No. VIII.
In the first number of these Notes attention was called to the growing importance to the student of explosives of the new study of thermo-chemistry, and frequent references have since then added force to this suggestion; but the elaborate work of M. Berthelot, now before us, bearing the title of "Sur la Force des Matieres Explosives d'apres la Thermochimie," shows most strongly the necessity of conducting our inquiries into the constitution and mode of action of explosive substances from this standpoint, if we wish to arrive at any rational conclusions and to discover the general underlying principles. This work is in two large octavo volumes, of over 400 pages each, and bears the imprint of Gauthier-Villars, Paris, 1883, which is a guarantee of excellent typographical form and workmanship. The work is announced as a "third edition, revised and considerably augmented," but one would be at a loss to recognize the thin little duodecimo entitled "Sur la Force de la Poudre, 1872," as the second edition of this work.
Although in civil life, yet Berthelot has had unparalleled opportunities for the study of explosive substances; for, in addition to having served as president of the Scientific Committee for the defence of Paris in 1870, and having been delegated to fill the place formerly occupied by Gay Lussac upon the Advisory Committee on Powders and Saltpetre, he is President of the Committee on Explosive Substances, created June 14, 1878, where he has associated with him some sixteen specialists from the Army, Navy, and the corps of Mining Engineers and Engineers of Powders and Saltpetre (among whom we note Sarrau, Sebert, Cornu, Vieille, Desortiaux and others), and in this position he has the assistance of a considerable laboratory force and all the resources of a highly centralized government at his command. It is then not surprising that, by April 5, 1883, his committee had completed twenty distinct researches, and had twenty three others in course of execution.
The work now under consideration is based principally upon these researches, together with those of Berthelot and others in pure chemistry. The first part deals with the general properties of explosive substances, their chemical composition, the heat disengaged in their explosion, and the pressure of the gases produced; the duration of the explosive reaction; explosions by influence, and the explosive wave. The second part treats especially of the thermo chemistry of explosive compounds, a chapter being devoted to the much-vexed question of the origin of the nitrates; and the third part considers the force of the explosive bodies. The work is replete with descriptions of the apparatus used, the methods pursued and the data obtained in these researches; but, as we have already noticed, in these Notes, many of the results as they have appeared in the professional journals, we will simply present the following tabular summary.
In citing these results of Berthelot's work, it may be proper to call attention to a recent review by Prof. Remsen, Am. Chem. Jour. 6, 423, Feb. 1885, of "A Treatise on the Principles of Chemistry," by M. M. Pattison Muir. He says: "Chapter IV is devoted to the Applications of physical methods to questions of chemical statics. Of the physical methods the first considered are the thermal methods. The subject is treated clearly; indeed, it would be difficult to find anywhere a more satisfactory statement in regard to the method and results of thermo-chemistry than we here have included within fifty odd pages. After showing that the thermal value of even the simplest chemical change is really the sum of various changes, some of which have a positive and others a negative value, and that in most, if not in all cases, it is impossible to assign values to each of the simple changes involved, the conclusion is drawn, that until 'the distinction' implied in the terms atom and molecule 'is practically recognized in thermal chemistry, we cannot expect any great advances to be made in applying the mass of data already accumulated to questions of chemical actions and reactions.'
‘Berthelot's work is saturated with the conceptions of the molecular theory; but, by some fatal perverseness, he refuses to apply this theory to chemical phenomena. While recognizing the existence of molecules, and building his generalizations on a molecular foundation, he refuses to accept the conception of atom, or rather he hopelessly confuses it with that of equivalent. The molecule is for him a definite and definable portion of matter, the parts of the molecules are only numbers.'
"This criticism may sound harsh, but it appears to be just. At all events we shall be glad to hear what the disciples of Berthelot may have to say in reply. We commend this part of the book to the careful attention of those who have blindly 'pinned their faith' to investigations in the field of thermo-chemistry. While something has been learned, and much more will be learned, in regard to the nature of chemical action by a study of thermal phenomena, it must be acknowledged that the results thus far reached are extremely meagre." In this connection consult the same Journal 5, 147 and 293, also 6, 202.
