[Reprinted from the Journal of the Franklin Institute, February, 1891.]
Members of the Institute and Ladies and Gentlemen :
In commencing my paper this evening I desire to call your attention to the fact that I am dealing with a subject which, though not theoretical, is still hardly practical, for, as a matter of fact, high explosives cannot be said to have yet been regularly used in warfare; and I hope you will pardon me if, in consequence, my statements appear in some respects unsatisfactory and my theories unsound. My subject, however, is no more obscure than future naval warfare generally. All civilized nations are spending millions of money for fighting purposes, directly in opposition to the higher feelings of the better class of their inhabitants. The political atmosphere of Europe is the cause of this, but its consequence is the development of theoretical plans of ships which are no sooner commenced than the rapid march of mechanical, chemical and electrical science shows them to be faulty in some particular feature, and others are laid down only to be superseded in their turn.
None of these crafts are obsolete (to use the popular expression of the day). All are theoretically better than any which have stood the test of battle; but each excels its predecessor in some particular feature. The use of high explosives is the direct cause of the very latest transformations in marine architecture, and is destined to work still greater changes; but it will require a war between the most civilized nations of the world, and a l.ong war, to either confirm or condemn the many theoretical machines and methods of destruction that modern science has produced. I say a war between the most civilized nations, since it is only they that can supply the educated intellect that is necessary to both attack and defense. Under other circumstances, false conclusions as to weapons and results are certain to be drawn. At the bombardment of Alexandria, the English armorclads, with their rifled guns, were not nearly as efficient against the feeble chalk fortifications as our wooden ships would have been with smooth-bore guns; on the other hand, I saw on shore, after the bombardment, hundreds of torpedoes and miles of cable that the Egyptians did not understand how to use. The French war with China was equally unsatisfactory from a military point of view. The Chinese at Foochow were annihilated because the French opened fire first, and the only shell that penetrated a French ironclad was filled with lampblack instead of powder. The national riots that we are accustomed to hear of in South America are likewise of little instructive value; they buy their weapons of more civilized people, but there is always something fatally defective about the tactics pursued in using them. It may be said in general terms that in these days of extreme power in fighting machines, the greater the efficiency the less the simplicity and the more knowledge required in the care of the weapons. When powder was merely powder, the advice of the old adage to "trust in God and keep your powder dry" was ample to maintain the efficiency of the powder for all purposes; but nowadays if you keep your powder dry you will burst your gun, and if you keep your gun-cotton dry you are liable to blow up your ship.
It is rather difficult to-day to define what high explosives are, in contradistinction to gunpowder. Thirty years ago we could say that powder was a mechanical mixture and the others were chemical compounds; but of late years this difference has disappeared.
The dynamical difference, however, still remains: gunpowder in its most efficient form is a slow-burning composition, which exerts a relatively low pressure and continues it for a long time and to a great distance; high explosives, on the contrary, in their most efficient form, are extremely quick-burning substances, which exert an enormous pressure within a limited radius. Ordinary black gunpowder consists of a mechanical mixture of seventy-five per cent of saltpeter, fifteen per cent of charcoal and ten per cent of sulphur. The most important of the high explosives are formed by the action of nitric acid upon organic substances or other hydrocarbons, the compound radical NO2 being substituted for a portion of the hydrogen in the substance. The bodies thus formed are in a condition of unstable equilibrium; but if well made from good material, they become stable in their instability, very much like Prince Rupert's drops, those little glass pellets which endure almost any amount of rough usage, but, once cracked, fly into infinitesimal fragments.
The power exerted by these nitro-substitution products is due to the fact that they detonate, i. e., they are instantaneously converted into colorless gas at a very high temperature, and in addition they have almost no solid residue. Nitro-glycerine actually leaves none at all, while gunpowder leaves sixty-eight per cent. The first departure in gunpowder from the old-time constituents of black powder just mentioned was for the purpose of obtaining less pressure and slower combustion than could be produced by mere granulating or caking; this was accomplished by using under burned charcoal, together with sugar and about one and one-half per cent of water. This is the brown powder most generally used at present, and with satisfactory results; but the abstraction of its moisture increases its rapidity of combustion to a dangerous degree, besides which the under burned charcoal is itself unstable.
The next change demanded is smokelessness, and to accomplish it, recourse is had to the high-explosive field, mechanically mixing various substances with them to reduce and regulate their rapidity of action. Just now some form of gun-cotton is most in use, mixed with nitrate of ammonia, camphor and other articles. The tendency of these mixtures is to absorb moisture, and the gun-cotton in them to decompose, and there is no smokeless powder which can to-day be considered successful. Such a powder, however, will undoubtedly be an accomplished fact in the near future. Military men seem to be a great deal at variance as to its value in the field, but there can be no doubt of its value for naval purposes; it is a necessity forced upon us by the development of torpedo warfare. First came the simple torpedo at the end of an ordinary boat's spar; then came the special torpedo-boat with its great speed; then the revolving cannon and rapid-fire gun to meet the torpedo-boat. At present the possible rapidity of fire is much greater than can be utilized on account of the smoke; hence the necessity of smokeless powder. Smokelessness is, however, principally a martial demand that has been made upon the science of explosives, and has attracted public attention on that account. The commercial demands for various other properties has been much greater than the military, and between gunpowder near one end of the line in point of power, and nitro-glycerine near the other, there are now over 350 different explosives manufactured, and most of these have been invented within the last twenty years.
