Popular Science Monthly/Volume 6/April 1875/Manufacture and Conveyance of Gunpowder
|MANUFACTURE AND CONVEYANCE OF GUNPOWDER.|
A LITTLE before five o'clock on the morning of the 2d of last October, a train of four barges was being towed by a steamer along the Regent's Canal, in the northwestern distract of London. The second of these barges was laden with a miscellaneous cargo, packed in such a manner, and containing such elements, that the barge was really a very efficient kind of torpedo. In her hold there were about five tons of gunpowder and a quantity of benzoline in kegs. This benzoline may be described as a very volatile species of petroleum. At ordinary temperatures it gives off a highly-inflammable vapor, and this, when mingled with the air in certain proportions, becomes explosive—the explosion running through it at the rate of about two feet per second when it is confined in a tube. In the case of the barge on the Regent's Canal, the cargo was closely covered with a tarpaulin, to protect it from the weather. From the moment, then, that this covering was put on by the bargemen, the vapor given off by the benzoline began to accumulate in the hold, and mingle with the air confined in the spaces between the various packages of the cargo. Thus the hold gradually became filled with a fiery explosive atmosphere, and all that was wanted to produce an explosion was contact with flame. In the little cabin, at the stem of the barge a fire was burning, and there was an aperture in the bulkhead, or partition, which divided the cabin from the hold. Through this the benzoline vapor entered the cabin, and the air in it was soon as vitiated as that under the tarpaulin in the hold. It was ignited by the fire; the explosion, beginning in the cabin, ran forward in a few seconds to the bow, and fired the gunpowder stowed there.
Every one knows what followed. Half London was awakened by the report, which was heard for miles around—to the northward as far as Finchley and Enfield, to the southward as far as Blackheath and Woolwich. Within a radius of from half a mile to a mile from the scene of the explosion houses were wrecked, windows blown in, doors burst open, ceilings shaken down, ornaments and furniture dashed to pieces. A massive bridge over the canal was destroyed, for hundreds of yards its embankment was displaced, and the house which stood nearest to it was so shaken that it had to be pulled down next day. The effect was more like that of a severe shock of an earthquake than any thing else. Fortunately no lives were lost except those of the crew of the barge, but the destruction of valuable property was enormous.
Much alarm has been caused not only in London but throughout the kingdom by this explosion in the heart of the metropolis, and it will have a useful effect in calling attention to the dangerous character of a material so largely employed as gunpowder, and the consequent necessity of carefully regulating its manufacture, storage, and transport, and seeing that these regulations are strictly enforced; for, no matter how perfect our precautions may be in theory, they are worse than useless if we cannot secure their practical efficiency. Without this, their only result must be to lull us into a false security. Gunpowder, and its manufacture and transport, are now subjects in which nearly every one is interested; and we purpose to devote the following pages to an account of the nature and action of this explosive, its manufacture, and the principles involved in it, and, finally, its transport, and the precautions necessary for our security against explosions like that of last October. We shall describe the process of manufacture in use at the Government mills, as these are probably the most perfect and efficient in the kingdom.
Fifteen miles to the northeast of London, between the sluggish stream of the River Lea and the northern heights of Epping Forest, stands the little village of Waltham, famous for its old abbey, founded by the last Saxon King of England, and destined to be his tomb after the fatal field of Hastings. On both sides of the high-road beyond the village extends a wide tract of flat alluvial ground, traversed by the branches of the Lea, and rich in plantations of willow and alder, with here and there stately rows of poplars. A tall chimney-shaft, the roofs of scattered buildings, and a range of houses near the road, indicate that these well-planted fields are the site of the Royal Gunpowder Factory.
The Waltham Abbey Mills are probably the oldest in Great Britain. They must have been established about the middle of the sixteenth century, for we know that before that time nearly all the powder used in England was imported from the Continent. But in 1561 we hear of John Thomworth, of Waltham, buying, as agent for Queen Elizabeth, saltpetre, sulphur, and staves for making barrels. In the following century the parish register shows entries of deaths resulting from explosions at the mills; and Fuller, who was Rector of Waltham, alludes in one of his works to the dangers of the manufacture, remarking that the mills were blown up five times during the seven years of his residence in the parish. The only wonder is, that explosions were not far more frequent in the old factories, where the elaborate precautions now adopted were utterly unknown. Powder was allowed to accumulate in heaps on the floor, spirits of wine was used instead of water to moisten the ingredients, under the impression that it made better and stronger powder, and the drying process was effected by heating the powder on metal plates over a fire without any means of regulating the temperature. Finally, all the workrooms were close together, and often under a single roof, so that, if the powder in one room exploded, that in the rest would follow, like a boy's train of crackers.