Through the courtesy of Manuel Eissler we have received a copy of his "Modern High Explosives," in the form of a large octavo of 495 pages, published by John Wiley & Sons, 1884. The book is arranged in three parts; the first of which describes the processes employed in the manufacture of high explosives, and the methods of analysis and chemical properties of these bodies and their constituents; the second treats of the methods for operating with these explosives; while the third deals with the principles of blasting, and gives the results obtained in many engineering, mining and military operations. Mr. Eissler designed, erected and managed the American Forcite Company's works, and has had considerable experience in the use of high explosives, and hence he has been able to gather together and present in the last two parts of his book a large amount of information and data which will be of value and use to the practical man in determining the amount of an explosive necessary to do a given work and the best way of applying it. The first part, on the other hand, is quite unsatisfactory, as it is so marred by errors in chemical terminology and technical phrases as to be quite bewildering.
One of the most novel and interesting chapters in the book is the one on "Big Blasts," where, among other instances, it is stated that it sometimes happens in the system of hydraulic mining in vogue in California, that banks of gravel are met with which are so firmly cemented together as not to yield readily to the action of the water, while other banks are so high that it becomes dangerous to bring the stream sufficiently close for the water to exert its disintegrating force. Under these circumstances recourse is had to what is termed "bank-blasting," in order to loosen the earth so that it will break down under the pressure of the water, and enormous quantities of explosives are used for this purpose. Mr. Eissler gives a detailed description of the method of laying these charges, and he says that in the diggings at Smartsville, San Juan, More's Flat, Bloomfield, and elsewhere in California, it is an almost daily occurrence to fire blasts where twenty, thirty, and even fifty thousand pounds of explosives are used in a single charge, and that this system of large blasts is even become common in hard rock excavations, such as quarries and railroad cuttings.
We learn that a second volume is in course of preparation, and from the scope of the projected work it promises to be of great value.
A circular from the Judson Powder Company gives the following instances of "big blasts," in which this powder has been used:
June 29th, 1882, 23,000 pounds were fired in a limestone quarry at Glendon, Pa., and it is estimated that it moved 150,000 tons of rock. January 11, 1883, 29,000 pounds of frozen powder were fired at the same place with good results. At Shell Rock, on the Oregon R. and N. Co. R. R., 10,000 pounds of powder, in one charge, moved 56,000 yards of solid rock. At Jacob's Ladder, on the same road, 21,000 pounds of powder moved 110,000 yards of solid rock.
Through the kindness of Gen. Abbot we are in receipt of Addendum II of his Report on Submarine Mines, in which the results of recent experiment on Atlas powder, Judson powder, Rackarock, Forcite, and explosive or blasting gelatine, are given.
Two grades of Atlasf powder, A and B, were used, of which the composition was:
Grade A.—Sodium nitrate 2
Wood fibre 21
Magnesium carbonate 2
Nitroglycerine 75
100
Grade B.—Sodium nitrate 34
Wood fibre 14
Magnesium carbonate 2
Nitroglycerine 50
100
The firing trial showed Atlas A to be precisely equivalent to dynamite No. 1, when fired under water; while Atlas B, under the same conditions, showed an intensity of action in the horizontal plane equal to 99 per cent, of dynamite No. 1, for equal weights.
