirely the effect of the lighter density of the interior, the inventor claims that the 'compensating' grain would burn in concentric layers, the gunpowder portion insuring the start of the projectile, which at a safe instant would be followed by the more rapidly expanding products of the combustion of the gun-cotton.
A little friendly criticism from the Hexagonal side of the question will not be out of place, and it would seem that the following objections possess a certain validity.
Passing over the insurmountable objections with which powder makers regard all explosives more violent than their own, the well established tendency of gun-cotton to change its condition in a varying temperature, unless kept in a moistened state, the cost of its preparation, the question of the injurious effect on the health of workmen, the mechanical difficulties in getting the apple inside of the dumpling, and again in attaining the requisite high density in the outer shell, with a yielding, elastic pulp, like gun-cotton, in the interior, it would appear quite probable, that were all these objections eliminated, there would still remain more serious elements which would render the principle of questionable value:
1. With but a quarter of an inch of material in the outer shell it seems extremely probable that the structure would be crushed in by the first effort of the expanding gases.
2. Were the shell to prove sufficiently strong, would not the guncotton, confined in a medium, perhaps two thousand times denser than the atmosphere, develop unsafe pressures when exploded?
3. In Professor Abel's valuable contribution to the "History of Explosive Agents," published in vol. 159 of the Phil. Trans, of the Royal Society, we find the principles of the detonation of the nitro-explosives fully developed. After showing that gun-cotton cannot easily be detonated by nitrogen chloride, whilst this change is readily effected by a much smaller quantity of the weaker mercuric fulminate, and
detailing a number of instances of this character all supported by experiment, Professor Abel concludes, "the vibrations produced by a particular explosive if synchronous with those which result from the explosion of a neighboring substance which is in a state of high chemical tension, will by their tendency to develop those vibrations, either determine the explosion of the substance, or at any rate greatly aid the disturbing influence of mechanical effect suddenly applied."
We have, in the Totten compensating grain, both of the elements referred to by Professor Abel. The explosion of gunpowder produces these vibrations, as we frequently hear of contiguous powder mills exploding without any communication by flame. The mechanical effect would be the impact of the powder gases against the solid remaining structure.
It is, however, a question in my mind whether Abel's ammonium picrate powder could not be used in some such manner as described by Lieutenant Totten. He would have here a substance of greater chemical stability, a solid which can be pressed into any form like gunpowder, less violence in explosion, and little liability to change of chemical condition with varying temperature. Experiment would be imperatively necessary to determine its capabilities in a detonating way. There would still remain the cost of manufacture, however, which, in all such matters, is a most important factor.
It seems doubtful, indeed, whether any of the nitro-explosives or their modifications can ever be utilized in a military way other than as bursting charges in shells, mines, or torpedoes. Their application to sporting purposes is being attempted, however, and we see in this country a preparation, called after its inventor, Ditmar powder, attaining a certain prominence. The polished interior surface of a fowling piece barrel and the yielding character of the shot-charge and cardboard wadding lessen the brutal element in the quicker burning explosive, and, were it not for the occasional "unaccountable" accident, it might become deservedly popular.
Of the characteristics enumerated above, which affect the burning qualities of gunpowder, that of the density is the most important, and perhaps the most difficult to secure within required limits. The difficulty is due mainly to the variations in the hygrometric state of the atmosphere, and, although many suggestions have been made to secure uniformity in the amount of moisture incorporated with the mill dust, they have generally failed in attaining their object. The so called Wiener powder, the invention of Colonel Wiener of the Russian Artillery, differs in its manufacture from the ordinary variety in the fact that all of the moisture is eliminated in the press mill, the mixture here being brought to a temperature of 240° Fah., the melting-point of sulphur. In this manner, equal volumes of the product of the wheel mills can be pressed to equal densities, but the resulting grain (besides having a great capacity for moisture) would, on theoretical grounds, seem to be too violent in its action for modern rifled guns. This may perhaps account for the rather local reputation of Wiener powder.
Moisture performs the part of a cushion to lessen the violence of the explosion, the role of nitrogen when a mixture of hydrogen and atmospheric air is exploded, of the kieselguhr in dynamite, of pulp and sawdust in the Ditmar and Schultze powders, and of the chamber air space in the Spezia trials.
Again, considering the great specific heat of water, and remembering that the work done is proportional to the heat developed, the presence of moisture would, through, this capacity for heat, have a direct and positive effect on the maximum pressure.