The simplest applications of high explosives in warfare is in connection with torpedoes, since within the same bulk a much more efficient substance can be obtained than gunpowder, and with reasonable care there is very little danger of premature explosions by reason of accidental shocks.
Torpedoes were made by the Chinese many years ago; they were tried in our war of independence, and also by the Russians during the Crimean war; but the first practical and successful use of them as a recognized weapon was during our war of secession, when thirty-seven vessels were either sunk or seriously injured by them. Gunpowder was used in these torpedoes, though it is stated that attempts were made to use other substances without success. Since that time all maritime nations have made a close study of the subject, and have adopted various high explosives, according to the results of their experiments. In general terms it may be stated that explosive chemical compounds have been found more suitable than explosive mixtures, because of the uniformity of direction in which they exert their pressure, and from the fact that water does not injure them. Mixtures may be very powerful, but they are erratic, and require tight cases. In the United States we use dynamite for harbor mines. It is composed of seventy-five per cent nitro-glycerine and twenty-five per cent silica; but blasting gelatine and forcite-gelatine will probably be adopted, when they can be satisfactorily manufactured here, as they are more powerful. The former is composed of ninety-two per cent of nitro-glycerine and eight per cent of gun-cotton, and the latter of ninety-five per cent of nitro-gelatine and 5 per cent un-nitrated cellulose.
For naval use we have adopted gun-cotton as being the most convenient. In Europe gun-cotton is generally used for both fixed mines and movable torpedoes; Russia, Austria and Italy use blasting gelatine also.
In actual warfare but little experience has been had; two Peruvian vessels were sunk by dynamite in the Chile- Peruvian war; one Turk by means of gun-cotton during the Turco-Russian war of 1877, and two Chinese by gun-cotton in the Franco-Chinese war of 1884,
In making experiments to determine the relative strength of the different explosives under water, very curious and puzzling results have been obtained. Nitro-glycerine being the simplest and most complete in its chemical decomposition, and apparently the most powerful in air, it was natural to suppose that it would be the same in submarine work; but it was found by Gen. Abbot at Willet's Point, after repeated experiments, as shown in his report of 1881, that it was not so powerful in its effect by twenty per cent as dynamite No. I, although the dynamite contained twenty-five per cent of an absolutely inert substance. His idea was that it was too quick in its action, and since water is slightly compressible, a minute fraction of time is required in the development of the full force of the explosive. Gen. Abbot's results for intensity of action per unit of weight of the most important substances, are as follows:
Blasting gelatine…142
Forcite gelatine…133
Dynamite No. 1…100
Gun-cotton, wet…87
Nitro-glycerine…81
Gunpowder…20 to 50
Col. Bucknill, of the Royal Engineers, in his publication of 1888, gives the following:
Blasting gelatine…142
Forcite gelatine…133
Dynamite No. 1…100
Gun-cotton, dry…100
Gun-cotton, wet…80
Gunpowder…25
In both tables, dynamite No. 1 is assumed as the standard of comparison. Col. Bucknill states that his gun-cotton results differ from Gen. Abbot's because he experimented with much larger quantities, viz: 500-pound charges. Gen. Abbot's experiments led him to believe that an instantaneous mean pressure of 6500 pounds per square inch would give a fatal blow to the double bottom of a modern armor-clad, and he developed a formula which gives this blow with blasting gelatine at the following distances under water, viz:
At 5 feet…4 pounds
10 …17
20 …67
30 …160
40 …3
Col. Bucknill's experiments caused him to believe that a pressure of 12,000 pounds per square inch is required, and his formula, which is somewhat different from Abbot's, gives widely different results at close quarters, but they approach each other as the distance increases.