Several grades of Judson powder| were tested. The one most commonly used, and which is sold at the price of common saltpeter blasting powder, is the R.R.P., which has the composition:
Sodium nitrate 64
Sulphur 16
Cannel coal 15
Nitroglycerine 5
100
It is manufactured at the New Jersey works by first grinding the niter, sulphur and coal separately, to a fine powder, and then thoroughly incorporating them in a barrel mixer to form a coarse mealed powder. This is then heated on a steam pan (150 pounds pressure and 350° F.), and constantly stirred until the sulphur melts and coats the particles of the niter and coal. The mass is then thrown out and allowed to cool, and it forms conglomerate grains, which are sorted by sieves and coated with nitroglycerine by stirring. The theory of this manufacture is the same as that claimed by Mr. Mowbray for his mica powder, viz.: the coating of a non-absorbent base with nitroglycerine, by which the quickness of explosion is promoted. The prime difference between the powders lies in the fact that Judson employs an explosive base, while Mowbray's is inert. The advantage sought for here is to obtain the maximum potential energy of the explosive base through the initial action of the small charge of nitroglycerine. The intensity of action of the R.R.P. of different makes, fired under water, was found to equal 38 to 39 per cent, of dynamite No. 1, while some of the other grades rose as high as 78 per cent. The lower grade seemed to be insensitive to the shock of the ball from a Springfield rifle at twenty paces, and when ignited by a match, in charges of 1 ½ pounds, it burned for about forty-five seconds with a strong yellow flame, but with no such flashing as occurs with ordinary gunpowder.
In addition to the rackarock mentioned in these Notes, Mr. Divine, the inventor, has patented several other formulas in which the potassium chlorate is finely powdered and mixed with nitro-benzol and sulphur, or with "dead oil" of tar and sulphur, or with "dead oil" of tar, sulphur and carbon disulphide in varying proportions. Several of these mixtures were tested and they gave results varying from 77 to 104 per cent, as compared with dynamite. Gen. Abbot seems to be favorably impressed with this explosive and recommends its trial in blasting and for use in shells.
The forcite gave results which varied, with the grade, between 88 and 133 per cent., and hence it compared favorably with Nobel's explosive gelatine. It was found that the higher grade was the more liable to sympathetic explosions, but that both those tested were much less sensitive than dynamite No. 1 under similar conditions. When the forcite-gelatine was exposed for several hours to a temperature of 100° F., a slight exudation of nitroglycerine was observed. The manufacturers state that the base for all grades of forcite is mixed with a special kind of "cellulose" whose formula is C6H7O2 , which permits them to operate and manufacture cold. As they state that in manufacturing Nobel's gelatine a temperature of 170° F. is necessary for incorporation, and that this temperature renders the nitroglycerine very sensitive to concussion and quite dangerous to transport, they claim the cold process as an advantage. A hydrocarbon is used as a solvent for the forcite mixture, and its action renders the product water-proof.
The blasting gelatine used in these experiments came from Nobel's Explosive Co. of Glasgow, Scotland. It was without camphor, but directions for easily camphorating it are given. The composition was 92 per cent, nitroglycerine and 8 per cent, nitro-cotton. The test gave the relative intensity as 142, showing this to be the most powerful explosive yet tested at the station. Its sensitiveness to sympathetic explosion was the same as that of the higher grade of forcite.
Gen. Abbot gives a tabular view and also a graphical diagram of relative intensity of the modern high explosives, as shown by the results obtained in his experiments, and he draws the following conclusions:
- The assumption sometimes made that the economic value of a dynamite is simply proportional to the known percentage of nitroglycerine it contains is shown to be erroneous. A judiciously selected base adds enormously to the energy developed by the nitroglycerine alone.
- There appears to be an advantage in gelatinizing the nitroglycerine before its absorption.
- The composition of the base is practically as well as theoretically a very important matter. For example, the lower grades of forcite and rendrock, which are very similar in composition, show nearly equal intensity; and the same is true of dynamite No. 2 and Vulcan No. 2; but both grades of electric powder fall far behind these rivals.
- It seems to be a general law that with any particular kind of base there is an economic gain in increasing the percentage of nitroglycerine up to a certain point, but that beyond that point the advantage ceases.