We will conclude our consideration of the modifications which have been proposed as improvements in gunpowder, with a description of the production of the Italian Mills at Fossano.
The abnormally high velocities obtained, coupled with moderate pressures, its adoption by several European powers, and its history in the remarkable trials with the 100-ton gun at Spezia, render it particularly interesting.
The manufacture presents certain peculiarities which, though suggested by the American Dr. Woodbridge some years ago, are to the general military public entirely novel.
After passing through the incorporating mill, the mixture is pressed into cakes of a density of 1.79. The cake is then broken up into grains of about one-eighth to one-fourth inch in diameter, which are mixed with a certain quantity of fine grained powder, and the mass again pressed to a mean density of 1.76. This second cake is finally broken up into tolerably regular grains about 2 ½ inches in length and breadth by 1 ¾ inches in thickness, thus forming an agglomeration of two densities.
I have not seen the probable action of the powder gases from the Fossano grain discussed in any of the military journals, but surmise the sequence of events may be something like the following: It is found that relatively slight differences in density produce considerable variations in pressure, and, we can therefore assume, in the rapidity of burning. In a grain of this character then, the lighter powder would first ignite, and its gases, at a high tension, would, as it were, entirely surround the partially burned denser granules. The projectile has started and furnished greater space for the expanding gases, which is filled at once by the great store of energy (due mainly to its conditions of confinement in a dense medium), probably all developed in the same instant, of the heavier powder.
Such a grain would, in the earlier stages, for the same reasons of unequal densities, burn with an increasing surface, as inequalities or indentations would be formed, which would materially enhance its already decidedly progressive action.
Fossano has the great merits of cheapness and simplicity, and, if uniformity in the form and size of the grain can be assured, I see no reason why it should not have a great future.
Lieut. Soley. The paper which we have just heard is particularly interesting in that it shows how extreme changes in the action of powder can be effected by slight changes in the methods of manufacture and by varying the size and shape of the grain while the proportions of the ingredients remain the same. There are two points mentioned in the paper, in regard to which I disagree with the lecturer, though I do it with extreme diffidence, as I believe there are few officers in the service more thoroughly conversant with the subject of the manufacture of powder than lie is; and I am quite ready to be corrected if I am mistaken. I was under the impression that, in the manufacture of the Fossano powder, the incorporating mill was only used for the preliminary mixtures of saltpetre-sulphur and sulphurcarbon, and that the process was continued by the use of mixing reels and hydraulic presses, an important feature being the fact that the incorporating mill is not used in the later part of the process. With regard to the Hexagonal powder, I am inclined to think that it somewhat resembles the Fossano powder, in that each cake is composed of large and small grains whose action would be very similar to that obtained with the Italian powder. I should have been glad if the lecturer had given some account of the Schaghticoke cubical powder, which has just been made the regulation powder for the large guns in our service, though I doubt if it is exactly a progressive powder, in the same sense as the Fossano is said to be. Any homogeneous powder of great density which may be considered as burning in concentric layers, is a progressive powder, in the widest acceptation of the term, and the velocity of combustion increases rapidly with the pressure developed by the gas, the diminution of the surface of inflammation being compensated, in the interior of the gun, by the velocity of combustion. The ordinary way of arriving at this result is by modifying the form and structure of the grain, and we have been told some of the methods of accomplishing this. Those powders which interest us most, in view of their actual or possible introduction into the service, are the Hexagonal, Schaghticoke, Totten's Compensating, and the Fossano; and I wish to consider the results of these changes on our ordnance.
An immediate consequence of putting armor on to vessels was the simultaneous increase in the weight of the projectile and the calibre of the gun; but, under these conditions, the tine-grained, quick-burning powder, which had been formerly used, was extremely dangerous to the piece, and it became necessary to modify the rate of burning. It is well known that when quick powder, under constant pressure, is burned, the disengagement of gas, at its maximum at the beginning, constantly decreases until the end of the combustion, giving its greatest force of disruption and translation before the inertia of the projectile is overcome. The experiments made by Rodman, Dahlgren, and others, on the pressures on the interior of the bore fully demonstrated this fact, and the results are expressed as follows:
At one calibre in rear of center of projectile, .98
At center of projectile, 1.00
At one calibre in front of projectile .81
At two … .68
At three … .63
At five … .53
At seven … .44
At nine … .40
At eleven … .37
At fifteen … .29
These figures show the relative strength necessary, at different parts, to resist explosion.