His results are as follows:
At 5 feet… 22 pounds
10…75
20…177
30…274
40…369
Regarding the comparative effects of gunpowder and the high explosives, I think Gen. Abbot's estimate of a varying value for powder is more admissible than the fixed value assigned by Col. Bucknill. Gunpowder gives a push, and detonating compounds a shock; as the quantities increase, the push reaches farther than the shock. According to Gen. Abbot, 100 pounds of dynamite No. 1 will have a destructive horizontal range of 16.3 feet, while the same amount of gunpowder will only have a range of 3.3 feet. Five hundred pounds of dynamite, however, will have a horizontal range of thirty-five feet, and 500 pounds of gunpowder will have 19.5 feet; the ratio has diminished from five to two. Whether 6500 pounds or 12,000 pounds per square inch is necessary to crush the bottom of an armor-clad will depend largely upon how far apart the frames of the ship are spaced and what other bracing is supplied, as well as many local circumstances. It is difficult to judge exactly of these matters. Some four years ago the Italian government adopted treble bottoms for their heaviest ships as a result of experiments with seventy-five pounds of gun-cotton (the charge of an ordinary Whitehead locomotive torpedo) against a caisson which was a facsimile of a portion of the proposed ships. Only two of the bottoms were broken through, and when the space between the two inner bottoms was filled with coal, only the outer bottom was broken. According to the formulae of either Abbot or Bucknill, there should have been a local pressure of at least 300,000 pounds per square inch on the outer skin, and yet judicious interior arrangements rendered it harmless to the target. It would not, however, be safe to conclude that the torpedo was thus vanquished—the immediate result was simply to create a demand for larger locomotive torpedoes for local application, and but little light was thrown upon the results which might be anticipated from a large mine at a greater distance, whose radius of explosive effect would embrace a larger portion of the ship, and especially if the ship were nearly over the torpedo. The local effect of a detonation is different from the transmitted shock. Experiments in England have shown that 500 pounds of gun-cotton at forty feet below any ship will sink her, and at a horizontal distance of 100 feet, damage to the interior pipes and machinery is to be expected.
The fact that the high explosives are so much heavier than gunpowder has an important bearing on the size of the containing case. Their sp. gr. is as follows:
Nitro-glycerine…1.6
Blasting gelatine…1.45
Forcite gelatine…1.42
Dynamite No. 1…1 .34
Wet gun-cotton…1.32
Dry gun-cotton…1.06
Gunpowder…0.9
Their relative efficiency under water per cubic foot, according to Bucknill, is as follows:
Blasting gelatine…1 .38
Forcite gelatine…1 .27
Dynamite No. 1…1.00
Dry gun-cotton… .66
Wet gun-cotton… .66
Gunpowder… .14
The wet gun-cotton has twenty-five per cent of added water.
Mines for harbor defense are of two kinds—buoyant and ground. The buoyant are usually spherical, and contain from 400 to 500 pounds of explosives. They bring the charge near to the ship's bottom, but are difficult to manage in a tide-way, and can be easily found by dragging. The ground mines can be made of any size, and are not easily found by dragging, but are of little value in very deep water. They are either cylindrical or hemispherical in shape, and contain from 500 to 1500 pounds of explosive in from thirty to eighty feet of water. Mines of any kind are exceedingly difficult to render efficient when the water is over 100 feet deep. On account of the tendency of all high explosives to detonate by influence or sympathy, and the liability of the cases to collapse by great exterior pressure, harbor mines are separated at a certain distance, according as they are buoyant or ground, and according to the nature of the explosive.
Five hundred pounds buoyant gun-cotton mines require 320 feet spacing.
Five hundred pounds buoyant blasting gelatine mines require 450 feet spacing.
Six hundred pounds ground gun-cotton mines require 180 feet spacing.
Six hundred pounds ground blasting gelatine mines require 230 feet spacing.
Of torpedoes, other than those described, we have several modern varieties: submarine projectiles, submarine rockets, automobile and controllable locomotive torpedoes. The first two varieties, though feasible, are not developed, and have not yet advanced beyond the experimental stage. Of the automobile, we have the Whitehead, Swartzkopf, and Howell. The first two are propelled by means of compressed air and an engine; the last, by the stored-up energy of a heavy fly-wheel. Generally speaking, they are cigar-shaped crafts, from 10 to 18 feet long, and 15 to 17 inches in diameter, capable of carrying from 75 to 250 pounds of explosive at the rate of 25 to 30 knots for 400 yards, at any depth at which they may be set. Of the controllable locomotive torpedoes, the three representative types are the Patrick, Sims, and Brennan. They are, in general terms, cigar-boats, about 40 feet long and 2 feet in diameter, carrying charges of 400 pounds of explosive. The Patrick and Sims are maintained at a constant depth under water by means of a float. The Brennan has diving rudders, like a Whitehead or a Howell. The Patrick is driven by means of carbonic-acid gas through an engine, and is controlled by an electric wire from shore. The Sims is driven by electricity from a dynamo on shore, through a cable to an electric engine in the torpedo. The Brennan is driven and controlled by means of two fine steel wires wound on reels in the torpedo, the reels being geared to the propeller-shafts. The wires are led to corresponding reels on shore, and these are rapidly revolved by means of an engine. A brake on each shore-reel controls the torpedo. The speed of all these torpedoes is about 19 knots, and their effective range one mile.
A Whitehead was successfully used in the Turco-Russian war of 1877. The Turkish vessel previously mentioned was sunk by one.