The instructors in the Department of Ordnance and Gunnery, U.S. Naval Academy, have prepared a work for use in instruction in that department entitled "Textbook of Ordnance and Gunnery," 1884, in which the most modern advancements in the construction and use of ordnance and ammunition are described by the aid of many excellent plates and cuts, while the latest data are discussed according to the most approved methods. The progress in the art of gunnery is so rapid that the best textbooks and treatises soon become imperfect, and it is for the purpose of bringing the instruction into accordance with modern views and methods that this book has been prepared. A new powder known as the Cocoa powder, and made by the Rottweil- Hamburg Powder Co. at their works at Duneberg, near Hamburg, has been exciting some attention abroad. It is formed into hexagonal prisms, with one canal, and is of the color of cocoa, whence the name. Two pamphlets issued by the firm, dated 1882 and 1883, and entitled "Trials executed with Prismatic Powder," have come to hand, from which we extract the following:
Powder in prisms of 50 mm. was tested in a 21 centimeter gun of 30 calibers length against ordinary prismatic powder. The Cocoa powder is designated as C/82, the ordinary prismatic as C/75.
While a charge of 40 kilos of the old pattern may be deemed too great, on account of the high pressure of gas it produces, a charge of the powder C/82 may safely be increased to 48 kilos; the pressure thereby produced, reached according to the Rodman gauge but 2615 atmospheres, according to the crusher gauge but 2570 atmospheres, the muzzle-velocity was 537 metres.
Other properties claimed for the new powder are comparatively little smoke and a slow rate of burning when unconfined. It was especially noticed at the trial that C/82 gave less and thinner smoke than C/75, which is of the greatest importance, as great quantities of dense smoke dispersing slowly may stop the firing, as was recently the case during the bombardment of Alexandria.
A grain of C/82 powder when ignited in air did not explode like C/75 and C/68 powder, but burned slowly, showing a red flame. A closed box containing 55 kg. of C/82 powder was ignited. The powder burned out in about 10 seconds without any detonation (?) whatever. It was found that the screwed lid had been loosened, without however being thrown off the box, and that with the exception of a few slight burnings the lid presented the same appearance as before the experiment. The box also escaped without any damage. This property of burning without explosion is of value in the use of powder. Another feature in the powder is that it attracts moisture less than powder of the old pattern. Experiments made with the 40 centimeter gun of 25 caliber length gave as good results as those above quoted. These experiments were made in 1882.
The pamphlet for 1883 contains data from tests with guns of various sizes. Among them we observe, that in the 28 cm. gun of 35 calibers length, with a projectile weighing 345 kg., 100 kg. of C/82 powder gave a muzzle velocity of 525 metres and a pressure by the Rodman gauge of 2325 atmospheres, and by the crusher gauge 2350 atmospheres. Both pamphlets contain tabulated results of many trials. It has been tried in Russia in small arms, and in France in large grains, and is reported to have given good results. A resume of some of the firing trials will be found in the Revue d'Artill. 21, 77 and 475, 1885.
In the Jour. Roy. United Service Inst. 28, 379, 1884, under the subject of "Gunpowder considered as the Spirit of Artillery," Col. Brackenbury, R. A., says that two firms in Germany are making the above mentioned powder, one calling it Cocoa and the other Brown Powder. The proportion of sulphur in its composition is small, and the charcoal, if we may so call it, is different from that generally made. When first brought forward it was irregular in its action, but later samples have given very good results, about the same as Waltham Abbey C-2, and with a less amount of powder. They are prepared to make it at Waltham Abbey if its value is established, but some claimed that its erosive action is too great.