It was in 1787 that Government bought the Waltham Mills from the last private proprietor, Mr. John Walton, supposed by some to have been a descendant of the family of old Izaak. Major (afterward Sir William) Congreve was the first superintendent. Horse and water power only were employed, most of the machinery was of wood, and the incorporating mills were, like mortar-mills at the present time, worked only by horses. Since then great improvements have been introduced into the manufacturing process; the factory has been widely extended, gun-metal and copper have been largely substituted for wood in the structure of the machines, refining-houses have been erected for purifying the saltpetre and sulphur, and retorts for preparing the charcoal. Machinery has been designed and erected for the preparation of the large cannon-powder introduced of late years, and in the mills iron runners, driven by steam, have taken the place of the stone runners, drawn by old horses. A complete code of rules and precautions has been introduced, and every building protected by a system of lightning-conductors. The factory gives employment to about two hundred men, and can produce twenty-four thousand barrels of gunpowder in the year, and the powder is believed to be at once the best and cheapest made by any existing factory.
Before describing the process of its manufacture at Waltham, it may be as well to note a few facts on its composition and action. Gunpowder may be regarded as a solid, which, by ignition, can be very rapidly converted into a large volume of gas at a high temperature. It is this quality which constitutes it an explosive, for the sudden expansion is what we call explosion, though the name is sometimes given to the loud report which accompanies it, caused by the outrush of the gas generating sound-waves in the air. When the explosion occurs in a confined space, the weakest portion of the confining bodies gives way before it. In quarrying, the rocks are rent, as the gas from the blasting-powder forces them apart. In blowing down walls and gates, the mass of earth heaped on one side to form the "tamping" offers a greater resistance than the wood or stone on the other, and the wall or gate gives way. In firing a cannon, the loose shot offers less resistance than the solid coils of the gun, and it is driven out to a distance proportioned to the force of the charge. If there is any defect impairing the strength of the cannon, or if the shot wedges in the bore, the gun bursts; for nothing we know of can resist the force of the gas. Recent experiments prove that this force, exerted in closed vessels unrelieved by expansion, is equal to a pressure of about forty tons on the square inch.
Of the three materials of which gunpowder consists—sulphur, charcoal, and saltpetre—only the last two are, strictly speaking, essential to it. The gas is actually generated from the charcoal and saltpetre, therefore a mixture of these only will explode. On ignition the charcoal decomposes the saltpetre, its combustion being supported by the oxygen of the latter, in combination with which it forms carbonic-acid gas, and this, mixed with the nitrogen from the saltpetre, is the gas which produces the useful effect. But, when gunpowder is thus made with saltpetre and charcoal only, the power developed by the explosion is comparatively trifling, and sulphur has to be added to increase it to such an extent as to make it really efficient. The sulphur acts in two ways to this end. In the first place, it ignites at a lower temperature than either charcoal or saltpetre, and its combustion accelerates both the decomposition of the saltpetre and the generation of gas, by combining with the potassium of the saltpetre and liberating the oxygen. Then, by heating the carbonic acid and nitrogen, it considerably increases their volume, and consequently their explosive force. The flash, and smoke, and the fouling of the gun, are the result of the decomposition of the saltpetre, and consist of sulphates and carbonates of potassa, resulting from the combination of potassium with the sulphur and carbon. The substances thus formed, swept out into the air, become flame and smoke, or remain in the bore of the gun as fouling, and it is these solid substances that blacken the faces of men engaged in close conflict.
Thus we see that of the materials of gunpowder saltpetre is the most important. Both saltpetre and sulphur arrive in England in a rough state, mixed with various impurities. It is generally the practice in private factories to purchase these materials after they have been refined elsewhere; but at Waltham the refining process is carried on within the works. By this means the materials are obtained of a uniform quality and perfectly pure. The saltpetre comes from various districts of India, chiefly from Bengal and Oude, where it is found mixed with the soil, and as an incrustation on the ground. In India it is boiled, and roughly crystallized by evaporation. When it is required for use in the Gunpowder Factory, it is purified by a process founded on the principle that hot water will receive in solution more of the saltpetre than of the impurities mingled with it. The saltpetre is boiled in water; the resulting solution is then filtered and allowed to cool in large vats, at the bottom of which the pure saltpetre is deposited in fine crystals. It is then washed, dried, and stored in bins, great care being taken that no sand or gritty particles are introduced, as they might cause an explosion when under pressure at subsequent stages of the manufacture, and the same precaution is taken with the sulphur and the charcoal. It is believed that many of the explosions which occur in private factories are caused by foreign substances being present in the materials.