Blasting gelatine, dynamite, and gun-cotton are capable of many applications to engineering purposes on shore in time of war, and in most cases they are better than powder. They received the serious attention of French engineers during the siege of Paris, and were employed in the various sorties which were made from the city, in throwing down walls, bursting guns, etc. An explosive for such purposes, and indeed for most military uses, should satisfy the following conditions:
(1) Very shattering in its effects.
(2) Insensible to shocks of projectiles.
(3) Plastic.
(4) Easy and safe to manipulate.
(5) Easy to insert a fuze.
(6) Great stability at all natural temperatures and when used in wet localities.
Neither blasting gelatine, dynamite nor gun-cotton fulfill all these conditions; but they satisfy many of them, and are more powerful than other substances. For the destruction of walls, trees, rails, bridges, etc., it is simply necessary to attach to them small bags of explosive, which are ignited by means of blasters' fuze and a cap of fulminate of mercury, or by an electric fuze.
We now come to the application of high explosives to warfare in the shape of bursting charges for shells. This is the latest phase of the problem, and it is undoubtedly fraught with the most important consequences to both attack and defense. Difficult as it has been to obtain an exact estimate of the force of different explosives under water, the problem is far greater out of the water and under the ordinary conditions of shell-fire; the principal obstacle being in the fact that it is physically impossible to control the force of large quantities in order to measure it, and small quantities give irregular results. Theoretically, the matter has been accomplished by Berthelot, the head of the French Government "Commission of Explosives," by calculating the volume of gas produced, heat developed, etc.; and this method is excellent for obtaining a fair idea of the specific pressure of any new explosive that may be brought forward, and determining whether it is worth while to investigate it further; but the explosives differ so much from each other in point of sensitiveness, weight, physical condition, velocity of explosive wave, influence of temperature and humidity, that we cannot determine from mere theoretical considerations all that we would like to know. Various methods of arriving at comparative values have been tried, but the figures are very variable, as will be seen by the following tables. Berthelot's commission, some ten years ago, exploded ten to thirty grams each in 300-pound blocks of lead, and measured the increased size of the hole thus made. The relative result was:
No. I dynamite… 1.
Dry gun-cotton… 1.17
Nitro-glycerine… 1.20
Powder blew out and could not be measured.
Mr. R. C. Williams, at the Boston Institute of Technology, in the winter of 1888 and 1889, tried the same method, but used six grams in forty-five-pound blocks of lead. He obtained a relative result
of—
No. I dynamite… 1.
Dry gun-cotton… 1 .37
Nitro-glycerine… 2.51
Explosive gelatine… 2.57
Forcite gelatine… 2.7
Warm nitro-glycerine… 2.7
Gunpowder… .1
The powder gave great trouble in this case, also, by blowing out. M. Chalon, a French engineer, obtained some years ago, with a small mortar, firing a projectile of thirty kilos and using a charge of ten grams of each explosive, the following ranges:
Blasting powder… 2.6 metres
No. I dynamite… 31.4
Forcite of 75 per cent N. G… 43.6
Blasting gelatine… 45
Roux and Sarrau obtained, by experiments in bursting small bombshells, the following comparative strengths of ranges:
Powder…1.
Gun-cotton… 6.5
Nitro-glycerine…10.0
In actual blasting work the results vary altogether with the nature of the material encountered, and with the result that is desired to be accomplished, viz: throwing out, shattering, or mere displacement.
Chalon gives for quarrying;
Powder… 1
Dynamite No. 2, containing 50 per cent nitro-glycerine… 3
For open blasting:
Dynamite No. 3, containing 30 per cent N. G…1.0
Dynamite No. i, containing 75 per cent N. G… 2.5
Blasting gelatine… 3.5
For tunneling:
Dynamite No. 3, containing 30 per cent N. G… 1
Dynamite No. 1, containing 75 per cent N. G… 3
Explosive gelatine… 19
Finally, Berthelot's theoretical calculations give a specific pressure of—
Powder… 1
Dynamite… 13
Gun-cotton… 14
Nitro-glycerine… 16
Blasting gelatine… 17
It will be observed that the practical results vary largely from the theoretical values, but they seem to indicate that gun-cotton and No. I dynamite are very nearly equal to each other, and that in the nitro-glycerine compounds, except where gun-cotton is added, the force appears to be nearly in proportion to the nitro-glycerine contained. From the foregoing it seems fair to estimate roughly the values of bursting charges of shells as follows:
Powder… 1
Gun-cotton and dynamite… 6 to 10
Nitro-glycerine… 13 to 15
Blasting gelatine… I5 to 17
Attention has been turned in Europe for more than thirty years towards firing high explosives in shells; but it is only within very late years that results have been reached which are claimed as satisfactory, and it is exceedingly difficult to obtain reliable accounts even of these. Dynamite was fired in Sweden in 1867 in small quantities, and a few years later it was fired in France. But two difficulties soon presented themselves. If the quantity of nitro-glycerine in the dynamite was small it could be fired in ordinary shells, but the effect was no better than with gunpowder. If the dynamite was stronger in nitro-glycerine, it took but a small quantity to burst the gun. As early as 1864, dry gun-cotton was safely fired in shells in small quantities, but when a sufficient quantity to fill the shell-cavity was used, the gun burst. Some few years ago it was found that if the gun-cotton