In view of these experiments, the well known Dahlgren shape was adopted, and has been adhered to not only in the small 8-inch and IX-inch smooth bores, but also in the 8-inch rifle and XV-inch smooth bores which use some of these progressive powders; that is to say, powders which, on first combustion, develop a small amount of gas to overcome the inertia of the projectile, and, as its movement becomes accelerated, give larger and increasing gas pressures. The pressure curves of the quick powders and of the progressive powders show how the rotations between certain parts of the gun and the strains exerted, have changed with the use of the progressive powders. Since the adoption of these powders has become a fixed fact in our service, and since we know that these new pressures will induce very different strains, the question presents itself with redoubled force;, are we proceeding in the right direction in converting our smooth-bore guns into rifled guns which are to use progressive powders? In our 8-inch rifles which have been converted from Xl-inch smooth bores, and which are being generally supplied to our ships, the cast-iron bore, which gave the gun its longitudinal strength, has been removed, and, in its stead, a coiled tube has been introduced, which is held in place merely by a small screw ring at the muzzle. The tube has only the longitudinal strength which it receives from the thoroughness of the weld; Armstrong was obliged to abandon the coiled tube A almost as soon as he adopted it, because he found that the bore of the gun must be the part which supplies the longitudinal strength; and, with the progressive pressure of the new powder, these guns of ours are weakest, both longitudinally and transversely, where they need the greatest strength. The lamentable disaster on board the Thunderer, whether the result of accident or carelessness, while it calls forth all our sympathies, must, at the same time, be instructive. An inspection of the diagram of the burst gun shows that it went in the very place where its comparatively small longitudinal strength was supplemented by the least transverse strength. As long as we confine ourselves to twenty or twenty-five pound charges, in our 8-inch rifles, there is no particular danger, but, if it becomes necessary to use these guns in action, with thirty five pound charges of progressive powder and battering shell, I venture to say that our makeshifts will make us regret that they were ever introduced.
Lieut.-Comdr. Folger. If I understand the question raised by Lieutenant Soley, as to the fact of the incorporation of the ingredients of Fossano powder. I do not exactly see how the necessary intimacy of mixture for perfect combustion can be obtained without the passage through the wheel mills. Were we to manufacture Fossano in this country, I hardly think the requirements of the Ordnance Bureau would be satisfied with any omission of this kind. The Schaghticoke Cubical is merely the mammoth grain in regular forms. If it were a flat grain instead of a cube, it would be more effective, as the surface would not diminish so rapidly in combustion. The regularity in shape is obtained by grooving the press cake in the desired lines of fracture. The homogeneous density, I consider, viewed in the light of the latest ideas on powder, is a defect rather than an advantage.
The Navy hexagonal is not a pressing of two densities, but simply of the product of the wheel mills. Of this I am quite positive, as I was present at the original experiments, in a manufacturing way. I think the present grain is too small for an application of the Fossano principle.
The converted Xl-inch gun is not by any means a model arm in its curves and lines of resistance, and of this I think none of us are more convinced than is the Bureau of Ordnance. I must, however, disagree with the gentleman in his opinion, which we are led to infer from his remarks, that it has little or no value as a part of the armament of our ships, or is so defective in its lines of resistance as to possibly be even an element of danger to our guns' crews.
Merely referring to materials, it is an acknowledged fact that American iron possesses more of the qualities requisite for gun-making than any other iron in existence; and, made of iron, these guns have withstood pressures in proof-firing of from thirty thousand to sixty thousand pounds per square inch.
Now, whilst the highest pressure ordinate with a progressive powder is somewhat further forward than with the quick-burning varieties, it at no time readies their dangerous limits (Fig. 3), and at the moment the projectile is passing the weaker points in the gun-cylinder referred to by the gentleman, the curve is doubtless falling rapidly by the movement of the projectile. Should the shot jam, as was first reported of the Thunderer's accident, there are good reasons for believing that no system of artillery can withstand the strain.
If we now utilize this capability of resistance of sixty thousand pounds' pressure with the latest ideas on progressive powders, as the Chief of Bureau intimates, in his last Report to the Secretary, that he is prepared to do, we have a powerful addition to our ships' batteries, and one which should inspire self-congratulation rather than any other sentiment, when compared with our antique smooth-bores.