The sulphur is all of the best quality, imported from Sicily. It is purified by a distilling process, which reduces it from its rough state to masses of handsome yellow crystals. It is then pulverized by being ground under iron runners, and sifted in a kind of revolving cylindrical sieve, called a "slope-reel." The sulphur refining-house is, of the whole factory, the least pleasant portion for a visitor, the air being always tainted with the fumes of the sulphur, which are so strong as even to burn and destroy the leaves of the trees near the building. The management of the process is, however, by no means an unhealthy labor. The workman last employed at it died as a pensioner at the ripe old age of eighty, after having worked forty years in the refining-house.
The charcoal is all made on the spot, chiefly from wood imported from Holland and Germany. The alders and willows in the plantations of the factory furnish but an insignificant supply, probably not enough to make a dozen barrels in the year. They are grown for the most part to form screens around and between the houses, so as to diminish the danger resulting from a possible explosion. The wood employed is of three kinds—alder and willow, which are used for common powder, and black dogwood for fine rifle-powder for the Snider and Martini-Henry. The latter wood is really a kind of buckthorn (Rhamnus frangula), of slow growth, and consequently close grain, which forms dense thickets in the forests of Germany, and is also found in the north of England and elsewhere. It is imported in bundles of slender rods about six feet long, and enormous quantities of these bundles may be seen stacked in the fields of the factory. There it is kept for at least three years, though generally it is allowed to lie in store for a much longer time. Some wood has been kept for twenty years, protected from the weather by a roof of thatch, and is still perfectly sound. Strange to say, comparatively little dogwood is used in the powder-factories of Germany, though it is quite certain that it supplies the best charcoal for the purpose. The old plan for charring wood was to burn it in pits, and this is still the practice abroad, but for many years the charcoal at Waltham has been manufactured by sawing the wood into short lengths, and packing it into iron cylinders called "slips," which are placed on a small carriage, and run into a retort very like those used in gas-works. Here the slip is exposed to the flames for a period varying from two and a half to three and a half hours, the gas issuing from the wood in the process being utilized as fuel; and the superintendent of the work knows when the wood is completely charred, by the peculiar color with which the combustion of the gas tinges the fire. As soon as this appears, the slip is withdrawn and cooled. The charcoal when taken out is ground in a machine like a colossal coffee-mill, and then, like the sulphur, sifted in a slope-reel.
The three ingredients are now ready for the regular process of manufacture to be commenced. Up to a certain, point (the formation of the "press-cake") the process is the same for whatever purpose the gunpowder is intended, but at that point it divides into two branches, according as it is to be used for heavy guns or smaller weapons. We shall, therefore, first trace the various stages of the manufacture up to the press-house, and then explain the method of making the various kinds of gunpowder, and the objects desired to be obtained by these modifications.
The first process is that of simply mixing the ingredients. For this purpose the proper quantities of each are accurately weighed out, allowance being made for a certain amount of moisture in the saltpetre. The proportions vary in different countries, and according to the purpose for which the gunpowder is to he used. For English Government powder of every kind it is—saltpetre 75 parts, sulphur 10, charcoal 15, the sulphur being reduced almost to a minimum, as its chief use is only to ignite the charge and accelerate its action. In France and Prussia the quantity of sulphur is larger, the scale being saltpetre 75 and 12.5 parts each of sulphur and charcoal, while in Chinese powder the amount of sulphur is between 14 and 15. It is remarkable that in all countries the proportion of saltpetre remains about the same.
The ingredients, being weighed for a charge of 50 pounds, are poured into a "churn." This is a revolving drum, placed horizontally, and having within it an axis revolving in a different direction from the drum, and furnished with eight rows of projecting arms, or "fliers." So rapid is the action of this apparatus, that when the charge has been three minutes in the revolving churn the ingredients are thoroughly mixed together. It is then known as a "green charge," and is ready for the incorporating mills, the object of which, as the name indicates, is to incorporate the materials, or to make the mixture so intimate that a new substance is produced, namely gunpowder.