Considerable space is devoted to the subject of blending powders, and cuts are given illustrating the method followed. In this connection he says an idea has lately been set afloat that this process can be got rid of and powder made so regular that it will need no blending, but up to the present time there appears no prospect of any such consummation. Gunpowder is such a nervous and sensitive spirit, that in almost every process of manufacture it changes under our hands as the weather changes. Sometimes its sensibility can be detected and allowed for, as in the process of pressing it into moulds, when we can by actual trial tell what densities we are getting, and give more or less pressure as is required. For instance, on the morning of the 13th June, 1882, the pressure had to be applied for 45 seconds to obtain the required density. Later in the day only 29 seconds were required to obtain the same density, so that in the morning of a June day half as much again time was required as in the afternoon. On the 30th June, 1882, during part of the day the time was as short as 26 seconds; on the 11th December the time varied between 98 and 84 seconds to get the same density as was obtained in June. In the other stages of manufacture we have no such indications as in the pressing process, but it is a fact that not only the warmth of summer and the cold of winter affect it greatly, but the morning mists, the sunshine of midday, the dews of evening, yea, even a passing cloud, tell upon its nervous temperament. As a mitigation of the weather difficulty, they are to try warming a set of houses with warm water.
Under the title "A Flashing Test for Gunpowder," Chas. E. Munroe discusses in the Jour. Am. Chem. Soc'y, 6, 7, 1884, the merits and defects of the method in use as described in the "Ordnance Instructions U. S. Navy," p. 345, and Smith's "Handbook of the Manufacture and Proof of Gunpowder," p. 83, and the suggested improvement of Marvin in his "Objects and Resources of the Naval Experimental Battery," p. 18, and also the pyrographic method invented by Colonel Chabrier, and described in the Comptes Re7idus, 78, 1138, 1874, and the Revue d'Arillerie, 4, 396, 1874. He then describes his own process, which consists in flashing the powder on a sheet of dampened paper which has been coated with Turnbull's blue—in fact, such blue paper as is produced in the "Blue Print Process" of photography. The result will be that if the paper is washed, after a half minute exposure to the action of the powder residue, it will be found covered with yellowish or white spots, that have been caused by the action of the alkaline salts which result from the combustion, and which have the power to discharge the blue color; and the author holds that the fineness of the spots and the uniformity of their arrangement about the explosive center are determined by the thoroughness of the incorporation of the powder.
The advantage which is claimed for this new method is, that as the test papers, when dried, remain unchanged for years, they may be filed at the factory with other data concerning a given powder, or that, in the case of the government, they could be enclosed with the quarterly returns of the inspecting officers at distant stations, to be examined by some expert in the Bureau. Specimens of the tests of standard powders could also be furnished inspecting officers, to guide them in the interpretation of the results of their tests, and finally, a sample of the required test might be attached to the specifications for a gunpowder to be purchased. A detailed account of the method can also be found in the Text-book of Ordnance and Gunnery, U.S.N.A., p. 93, 1884, and Van Nostrand's Eng. Mag. 32, 427, May '85.
We are in receipt from Luckhardt & Alten, Cassel, Germany, of a thin pamphlet, entitled "A Few Words on the Present State of the Manufacture of Gunpowder," in which it is claimed that the German gunpowders are now superior to those of all other countries, and are largely purchased by foreign governments, and that the superiority is due solely to the severity of the tests applied to the materials and the manufactured product, and to the delicacy and accuracy of the instruments employed in making these tests. The pamphlet then describes, with illustrations, a considerable assortment of apparatus which this firm supplies, and among them, besides the densimeters, chronographs, pressure gauges, and the like, we especially note an apparatus for measuring the length of cylinders in crusher gauges, which it is claimed will measure a difference of 1/40 of a millimeter (1/1000 of an inch), another for measuring the length of cut in the Rodman method, which will read to the 1/100 millimeter (1/2500 inch), and another still for measuring the thickness of wire and width of mesh in metallic powder sieves, which also reads to the 1/100 millimeter.
The newspapers for some time past have contained notices of the dynamite gun now on trial at Fort Hamilton. One of the fullest descriptions, with an illustration, is to be found in the Scientific American 50, 214, April 5, 1884. This represents the 4-inch gun building at the Delamater Iron Works, N. Y. It consists of a brass tube, 40 feet in length and ¼ inch thick, mounted on a high steel girder. The latter is trunnioned and is pivoted on a cast-iron base, thus enabling it to be swung into any desired position and range. To assist in the latter operation guys are placed on either side of the base, and their length can be altered and fixed by means of hand wheels.