We are not proceeding in a wrong direction, for at this moment the Bureau of Ordnance (which all along has presented the 8-inch rifle as a makeshift to tide over an economical Congress) has prepared drawings and specifications of a 6-inch and a X-inch steel gun, to be undertaken with the first funds appropriated for the purpose. Our makeshift 8-inch rifle has been paid for by the sale of condemned material; it costs but $2,000, while the X-inch steel gun will cost more than twenty times this amount.
Lieut. Kennedy. In Professor Hill's lectures on explosives delivered at the Torpedo Station, I find the statement, that when powder is pressed, if it be too dry it will not bind or retain its form; and if too moist it cannot be much pressed on account of the incompressibility of water. "Without going to either of these extremes, the density of powder may be greatly varied by a slight change in the moisture contained. In the Fossano powder mentioned in the paper just read, the powder is first pressed to a density of 1.79 and then broken up, mixed with other powder, and again pressed to a density of 1,776. These limits are very small, and still on this difference depends the progressive action of this powder. Plow is the moisture kept at exactly such a point that the same pressure shall always produce the same density; and, in general, by what means is the moisture controlled daring manufacture, particularly in damp weather, so that different lots of powder shall always come up to the required standard?
Lieut.-Comdr. Folger. As I intimated in my notes, it is a matter of considerable difficulty, to reproduce densities, and I have repeatedly seen the production of the work of days thrown back on contractors' hands as showing too great variations in density. The Bureau usually limits the manufacture to 0.01 on each side of the required point, and this they consider very close work. The matter requires the greatest care, and experience alone is the best guide. The hygrometric state of the atmosphere is noted and the quantity of water added in the wheel mill is regulated accordingly. The behavior in the press, the sounds given out by the mass at the close of the process, in connection with normal marks on the press frame are among the points which govern the judgment of the pressers.
It is remarkable however what experience or good judgment will sometimes accomplish in this respect. I witnessed the manufacturer's inspection (Oriental Mills) of an army contract last year in which with twenty densimeter samples the extreme variation was but 0.007.
Lieut. Miller. I should like to ask if the lecturer considers the Rodman cutter a reliable instrument for measuring the pressures which take place upon the explosion of the charge.
There are many instances where the indentations upon the copper disk have recorded pressures far beyond the tensile strength of the gun metal. Last year at the Experimental Battery, the XI-inch gun was loaded with service charge and shot—the powder being that known as oriental cannon, which I think had been twice reworked. The pressure indicated was on one occasion as high as one hundred and fifty thousand pounds, and on others about ninety thousand. Now the tensile strength of the very best cast-iron is never over fifty thousand pounds per square inch.
Therefore were the enormous figures quoted above due
I. To an error in the Rodman cutter itself,
II. To the non homogeneousness of the copper disk, or
III. To the effect of gas waves reacting from the base of the shot, acting as successive blows to drive the knife deeper and deeper into the disk?
I am well aware that the opinion is held that if it requires a force, say of forty thousand pounds, to drive the cutter a sixteenth of an inch into the copper it will require a greater force than forty thousand to drive it any further at any subsequent time, but it strikes me that the theory is not altogether correct; especially as regards the action of explosive gases in a confined space, and I would especially ask if the resultant cut in the copper, may not be the combined effect of several pressures, acting at different times and each less than that necessary to rupture the gun.
Lieut.-Comdr. Folger. If I remember correctly. General Rodman believed his method of measuring pressures to be reliable within one thousand pounds per square inch, even at the maximum, but my own limited experience would lead me to a contrary opinion, at least as regards its measuring absolute pressures. There should be some account taken of the time element, for, with the testing machine, the particles of copper have a chance to "flow," or in other words, arrange themselves.
Again, we are taught, considering the metal homogeneous, that the resistance to the cutter varies with the square of its velocity.
The pressure of a hundred and fifty thousand pounds per square inch referred to was probably the total of a number of local pressures, which always occur, according to a number of writers, (particularly with a quick burning powder.) and which are unquestionably due to the wave-like vibration mentioned by Lieutenant Miller. This action of the gases probably continues during the whole of the interval occupied by the shot in its passage to the muzzle. With a uniformly-burning progressive powder, the Rodman gauge will register much nearer the true pressure.
As regards the point referred to by Lieut. Miller that the XI-inch gun did not burst with a registered pressure much beyond the tensile strength of iron, I think the terse remark of General Rodman concerning such cases, that it "did not have time to burst" (before the pressure was relieved) is
applicable. Also, we might say, the case is analogous to the simple experiment of the breaking of a small iron wire. A sudden jerk will not effect a rupture whilst a continued effort in which much less force is expended will accomplish the object. Tensile strength is measured by the slow process.