The incorporating-houses at Waltham contain at present thirty-two separate mills. Each mill consists of a pair of runners, coupled together by a strong axle. This axle rests in the socket of an upright shaft, which, passing down through the mill-bed, is connected by bevel-wheels with a revolving horizontal shaft, driven by steam or water-power. The runners are either of black Derbyshire limestone or of iron, and weigh from three and a half to four tons. Iron runners are now generally used, and their size varies from three and a half to seven feet in diameter. The mill-bed, a large circular vat with a flat bottom and sloping sides, is of stone or metal, according to the material of the runners. On this bed 50 pounds of the green charge is spread out and moistened with water, and the mill is then set going. The length of time required for the incorporation of the powder varies according to the use to which it is to be applied. Thus cannon-powder is left under the mills for three hours; while for rifle-powder, which requires a closer incorporation on account of its more rapid action, the time is five hours. The power of a gunpowder-factory is measured by the number of pairs of runners it possesses, for, as the law allows no more than 50 pounds to be placed in any mill at one time, the amount which can be incorporated in a year is easily calculated. A pair of iron runners, driven by steam and working day and night, will incorporate in a year nearly 100,000 pounds of cannon-powder, or about half that quantity of rifle-powder.
This part of the process is more dangerous than any other, and explosions in the incorporating-mills are very frequent. The houses are built of light planking, nailed on a strong framework, so as to diminish the force of the explosion by yielding easily before it. The men are forbidden to remain in them while the mill is in motion, and a very simple arrangement has been devised for preventing an explosion from extending from one mill to another. A shaft runs horizontally through the upper part of the walls of each row of mill-houses. A shutter, balanced by a weight on the other side of the shaft, projects from it over each mill, and this shutter supports one side of a water-tank, the other resting on a pivot. Now, if an explosion takes place in any of the mills, the shutter above it will be blown up, turning the horizontal shaft, and raising all the shutters attached to it; so that the tanks, being left unsupported, turn over, and drench the contents of the mill-beds below.
On leaving the mill, the gunpowder is in the form of a soft cake, which easily breaks up into meal and dust. The old plan for making gunpowder, still followed in some places, was to moisten this mill-cake and force it through fine sieves, so as to break it into grains; but the moisture partly dissolved the saltpetre, and thus, to some extent, destroyed the previous incorporation, and the result was an inferior gunpowder, which, on account of the softness of the grains, often broke up into dust in transport. In the modern process, the mill-cake is firstin layers between plates of copper or gun-metal, to increase its hardness and density, and then made into grains of the required form by machinery. As a preparation for the press, the mill-cake is roughly broken down into meal and dust by being passed between grooved gun-metal rollers. It is then ready to be poured into the press-box.
This is a large box of gun-metal, lined with oak, and capable of holding about 800 pounds of powder. The sides are hinged to facilitate unloading, and by means of a small crane it can be swung into or out of the hydraulic press. To be loaded it is turned on one side, a wooden cover placed on the top, and the uppermost side is turned back on its hinges. Then, by means of gun-metal racks, the plates are arranged in the box, with the proper intervals between them to produce a thick cake for cannon-powder, or a thin one if rifle powder is to be made. The powder-meal is then poured in between the plates, the racks withdrawn, the side closed and bolted down. It is then swung by the crane on to the table of the press, and the cover taken off. The press is an ordinary hydraulic one; the table which supports the box is placed on the head of the ram, and as it rises a block of oak fixed overhead enters the box, and presses the powder, the amount of the pressure being measured by the extent to which the block enters the box. The pumps which supply the press with water are fixed in an adjoining room, and worked by a water-wheel; and, in order that the men may know when the pressing is complete without having to enter the press-room, a kind of catch and trigger is attached to the side of the block, and, as soon as the press-box has been forced up to the required point, the catch is liberated by coming in contact with it, and rings a bell in the pump-room. The pumps are then stopped, the ram falls by its own weight, and the box is unloaded, the gunpowder being taken out in large cakes the size of the metal plates, and as hard as slate. It is in the pressing of the gunpowder that the most serious explosions occur, for, if by any chance the pressure becomes too severe, and the powder explodes in the box, its force is much greater than if it were ignited in the open air. Seven men were killed by an explosion in the press-house at Waltham Abbey in 1843, and by a similar accident on June 16, 1870, five were killed and seven injured.