was either wet or soaked in paraffin, it could be fired with safety from powder-guns in ordinary shells, provided the quantity was small in proportion to the total weight of the shell, (say) five to six per cent; but a new difficulty arises from the fact that it breaks the shell up into very small pieces, and it is an unsettled question among artillerists whether more damage is done to an enemy by breaking a shell into comparatively large pieces and dispersing them a long distance with a bursting charge of powder, which has a propulsive force, or by breaking it with a detonating compound into fine pieces which are not driven nearly so far. Wheh used against troops there is also the objection to the high-explosive shell that it makes scarcely any smoke in bursting, and smoke at this point is useful to the artillerist in rectifying his aim. In the matter of shells for piercing armor, however, there are no two opinions regarding the nature of the bursting charge. To pierce modern armor at all a shell must be made of forged steel, so thick that the capacity of the cavity for the bursting charge is reduced to one-fourth or one-fifth of what it is in the common shell; the result is that a charge of powder is frequently not powerful enough to burst the shell at all; it simply blows the plug out of the filling-hole in the rear. In addition it is found that in passing through armor, the heat generated is so great that the powder is prematurely ignited. If then we can fill the small cavity in the shell with an explosive which will not ignite prematurely, and yet will burst the shell properly after it has passed through the armor, the problem will be solved. Wet or paraffined gun-cotton can be made sluggish enough to satisfy the first condition; but at present the difficulty is to make it explode at all. The more sluggish the gun-cotton, the more powerful must be the fuze -exploders to detonate it, and such exploders are themselves liable to premature ignition in passing through the armor. The Italians and Germans claim to have accomplished the desired result up to a thickness of five inches of armor; gun-cotton and fuze both working well. But the English authorities say that no one has yet accomplished it. The Austrians claim to have succeeded in this direction within the last year with a new explosive called ecrastite (supposed to be blasting gelatine combined with sulphate or hydrochlorate of ammonia, and claimed to be one and one-half times as powerful as dynamite). With a gun of 8.24-inch caliber and an armor-piercing shell weighing 206.6 pounds, containing a bursting charge of 15.88 pounds of ecrastite, they are said to have perforated two plates four inches thick, and entered a third four-inch plate where the shell exploded. There is a weak point in this account in the fact that the powder capacity of the shell is said to be 4.4 pounds. This amount is approximately correct, judging from our own eight-inch armor piercing shell; but if this is true, there could not have been more than nine pounds of ecrastite in the shell, instead of sixteen, or else there is an exceedingly small proportion of blasting gelatine in ecrastite, and if that is the case it is not one and one-half times as powerful as dynamite. If it is weak stuff it is probably insensitive; and even if it were strong, one swallow does not make a summer. The English fired quantities of blasting gelatine from a two-inch Nordenfeldt gun in 1884, but when they tried it in a 7-inch gun in 1885, they burst the gun at once. I have only analyzed this Austrian case because the statement is taken from this year's annual report of the Office of Naval Intelligence, which is an excellent authority, and to illustrate the fact that of the thousands of accounts which we see in foreign and domestic newspapers concerning the successful use of high explosives in shells, fully ninety per cent are totally unreliable. In many cases they are in the nature of a prospectus from the inventors of explosives or methods of firing, who are aware of the fact that it is almost impossible to dispute any statements that they may choose to make regarding the power of their new compounds, and thinking, as most of them do, that power alone is required.
Referring to the qualities that I have previously cited as being required in a high explosive for military purposes, it is sooner or later found that nearly all the novelties proposed lack some of the essentials, and soon disappear from the advertising world only to be succeeded by others. The most common defect is lack of keeping qualities. They will either absorb moisture or will evaporate; or further chemical action will go on among the constituents, making them dangerously sensitive or completely inert, or they will separate mechanically according to their specific gravities.
The chief efficiency of small quantities of high explosives having reduced itself to the case of armor-piercing projectiles, it next became evident that there was an entirely new field for high explosives into which powder had entered but little, and this was the introduction of huge torpedo-shells which did not rely for their efficiency upon the dispersion of the pieces of shell, but upon the devastating force of the bursting charge itself upon everything within the radius of its explosive effect. It is in this field that we may look for the most remarkable results, and it is here that the absolute power of the explosive thrown is of the utmost importance, provided that it can be safely used. Attention was at once turned in Europe to the manufacture of large projectiles with great capacity for bursting charges, and it has resulted in the production of a class of shells 4 ½ to 6 calibers long, with walls only .4 of an inch thick. (If they are made thinner, they will swell and jam in the gun when fired.)