Compressed air is introduced to the gun from below and passes up through the center of the base, the pipe connecting with one of the trunnions (which are hollow); it is then carried into a pipe at the side of the gun which leads into the valve. This valve is a continuation of the breech of the gun, with which it is connected by a short passage.
An important feature of the system, and one upon which the success of the undertaking greatly depends, is the projectile, or dart. It consists essentially of two parts, and while several different modifications have been tried, the principal features are alike in all of them. The forward part of the dart consists of a thin brass tube, into which the charge of dynamite is inserted. At the rear the tube is enclosed by a wooden plug, which flares out towards the rear until its diameter equals that of the bore of the gun. The forward end of the brass tube shows a mass of some soft material, into which is inserted a pin firmly held in place, the end being closed by a conical metal cap. Provision has also been made to allow a certain amount of air to act as a cushion for the dynamite cartridge, thus lessening the shock due to a sudden discharge. It is therefore claimed, that under ordinary circumstances there is little danger of the charge exploding, since the pin cannot reach it and ignite the fulminate at its end; but when thrown from the gun, the impact against a body will displace the soft material and drive the pin home, causing an explosion. Another feature of the projectile is the power which it possesses to correct, to a certain extent, the deflection due to a side wind. It will be noted, that with the present construction, the centre of gravity of the dart is some distance forward of its centre of figure. A side wind acting upon the lighter rear part would therefore have the tendency to deflect it so as to turn the head of the dart into the wind, which action would, in a measure, tend to keep it in the line of its trajectory.
The firing of the gun, if the expression may be used, is accomplished in the following manner: The dart is inserted in the breech, and a gas check placed in position; a lever then being moved, the valve is opened and the air pressure admitted. This method of discharge will, it is thought, obviate the danger of the shock, which had heretofore proved a stumbling block to success; and in addition, the valve-controlling mechanism is automatically arranged to admit the air, gently at first to overcome the inertia of the projectile, following with full pressure, and finally closing at the proper time as the dart leaves the gun.
Experiments made thus far have shown that the apparatus can be depended upon for a fair degree of accuracy and rapidity in firing. As regards the range attainable, the two-inch gun now being tested has attained 14 miles with a pressure of 420 pounds to the square inch. In the four and six-inch gun which are in course of construction, it is intended to use pressures of 2000 pounds and over, by the use of which they hoped to attain a range of three miles. Advantages claimed for these guns are lightness and ease of manufacture.
The Washington Sunday Herald of March 17, 1884, states that the experiments with dynamite shells have proved less efficient than was anticipated, as they have no penetration, and the explosion takes place on striking. The issue of March 30 narrates that a workman at the Delamater Works tried the 4-inch gun with a piece of cotton waste. Although only a 100 pounds pressure was put on, the waste was blown through a wooden door two inches thick, making quite a large breach.
The idea of employing the expansive force of steam, or of compressed air, for propelling projectiles is not a new one, but its application has not heretofore met with success, since penetration has been sought for. Among other contributions to the subject is a paper "On the Numerical Expression of the Destructive Energy in the Explosions of Steam Boilers, and on its Comparison with the Destructive Energy of Gunpowder," by G. B. Airy, Phil. Mag. [4] 26, 329, 1863. He reaches the conclusion that the destructive energy of one cubic foot of water, at the temperature which produces the pressure of 60 pounds to the square inch, is equal to that of one pound of gunpowder.