The question of the density given to gunpowder by pressing and its effects is one which is only now being worked out. Formerly the density of the powder was roughly ascertained by weighing a cubic foot of it, and then its quality was tested by observing to what distance it would fire a shell from a mortar. The primitive methods are now superseded by a testing apparatus, which gives scientifically accurate results. The density is determined by reducing a small quantity of the powder to dust in a mortar, and then placing it in a glass globe provided with stopcocks, one of them connected with an air-pump, and the other with a tube dipping into a vessel of mercury. On exhausting the air, closing the first cock and opening the second, the mercury is forced into the globe, and completely fills it. It is then weighed in a delicate balance, and, its weight when filled with mercury only being known, it is easy to calculate the density of the gunpowder.
Its force is ascertained by observing the initial velocities which it will give to a shot fired from a cannon. These velocities are measured with Bashforth's Chronograph, as explained in a former article in the Popular Science Review; and with the Noble Chronoscope, the invention of Captain A. Noble, of the Elswick Works, by means of which we are enabled to ascertain what takes place in the bore of the gun on the explosion of the charge, and what is the velocity of the shot, not only in the whole length of its course within the gun, but also in each portion of that short distance, thus determining the velocity within very small limits both of time and space, and this with the most perfect accuracy.
It is difficult to describe the chronoscope without a diagram, but it is easy to indicate the general principles of its action in a few words, and this will be sufficient for our purpose. A gun having been selected for the experiment, six or eight holes are drilled in one side of it, penetrating to the bore, at intervals along its length from the seat of the shot to the muzzle. Through each of these holes an insulated wire enters the gun, its lower extremity being in contact with, but insulated from, a sharp cutting edge, so arranged in the bore of the gun that the passage of the shot would force it down upon the wire and destroy the insulation. Each of the wires is connected with the secondary wire of an induction coil. The recording apparatus consists of a series of disks of polished silver, coated with lamp-black, and made to revolve simultaneously by the action of a falling weight and multiplying wheels at a very high velocity. One of these disks corresponds to each of the wires, the end of which is placed in a small discharger close to its circumference. On firing the charge, the shot cuts the insulation of wire after wire in rapid succession, and as each is cut a current passes, and a spark darts from the discharger to the edge of the revolving disk, striking off a speck of the lamp-black, and leaving the bright silver bare. Now, supposing the velocity of the circumference of the disks to be 1,000 inches per second, and the mark of the electric spark on the second disk to be one inch farther on than that upon the first, this would show that the shot took the 1 part of a second to pass from the first wire in the gun to the next. Similarly, if the distance between the marks on the first and last disks were five inches, this would indicate that the time the shot took to traverse the whole length of the gun was five-thousandths, or 1, of a second. In reality, the time is even shorter than this. In the 10-inch gun, a 300-pound shot, with a charge of 43 pounds of powder, passes down the bore in something less than the 1 part of a second. So delicate is this apparatus that, by dividing each inch of circumference of the disks into thousandths with the help of the vernier, the 1 part of a second would become an appreciable quantity.
It is found, by careful experiment with these appliances and the crusher gauge (by which pressure is estimated by the compression of a copper cylinder placed in the bore of the gun), that the denser the powder is the slower it burns, giving a lower initial velocity to the shot, and exerting a smaller strain on the gun. As an instance of the great differences caused by the smallest variations in density, we give the following results of an experiment with the 10-inch gun, with a charge of 70 pounds:
Feet per Second.
Tons on Square Inch.
Here an increase of .05 in density reduced the velocity by forty-two feet, and the pressure by eight tons. This shows the importance of obtaining a uniform density in the manufacture. For this purpose it is not sufficient to use a uniform pressure in the press-house, as even then the density of different pressings will vary on account of the changing state of the atmosphere, the different degrees of moisture in the powder-meal, its varying bulk and elasticity, and other minor causes. The only practicable method of securing an approximately uniform density is to test the product of various pressings and then mix them, so as to reduce the whole to the average density required, and this is the constant practice at Waltham.