The mathematicians have demonstrated the fact that a homogeneous inelastic cylinder will burst if the internal pressure per unit area exceeds the tensile strength of unit section, but in our case the limit of elasticity of our limit section is not reached with its rupturing pressure, as measured by the testing machine, The curve of resistance falls very rapidly however, and we reach a point beyond which no increase in thickness of metal will add strength to the gun cylinder.
The point of the non-homogeneous character of the copper disks enters largely into the question, and is doubtless a frequent cause of unreliability in the registered pressures. This fact was one of the strongest arguments in favor of the adoption of the spiral gauge, as the copper disks used with the latter are much smaller, and variations in hardness and density do not produce so marked an effect.
The Chairman. I have listened with extreme interest to the able paper which has just been presented to us, and I find in it nothing to criticize and feel that there is little to be added to it. It suggests, however, one or two thoughts.
The lecturer has pointed out certain objectionable features in the Totten powder which would interfere with its use. Besides these, there is another which is quite as important. It is well known that explosives such as guncotton and nitro-glycerine, if ignited when exposed to the atmosphere, simply burn freely; but if ignited when they are closely confined, they explode with detonating violence. When the Totten powder burns in the chamber of a gun, we have the gun-cotton ignited while it is closely confined, and it is quite probable that a detonating explosion would result.
I have been pleased to see that the lecturer places more reliance on the pressures as determined by Rumford and accepted by Piobert, than on those obtained by later experimenters, and I am disposed to follow him for several reasons. First, because, as he has pointed out, Rumford is the only experimenter who has directly measured the pressure. Second, because I have yet to learn that it has been shown that Rumford's apparatus was defective, that his method was faulty, or that there were any errors of observation in his experiments. The chief objection urged against his measurements, is that the results did not agree closely enough among themselves, but Noble and Abel have shown that closely agreeing results can only be obtained when the different specimens of powder are of normal type and uniform make, and it is doubtful whether Rumford possessed such powders. In the case in which Rumford estimated the pressure under which his gun burst, there can be no doubt that his results were too high, and there is no question that the fact of his having done this, and attempted also to explain the high pressures obtained in all the experiments by an erroneous theory as to the action of aqueous vapor at high temperatures, has done much towards throwing discredit upon all his work. Between the results of Rumford and those obtained by Noble and Abel, there is a difference so great that it is difficult to explain it. These last pressures having been obtained by calculation, and by comparison of the calculated pressures with those obtained by the crusher gauge, are more likely to be in error than those obtained by direct experiment. The fact that estimates by the two methods agreed closely, confirms me in the belief that they are too small; I agree with the lecturer in believing that the crushing machines register too low a pressure. To calculate the pressure exerted by powder, we must know the quantity and character of the products of its combustion, the temperature of combustion, and the action of the products at the high temperatures which obtain in the experiments, and, notwithstanding; the interesting investigations of Bansen and Schischkoff, and the more elaborate researches of Noble and Abel, I do not think that these data have yet been determined with sufficient accuracy.
I would like to ask the lecturer how he accounts for the increased velocity and diminished pressure obtained with a small air space, and also the action of the gases with a large space, as for instance, when the projectile has slipped forward.
Lieut.-Comdr. Folger. There are so many opinions and theories on this subject since the Thunderer's accident, that I fear one from myself will throw no additional light on the subject, but I believe the action of the gases to be something of this character:—
When the projectile is pressed close against a charge which entirely fills the chamber, the latter burning more rapidly in a confined space develops a greater portion of its energy, in a given time, than when air space is furnished. With a limited air-space, a smaller portion of the charge overcomes the inertia of the projectile, leaving the remainder to develop a progressive action on an already moving obstacle. This perhaps accounts for the increased velocity. Concerning the reduced pressure, we have the same argument of less powder burned in a given time, and to go into details, I think the cushioning effect of the air particles assists in starting the projectile when the space is small. Perhaps a portion, at least, of the oxygen in the air is utilized in combustion, though that from the saltpeter is, the chemists tell us, more apt to perform this part of the work, but there still remains the nitrogen. Again, the air particles would use a portion of the energy in their own rise in temperature.
With a large air space, the cushioning effect, more apparent, permits the entire combustion of the charge before the projectile starts, and the developed gases being given direction by the tube—focus, we might say,—with great inertia on a projectile at rest, and a rupture of the walls of the gun is the possible result.