These shells are used in long guns up to 6 and 8 ½ inches caliber, and in mortars up to 11. 2 inch. They are made from disks of steel, 3 to 4 feet in diameter and 1 inch thick, and are forced into shape by hydraulic presses. The base is usually screwed in, but some of the German shell are made in two halves which screw together. The Italians were the first in this new field of investigation, but the Germans soon followed, and after trying various materials, were at length reasonably successful with gun-cotton soaked in paraffin. Their 8.4-inch mortar shells of 5 calibers contain 42 pounds; those of 6 calibers contain 57 pounds; and the 11.2-inch mortar shells of 5 calibers contain no pounds.
The projectile velocity used with the mortars is about 800 f. s. The effect of these shells against ordinary masonry and earth fortifications is very great. The charge of forty-two pounds has broken through a masonry vault of three feet, four inches thick, covered with two feet, eight inches of cement, and with three to five feet of earth over all. The shell containing fifty-seven pounds, at a range of two and one-half miles, broke through a similar vault covered with ten feet of earth; but with seventeen feet of earth the vault resisted. In 1883, experiments at Kummersdorf showed that a shell containing the fifty-seven-pound charge would excavate in sand a crater sixteen feet in diameter and eight feet deep, with a capacity of twenty-two cubic yards. The Italians have had similar experiences; but it is notable that in both Germany and Italy several guns and mortars have burst. The velocity in the guns is not believed to exceed 1200 to 1300 f. s., and it is not thought that the quantity of gun-cotton is as great in the gun-shells as in the mortars. I have lately been informed on good authority that the use of gun-cotton shells has been abandoned in the German navy as too dangerous.
The French, in their investigations in this field, found gun-cotton too inconvenient, and decided upon melenite. This substance has probably attracted more attention in the military world than all others combined, on account of the fabulous qualities that have been ascribed to it. Its composition was for a long time entirely a secret; but it is now thought to consist principally of picric acid, which is formed by the action of nitric acid upon phenol or phenyllic alcohol, a constituent of coal-tar. The actual nature of melenite is not positively known, as the French government, after buying it from the inventor, Turpin, are said to have added other articles and improved it. This is probable, since French experiments in firing against a partially armored vessel, the Bellequesne, developed an enormous destructive effect, while the English, who afterwards bought it, conducted similar experiments against the Resistance and obtained no better results than with powder. The proof that the Bellequesne experiments were deemed of great value by the French lies in the fact that they immediately laid down a frigate—Dupuy de Lome—in which four-inch armor is used, not only on the side, but about the gun-stations, to protect the men; this thickness having been found sufficient to keep out melenite shell. In most armorclads the armor is very heavy about the vitals, but the guns are frequently much exposed.
The best authenticated composition for melenite consists of picric acid, gun-cotton and gum-arabic, and lately it is stated that the French have added cresilite to it. Cresilite is another product of coal-tar. Melenite is normally only three times as strong as gunpowder, but it is said to owe its destructive qualities in shells to the powerful character of the exploder which ignites it. It has been known for some years that all explosives (including gunpowder) are capable of two orders of explosion, according as they are merely ignited or excited by a weak fuze, or as they are powerfully shocked by a more vigorous excitant. Fulminate of mercury has been found most serviceable for the latter purpose. With melenite the French have reproduced all the results that the Germans have effected with gun-cotton, and have found that a shell containing 119 pounds of it will penetrate nearly ten feet of solid cement, but will not penetrate armored turrets six to eight inches thick. The French claim that melenite has an advantage over gun-cotton in not being so dangerous to handle and being insensible to shock or friction, and they have obtained a velocity of 1300 f. s. with the 8.8-inch mortar, and claim to have obtained 2000 f. s. in long guns up to 6.2-inch caliber. However this may be, they are known to have had severe accidents at the manufactory at Belfort, and at least one 5.6-inch gun was burst at the Bellequesne experiments in firing a sixty-six-pound shell containing twenty-eight pounds of melenite. The French are said to have large quantities of melenite shells in store, but they are not issued to service.