We have alluded incidentally to the decrease of pressure in the gun as the density of the powder increases. With the immense guns constructed in recent years, it is important to reduce the strain on the metal as far as possible, as this is the only way in which the gun can be safely fired. But it must be remembered that, by seeking to accomplish this by indefinitely increasing the density of the powder, we would at the same time decrease the velocity of the shot, or, in other words, its useful effect. Artillerists have, therefore, had recourse to the expedient of using a powder, each grain of which is a lump of press-cake. The effect of this is to make it burn slower than grain-powder; for these lumps, when ignited at the surface, burn, as it were, in concentric layers, until the whole is consumed; and by this means the explosion, though to all appearance instantaneous, is in reality much more gradual than that which follows the ignition of smaller grains. In other words, the explosion of the charge in a heavy gun is made to be less of the character of a violent blow on the sides of the bore and the base of the shot, and more like a gradual shove given to the shot with a corresponding pressure on the gun.
The first form proposed for cannon-powder for heavy guns was that invented in America by Dr. Doremus, in which the whole charge was made into a solid disk, the size of the bore of the gun. This, however, was found to give very unsatisfactory results. Then the Russian Government adopted a powder compressed into large hexagonal prisms, and in Belgium another powder was made in the form of cylindrical pellets. This was adopted by our Government, and an immense sum was spent on erecting machinery at Waltham for its manufacture, but the pellet-powder has since been superseded by a simpler form, much easier to make, and giving better results; and the pellet machinery has been altered, and, we believe, is now used in the manufacture of gun-cotton.
The pebble-powder now in use consists of cubes of compressed gunpowder, with sides about four-fifths of an inch square. These pebbles are made by passing press-cake of that thickness between two pairs of rollers, armed with sharp-cutting edges. The first pair of rollers, by means of these edges, cuts the cake into several small bars, about four-fifths of an inch square at the ends, and these bars, on passing between the second pair of rollers, are divided into cubes or pebbles. After having been rolled in a hollow cylinder, or reel, to round off the sharp edges and get rid of the dust, the pebbles are carried to the drying-house to be freed from the moisture they contain.
The drying-house is a large room with double doors, and fitted with racks from floor to ceiling. On these racks copper and wooden trays are placed, containing the powder, spread out in thin layers. Steam-pipes are introduced from a boiler in an adjoining building, and thus the air of the room is kept at a temperature of about 135°. The fire-men in charge of the drying are forbidden to enter the room for fear of carrying in a spark in their clothes, but they ascertain the temperature by a register thermometer placed inside a small window, and this thermometer also acts as a telltale, by showing if the temperature has at any time been allowed to become too high or too low. So perfect are all the arrangements at Waltham, that no explosion has ever occurred in the drying-house.
The dried pebbles are finished by being placed in a revolving barrel (called a glazing-barrel), with a certain amount of powdered black-lead. On being taken out, every pebble is found to have a perfectly smooth surface coated with black-lead, the effect of which is still further to diminish the rate of burning. The pebbles are then thrown into sieves to separate small fragments; all irregular pieces are picked out by hand, and the remainder is packed in ordinary powder-barrels, which would hold 100 pounds of rifle-powder, but contain 125 pounds of the pebble-powder, on account of its greater density.
The following results of experiments with the 8-inch gun will give the reader an idea of the effects of the different kinds of powder. We need only explain that R. L. G. means the old "Rifle Large Grain" powder, still in use for field-artillery, and draw attention to the fact that the pebble-powder gives at once the highest velocity and the lowest strain:
Feet per Second.
Tons on Square Inch.
|R. L. G||30||1324||29.8|
Visitors to the laboratory at Waltham can see there a number of experimental varieties of pebble-powder, the largest of which consists of cubes as hard as stone, each side of which is two inches square. A shower of this alone fired from a gun would be quite as effective as grape, and it is possible that 300 pounds of this tremendous powder will form the charge of the new 80-ton gun.
For rifle-powder the meal is pressed into thin cakes; these are broken up into irregular pieces by hand, and carried to the granulating machine. This machine consists of four pairs of toothed cylinders, between which the broken cake is passed. As it falls from them in grains, it is received upon a series of screens of net-work. There are three of these, the texture of each being closer than that above it, so that large-grain powder is retained on the first, while fine-grain and dust fall through it. The fine-grain remains on the second, and the dust passes on to the third. All the screens are placed in an inclined position, so that the powder runs down them into tubs arranged at the lower end, one of which receives the large, another the fine-grain, and a third the dust.
The powder is then rolled for some hours in the glazing-barrels, to break off all minute irregularities, and give it a smooth surface. Then it is dried, and finally freed from dust in the slope-reel. This done, it is finished by being passed once more through the glazing-barrels, and it is then packed in barrels of 100 pounds each.