In Richardson & Watts' "Chemical Technology," Vol. I, Part IV, page 523, London, 1865, we also find the following: "High pressure steam is exceedingly well adapted to the performance of this kind of work; unluckily it would require high pressure steam of 400 atmospheres or 5000 lbs. pressure on the round inch to perform this duty, and as such steam could only be generated in a furnace intensely heated, it is scarcely probable that boilers will be found sufficiently strong and durable to work continuously under such pressure. If they were found to be practicable, nothing more would be necessary than to bring a steam pipe from the boiler to the breech of every gun in a fortress or a ship, and the admission of the charge of such steam into the chamber by a valve would be sufficient to discharge the missile of the 68-pounder with a speed of 1600 feet a second. The well-known Mr. Perkins studied this subject carefully, but applied it somewhat differently. He found that steam of this pressure could be generated only by water nearly red-hot; and instead of throwing the steam into the breech by a pipe, he threw the red-hot water into the breech of his gun, allowing it when there to expand itself into steam and expend its force in giving speed to the ball. This expedient of Perkins is well worthy of study. It has both the defects and advantages of a gunpowder gun. The red-hot water thrown into the barrel would have the fault of being too powerful at the beginning of its expansion and too weak at the end. The barrel would be filled partly with water and partly with steam; and as the water grew into steam it would lower its temperature and its pressure, so that the explosive force would fall off very much towards the end of the stroke. This is the inevitable evil of allowing the water to become vapor in the gun. When the steam is generated in a separate boiler, and freely admitted into the breech of the gun, there is reservoir enough of heat and steam to maintain the even pressure in following up the ball from the breech to the muzzle. It is the evil of charges converted into gas within the breech of the gun, that their temperature and pressure are too high at starting and too low at the end. The steam-gun would in this respect be the best of our projectile forces.
"Compressed air has many of the advantages and some of the defects of steam; and the frequent use of the air-gun has shown its convenience as well as its efficiency. Air can be compressed into a reservoir by mechanical force, just as steam can be raised in a boiler by heat; and by compressing 400 times the natural quantity of air into a given space, a pressure of 400 atmospheres might possibly be obtained in this way. If an air pipe communicated from this reservoir to the breech of our gun, air of 400 atmospheres of pressure would certainly be able to follow up the 68-pounder shot, with pressure and velocity able to discharge it with a speed of 1600 feet per second, and, therefore, to do our work; but the apparatus would be full of mechanical difficulties.
"Liquid gases are known to be receptacles of enormous mechanical power. Carbonic acid gas, liquefied and shut up in a reservoir, generates large volumes of gas with great rapidity the moment it is permitted to expand. Other gases expand with still greater rapidity and force; and if we could conceive liquid gases to be easily made, safely carried, and comfortably handled, a charge of liquid gas bottled up in the breech of a gun would be a very effectual propelling power, and quite able to generate the force we want, and to apply it within the time we require. This system, however, is also beset with mechanical difficulties.
"The preceding illustrations of steam, compressed air, and liquid gases lead us on very instructively to the manner in which fire has become necessary to do the work of a gun. A supply of heat is essential to the expansion of a gas, and a rapid supply is indispensable to the rapid performance of the work. In steam, the fire is not only external to the gun, but external to the boiler in which the steam is generated. In gunpowder, the fire is introduced into the inside of the gun, for the purpose of supplying the heat that is wanted to raise the gases to their elastic pressure, and to maintain them at that pressure while expanding. Red-hot steam introduced into the breech of a gun rapidly cools down and loses its heat and power in expanding. If we could introduce fire into the breech of the gun at the same time, to maintain the heat of the steam and the water, the steam would become an admirable propelling force. Carbonic acid gas expanding rapidly from the liquid into the gaseous state cools down so suddenly as not only to lose its mechanical power, but to freeze into solid flakes of snow. If we could charge the breech of the gun with fire as well as with liquid gas, the fire would give it the heat it wants, prevent its congelation and maintain its power to the end of the discharge. What gunpowder and gun-cotton do is really to provide a reservoir of gas and a fire to heat it simultaneously and in the same chamber. In the case of gunpowder the fire is fed with charcoal, in the case of gun-cotton the fire is fed with gun-cotton wool—another form of carbon. In gunpowder, large quantities of carbonic acid gas are generated, possibly in the liquid state, and are heated by the internal furnace of the charge, possibly red-hot. In like manner in a gun-cotton charge, red-hot water or steam is introduced with other gases, possibly also liquids, together with an internal furnace of flame; and thus the work is done—first, by the release of the gases themselves, and, secondly, by the continuance of the elasticity of those gases by the internal supply of heat. This is how gunpowder and gun-cotton really do the work of a steam-gun, a carbonic acid gun, or any other kind of gas gun."