Probably one reason why we have so many conflicting yet positive accounts of great successes in Europe with torpedo-shells is because each nation wishes its neighbors to think that it is prepared for all eventualities, and they are obliged to keep on hand large quantities of some explosive, whether they have confidence in it or not. Fortunately we are not so situated, but, singularly enough, what we have done in the field of high-explosive projection has been accomplished by private enterprise, and we have attacked the problem at exactly the opposite point from which European nations have undertaken it. While they have assumed that the powder-gun with its powerful and relatively irregular pressures was a necessity, and have endeavored to modify the explosive to suit it, we have taken the
explosive as we have found it, and have adapted the gun to the explosive. At present the prominent weapon in this new field is the pneumatic gun, but it is obvious that steam, carbonic acid gas, ammonia, or any other moderate and regulatable pressure can be used as well as compressed air; it is merely a question of mechanical convenience. In throwing small quantities of certain high explosives, powder-guns can be used satisfactorily, but when large quantities are required, the mechanical system of guns possesses numerous advantages. All the high explosives are subject to premature detonation by shock; each of them is supposed to have its own peculiar shock to which it is sensitive, but what this shock may be is at present unknown. We do know, however, that premature explosions in guns are more liable to occur when the charge in the shell is large than when it is small; this is due to the fact that when the gun is fired, the inertia of the charge in the shell is overcome by a pressure proportional to the mass and acceleration, which pressure is communicated to the shell-charge by the rear surface of the cavity, and the pressure per unit of mass will vary inversely as this surface. If then the quantity of explosive in the shell forms a large proportion of the total weight of the shell, we approach in powder-guns a condition of shock to it which is always dangerous and frequently fatal. The pressure behind the projectile varies from twelve to fifteen tons per square inch, but it is liable to rise to seventeen and eighteen tons, and in the present state of the manufacture of gunpowder we cannot in ordinary guns regulate it nearer than that. It is not a matter of so much importance so far as the guns are concerned, when using ordinary projectiles, as the gun will endure a pressure of from twenty-five to thirty tons per square inch; but with high explosives in the shell it is a vitally serious matter. From all I can learn regarding European practice, it appears that not only are the explosives made sluggish, but the quantity seldom exceeds thirty per cent of the weight of the shell, and the velocities, notwithstanding, are kept very low. In the pneumatic gun the velocity is low also, but so is the pressure in the gun. The pressure in the firing reservoir is kept at the relatively low figure of 1000 pounds per square inch or less, and the air is admitted to the chamber of the gun by a balance-valve which cuts off just the quantity of air (within a very few pounds) that is required to make the shot. The gun is long, and advantage is taken of the expansion of the air. In no case can the pressure rise in the gun beyond that in the reservoir.
Up to the present time there have been no accidents in using the most powerful explosives in their natural state, and in quantities over fifty per cent of the weight of the projectile. I have seen projectiles weighing 950 pounds, and containing 500 pounds of explosives (300 pounds of the blasting gelatine and 200 pounds of No. 1 dynamite) thrown nearly a mile and exploded after disappearing under water. According to Gen. Abbot's formula, such a projectile would have sunk any armor-clad floating within forty-seven feet of where it struck. Apparently there is no limit to the percentage of explosive that can be placed in the shell, except the mechanical one of having the walls thick enough to prevent being crushed by the shock of discharge. In the large projectiles a transverse diaphragm is introduced to strengthen the walls and to subdivide the charge.
The development of the pneumatic gun has been attended with some other important discoveries which may be of interest. It is well known that mortar fire is very inaccurate, except at fixed long distances, in consequence of the high angle, the slowness of flight of the projectile, the variability of the powder pressure, and the inability to change the elevation and the charge of powder rapidly. In the pneumatic gun, the complete control of the pressure remedies the most important of the mortar's defects, and makes the fire accurate from long ranges down to within a few yards of the gun. It is obvious that the pressure can be usefully controlled in two ways: (1) by keeping the elevation of the gun fixed, and using a valve that can be set to cut off any quantity of air according to the range desired; (2) by keeping the pressure in the reservoir constant, and using a valve which will cut off the same quantity of air every time, changing the elevation of the gun according to the distance. Another important discovery consists in the application of subcalibered projectiles for obtaining increased range. The gun is smoothbored, and a full-sized projectile is a cylinder with hemispherical ends, to the rear of which is attached a shaft having metal vanes placed at an angle, which cause the projectile to revolve round its longer axis during flight. A sub-calibered projectile, however, being of less diameter than the bore of the gun, has the vanes on its exterior, and is held in the axis of the gun by means of gas checks which drop off as the projectile leaves the muzzle. The shock to the explosive is, of course, greater than in the full-sized projectile, but the increase can be calculated, and so far a dangerous limit has not been reached. From the fifteen-inch gun, with a pressure of 1000 pounds per square inch, and a velocity of about 800 f. s., a range of 4000 yards has been obtained at an elevation of 30° 20, with a ten-inch sub-calibered projectile, about eight calibers long and weighing 500 pounds. This projectile will contain 220 pounds of blasting gelatine. With improved full-sized projectiles weighing 1000 pounds, a range of 2500 yards will doubtless be obtained. At elevations below 15° these long projectiles are liable to ricochet, and what is now wanted is a projectile which will stay under water at all angles of fall, and will run parallel to the surface like a locomotive torpedo. Such a projectile has yet to be invented; but I have seen a linked shell which has been experimented with from a nine-inch powder-gun that partially meets this condition. It is made of several sections, united by means of rope or electric wire in lengths of 100 or 150 feet. When fired, all sections remain together for some distance; the rear section then first begins to separate; then the next, and so on. It is primarily intended to envelop an enemy's vessel, and to remedy the present uncertainty of elevation in a gun mounted in a pitching boat; but it is found that when it strikes the water in its lengthened-out condition, it will neither dive nor ricochet, but will continue for some distance just under the surface until all momentum is lost, when it will sink. This projectile is at present crude, and has never been tried loaded, but it will probably be developed into something useful in time.