Such is the process of the gunpowder manufacture in the Royal Factory at Waltham. We have only briefly indicated the principles of each process, for to go into detail would occupy far greater space than is at our command; but even this sketch will show how at each step science has been called in to aid art in bringing the manufacture to its present high state of perfection. No expense is spared in procuring the best materials, the most efficient machines, and the most accurate tests; yet the cost of manufacture is only about seven-pence per pound. What a contrast to the early days of gunpowder-making, when in France, in 1375, a pound of gunpowder cost a sum equal to ten pounds of our money!
In every department the greatest care is taken to prevent the danger of explosions. The houses are built from 200 to 400 yards apart. Wood, copper, and gun-metal, are the only materials used in the structure of the machines, except where, and that rarely, a bolt of iron is introduced for the sake of strength, and then the metal is encased in leather. The floors are covered with hides secured with copper nails, and these, as well as the wooden platforms round the houses, are kept constantly wet. All loose powder is swept away from the floors, damped, and carried to a magazine, where it is collected and the saltpetre subsequently extracted. No one is allowed to enter a room without putting on a pair of leather "magazine shoes" made without nails, as the iron nails in ordinary boots might lead to an explosion if one trod on the loose powder; and, moreover, one would be certain to bring in grains of grit, which are so dangerous if they become mixed with the powder. The men wear a kind of fire-proof clothing, and in the incorporating-houses leather caps and gloves. Fire-engines are stationed in various parts of the factory, and every man has his post assigned to him in the event of an alarm of fire. To such an extent are these precautions carried that the roofs and eaves of the buildings are searched for birds' nests, and they are pulled down whenever they are found, lest the birds, in building or bringing food to the young, might drop grains of grit or sand on the platforms round the houses. Every building is protected by lightning-conductors, and, as soon as a thunder-storm approaches, the men have orders to stop the machinery, leave the houses, close the doors, and cease all work until it has passed over. But the best security for the safety of the factory is that the workmen are a body of steady, industrious, intelligent men, and bear so high a character that a dismissal is a rare event, though it is the penalty of any breach of a necessarily strict code of rules.
The powder manufactured at Waltham is carried down the river Lea to be stored in the great magazines at Purfleet. A useful lesson can be learned from the method of transport. The gunpowder is conveyed in barges specially constructed for the purpose. They are about half the length of an ordinary canal-boat, and are covered with a semicircular roof, with a door at the side, which is kept closed, except when the boat is being actually loaded or unloaded. Every powder-barge is considered a magazine, and the same rules apply to it as to the Government magazines. No fire or light is allowed on board; nothing but powder is to be placed in the hold; and no one is allowed to enter it without wearing the ordinary magazine shoes. In this way it may be said that every chance of an explosion is carefully guarded against. It would be well if the same method of transport were used on canals where(as on the Regent's Canal) gunpowder is being continually carried to and fro. The extra expense of having special barges would not amount to one-hundredth part of the loss caused by an explosion.
We have also to consider the transport of gunpowder by road. It is said that it is a common thing for cart-loads of gunpowder to pass through the crowded streets of London, sometimes several carts closely following each other, and crowding together if there is a block in the traffic. This is unquestionably a very dangerous practice. It would be well if carts laden with any considerable quantity of gunpowder could be prevented from entering the streets of a town; but in many cases this would be impossible. The transport, however, might easily be rendered much safer, by forbidding gunpowder-carts to pass along the streets except during a few hours in the morning, allowing only covered vans to be used, and fixing certain intervals within which no van should approach another. Finally, powder should never be packed in the light kegs used by some manufacturers, which are continually liable to leak, for loose powder is always exposed to ignition by any one of a hundred accidents. There are cases recorded of explosions having taken place through powder leaking from a tumbril, and forming a train upon the ground, which was fired by a spark struck from the shoes of one of the horses drawing it. Good, strong barrels should always be used, and they could of course be returned when empty.