Prof R. H. Thurston treats of the same subject in the Jour. Frank. Inst. 88, 427, December 1884, under the title "Steam Boilers as Magazines of Explosive Energy," and after referring to the above mentioned paper of Airy's, and that of Rankine "On the Expansive Energy of Heated Water," Phil. Mag: [4] 26, 1863, he has computed, by the aid of the formulas of Rankine and Clausius, the magnitude of the quantities of energy residing in available form in both steam and water, for the whole usual range of temperatures and pressures familiar to the engineer, and also for those beyond, which have only been attained experimentally, but which are likely to be reached in the course of time, and he has plotted curves of the available energy of heated water, of latent heat and of steam, and the explosive energy of many different forms of boilers.
The dynamite air-gun is also described with illustrations, and discussed by Chas. E. Munroe in Van Nostrand's Eng. Mag, 32, 1, January, 1885, in a paper entitled "Some Recent Experiments on the Use of High Explosives for War Purposes."
In determining the efficiency of the air-gun projectile against an armor-clad vessel, the author assumes that one of four effects may be produced, depending on the resistance of the armor to penetration, and on the material, thickness of wall, profile, weight and velocity of the projectile.
(1). The projectile may either penetrate the armor partially and explode in place, or pierce it completely and burst inside of the ship. This is the condition of greatest efficiency.
(2). It may explode immediately upon impact, and before breaking up. Then the explosive will exert the energy which it develops through explosion in a resisting receptacle.
(3). It may rebound before exploding. Then the effect will be reduced by the interposed cushion of air.
(4). It may break up on impact before the explosion takes place. Then the energy of the explosive will be simply that which it develops when exploded unconfined.
The resistance of an armor to penetration depends upon its hardness, its tensile strength (that due to bolting as well as that inherent in the metal itself), and its inertia. The latter is augmented by the thickness and weight of the armor, and by the rigid system of bracing which now obtains in practice. How great this resistance is can best be illustrated by an example. While, from the fact that very rapid progress is being made in the improvement of armor plates, we may have not chosen the best example, let us take the steel plates designed for the Furieux. One of these, weighing 23 tons, 9 cwt., and averaging over 17 inches in thickness, was tested at Gavre, July 13, 1883. Three shots were fired against this plate from a 12.6-inch rifle using chilled iron projectiles, weighing 759 pounds each. The first and second shot struck with a velocity of 1403 feet each. The third struck with a velocity of 1438 feet. The projectiles were all broken up, all of the twenty bolts through the plate remained intact, and no portion of the plate fell from the backing, although it was somewhat indented and cracked.
Although we are not yet informed concerning the air-gun projectile, except for the weight given above and the pressure of the air also cited, yet when we remember that in the Gavre experiments the pressure of the powder gas probably approached 40,000 pounds to the square inch, it is not unfair to infer that with a pressure of 500 pounds to the square inch a projectile will possess little or no penetrative power against the Furieux plates at a distance of 1 ¼ mile. Whether then the projectile would explode on impact or after rebounding, or whether it would break up before exploding, is a matter for speculation and conjecture. If the last condition prevails, then we can judge from some experiments recently made at the Naval Experimental Battery, what the destructive effect would probably be.
In these experiments Commander Folger detonated upwards of 400 pounds of dynamite, in charges of from 5 to 100 pounds, against a wrought-iron target, eleven inches thick, without damaging the plates. These charges were enclosed in cartridge bags and suspended against the plates. Commander Folger concludes from these experiments "that it is a matter which hardly admits of doubt, that a modern armor-clad will not be materially injured by the explosion, in superficial contact with her over-water plating, of charges of more than 100 pounds of dynamite."