I have confined my remarks in the foregoing discussion principally to such methods of using high explosives in shells as have proved themselves successful beyond an experimental degree, and practically they reduce themselves to two, viz: using a sluggish explosive in small quantities from an ordinary powder-gun, and using any explosive from a pneumatic or other mechanical gun. Naturally, the success of the latter method will soon induce the manufacture of powders having an abnormally low maximum pressure. There is undoubtedly a field for the use of such powders in connection with an air-space in the gun to still further regulate the pressure; but nothing of this sort has yet been attempted. Many methods of padding the shell have been devised for reducing the shock in powder-guns, but the variability of the powder-pressure is too great to have yet rendered any such method successful. A method was patented by Gruson, in Germany, of filling a shell with the two harmless constituents of an explosive, and having them unite and explode by means of a fulminate fuze on striking an object. He used for the constituents nitric acid and dinitro-benzine, and was quite successful; but the system has not met with favor on account of the inconvenience. The explosive was about four times as powerful as gunpowder.
That the advantage of using the most powerful explosives is a real one can be easily shown. The eight-inch pneumatic gun in New York harbor, with a projectile containing fifty pounds of blasting gelatine and five pounds of dynamite, easily sunk a schooner at 1864 yards range, from the torpedo effect of the shell falling alongside of it. This same shell, if filled with gunpowder, would have contained but twenty-five pounds, and have had but one-ninth the power.
The principal European nations are now building armored turrets sunk in enormous masses of cement, as a result of their experiences with gun-cotton and melenite. The fifteen-inch pneumatic projectile, which I described as being capable of sinking an armor-clad at forty-seven feet from where it struck, would have been capable of penetrating fifty feet of cement had it struck upon a fortification. It was not only a much larger quantity of high explosive than Europeans have experimented with, but the explosive itself is probably more than twice as strong as their gun-cotton, and five or six times as Strong as their melenite. In the plans of Gen. Brialmont, one of the most eminent of European engineers, he allows in his fortifications about ten feet of cement over casements, magazines, etc. It is evident that this is insufficient for dynamite shells such as I have described.
At Fort Wagner, a sand work built during our war, Gen. Gilmore estimated that he threw one pound of metal for every 3.27 pounds of sand removed. He fired over 122,230 pounds of metal, and one night's work would have repaired the damage. The new fifteen-inch pneumatic shell will contain 600 pounds of blasting gelatine, and judging from the German experiments at Kummersdorf, which I have cited, one of these fifteen-inch shells would throw out a prodigious quantity of sand; either 500 pounds to one of shell, or 2000 pounds to one of shell, according as the estimate of Gen. Abbot or of Capt. Zalinski is used. The former considers that the radius of destructive effect increases as the square root of the charge; the latter, that the area of destructive effect for this kind of work is directly proportional to the charge.
The effect of the high explosives upon horizontal armor is very great, but we have yet to learn how to make it shatter vertical armor. No fact about high explosives is more curious than this, and there is no theory to account for it satisfactorily. As previously stated, the French have found that four inches of vertical armor is ample to keep out the largest melenite shells, and experiments at Annapolis, in 1884, showed that masses of dynamite No. 1, weighing from 75 to 100 pounds, could be detonated with impunity when hung against a vertical target composed of a dozen one-inch iron plates bolted together.
In conclusion, I may say that in this country we are prone to think that the perfection of the methods of throwing high explosives in shell is vastly in favor of an unprotected nation like ourselves, because we could easily make it very uncomfortable for any vessels that might attempt to bombard our sea-coast cities. This is true as far as it goes, but unfortunately the use of high explosives will not stop there. I lately had explained to me the details of a system which is certainly not impossible for damaging New York from the sea by means of dynamite balloons. The inventor simply proposed to take advantage of the sea-breeze which blows toward New York every summer's afternoon and evening. Without ever coming in sight of land, he could locate his vessel in such a position that his balloons would float directly over the city and let fall a ton or two of dynamite by means of clock-work attachment.
The inventor had all the minor details very plausibly worked out, such as locating by means of pilot balloons the air-currents at the proper height for the large balloons, automatic arrangements for keeping the balloon at the proper height after it was let go from the vessel, and so on. His scheme is nothing but the idea of the drifting or current torpedo, which was so popular during our war, transferred to the upper air. An automatic flying-machine would be one step farther than this inventor's idea, and would be an exact parallel in the air to the much dreaded locomotive water-torpedo of to-day. There seems to be no limit to the possibilities of high explosives when intelligently applied to the warfare of the future, and the advantage will always be on the side of the nation that is best prepared to use them.