After the Regent's Park explosion there were some fears expressed as to a possible explosion at Purfleet, where about 50,000 barrels of gunpowder are stored in five large magazines. If five tons on board the canal-barge could do so much damage, what, it was asked, would be the effect of an explosion of the 2,000 tons at Purfleet? Attempts were made to calculate the radius of destruction by Mallet's formula for the effect of bursting shells—the fact being disregarded that the bursting of a shell and the blowing up of a magazine are essentially different affairs. We were told how all East London would suffer from the shock, how several villages in Kent and Essex would be destroyed, and how trains would be thrown off the railway-lines, gasometers wrecked, and a wide district plunged into darkness. We must not forget that an explosion in the open country would have relatively much less force than an explosion in the midst of closely-built streets like those about Regent's Park. An explosion at Purfleet would be very terrible, but probably not half so destructive as one might expect at first sight. Then the Government must keep this large store of powder somewhere; 50,000 barrels could not be manufactured on an emergency, and Purfleet offers advantages in the way of safe and easy transport from Waltham, and shipment to India and the colonies, which mark it as a good site for our chief magazines. The gunpowder might indeed be distributed in numerous magazines, at various points along the lower part of the Thames, but this would really be to increase the chances of an explosion; for, the more numerous is the staff of superintendents and store-keepers, the greater is the danger of carelessness on the part of some among them.
Many a one has said, with the foppish young lord, who so much excited the anger of Hotspur at Holmedon:
That villainous saltpetre should be digged
Out of the bowels of the harmless earth,
Which many a good tall fellow had destroyed."
But, strange as it may seem at first sight, gunpowder and such compounds are as much used in peace as in war. What with practising, salutes, experiments, and reviews, our army, navy, and volunteers, burn every year as much gunpowder as would be required for half a dozen battles and a siege or two. But it is in mining, quarrying, and engineering works, in a word, for industrial purposes generally, that gunpowder is chiefly used; and as strife and peaceful industry cannot exist together, a war, on the whole, tends to lessen rather than increase the consumption of explosive substances. During the great conflict in America the sale and import of gunpowder fell off enormously. It is said that the same thing was noticed in France during the Crimean War; and probably the present war in Spain, by stopping the iron-mines of the north, has diminished the import of blasting-powder to a greater extent than it has accelerated that of powder specially manufactured for military purposes.
The following figures will give an idea of the amount of gun-powder employed in mining operations: It is estimated that in coal-mines about 80 pounds of powder are used for every thousand tons of coal raised. In mines of lead and other minerals, which are found in hard crystalline rocks, about 7,000 pounds of blasting-powder are required for every thousand tons of ore. To quarry a similar quantity of sandstone 170 pounds would be used; while for the harder granite the amount would be 650 pounds.
The quantity of gunpowder exported from England has not increased very rapidly of late years. In 1860 it was 11,078,436 pounds, of a declared value of £353,101. In 1865 it had risen to 16,833,723 pounds, valued at £457,078; and in 1870 it was 17,357,668 pounds, valued at £427,229. The increase in weight, with a decrease in value from 1865 to 1870, is due in a great measure to the fact that we export an immense quantity of gunpowder of an inferior quality to non-British ports in Western Africa; and it is in this cheap sort of gunpowder that the chief increase has taken place, while there has been a falling off in the more valuable kinds. Thus, in 1870, no less than 4,637,066 pounds, or more than 25 per cent. of the whole export, went to Western Africa, chiefly to satisfy the warlike propensities of woolly-headed kings; but it will be seen at once what the quality of the powder was, when we add that its declared value was only £83,657, while the comparatively small quantity of 1,173,762 pounds, exported to France, was worth £75,522, or about four times as much in proportion to its weight. Heavy as our loss was at Amoaful, it would have been much more severe if the Ashantees had been provided with something better than this worthless powder. As it was, several of the men in the front line were struck five or six times without being wounded, the bullets having such little force that they fell harmlessly to the ground.—Popular Science Review.
- M. Proust's experiments on charcoal, made from various woods, give the following results: 12 grains of charcoal of each wood, mixed with 60 grains of saltpetre and ignited, yielded the following proportions of gas in cubic inches: Dogwood, 80 to 84; willow, 76 to 78; alder, 74 to 75; filbert, 72; fir, 76; elm, 62; oak, 61 to 63. The importance of not overheating the wood is shown by the fact that, when the charcoal consisted of overheated willow, the yield of gas was only from 59 to 66 cubic inches.
- "On the Striking Velocity of Shot." By W. Royston Pigott, M. D., P. S. R., January, 1871.
- The explosion of a large magazine is really the successive explosion of various portions of its contents, not the detonation of the whole mass. This is why it is fallacious to attempt to estimate the effect of the explosion of 2,000 tons by comparing it with the explosion of a large shell, or of a few barrels on board a barge.