Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/349

tlie space occupied by the gases increases as the shot travels along the bore. This is exactly the opposite of what should theoretically take place, and causes the maxi mum pressure to be exerted before the inertia of the pro jectile has been overcome. So far back as 1860, these con siderations led General Rodman, the eminent American artillerist, to employ, in the experiments with his 15-inch and 20-inch cast-iron guns, what he termed a "perforated cake cartridge," composed of discs of compressed powder from 1 to 2 inches thick, and of a diameter to fit the bore, pierced with holes running parallel to the axis of the gun. In his Properties of Metal for Cannon, and Qualities of Cannon-Powder (Boston, 1861), Rodman demonstrated mathematically that such an arrangement of the charge would relieve the initial strain by exposing a minimum surface at the beginning of the combustion, while a greater volume of gas would be evolved from the increasing sur faces of the cylindrical hollows as the space behind the pro jectile became larger ; this would tend to distribute the pressure more uniformly along the bore. The results of experiment perfectly bore out his theory, but he found it more convenient for several reasons to build up the charge in layers of hexagonal "prisms" of comparatively small size, fitting closely to one another, instead of having the cakes or discs as large as the bore. The civil war in America most probably interfered with the further development of General Rodman s powder, but . j.j ie c ^ ea wag t a k en U p by a Russian military mission in the United States, and resulted in the manufacture on a large scale by Russia of what is now known as " prismatic powder." It has since been adopted in Germany for use in all heavy rifled ordnance ; for the very largest guns, such as Krupp s 70-ton gun. this powder has been recently made with one central hole, having a higher density (1 78) than the original seven-hole prisms, which were about 1 68; the external dimensions are the same, 1 inch high by 1 36 in diameter. The prisms are so arranged in the cartridge that the hollows are continuous throughout. Prismatic powder represents a distinct class, the peculiarity of which is that each grain or piece is pressed separately in a metal mould. In the British service, powder for heavy rifled guns, in the shape of small cylindrical pellets, with a hollow half-way through, was made some twelve years since, but has been superseded by the pebble or cubical powder cut up from press-cake. To make this class of gunpowder, whether prismatic or cylindrical, we need (1) a mould in which to place the meal or granulated powder ; usually a number of moulds are contained in one plate ; (2) a punch to fit each mould accurately with which to compress the powder, and needles to form the perforations ; (3) an appliance for pressing the finished pellets or prisms out of the moulds; or this may be done by the punches themselves, if the moulds are closed by a removable upper plate. The requisite pressure may be given either by hydraulic machinery, as at Walcham Abbey, or by means of a cam or eccentric on a shaft driven by steam or water power, as in the Russian and German prismatic machines. By the former plan a large number of pieces may be pressed slowly at one operation, but by the latter only about six prisms can be formed at a time ; the machine, however, works very quickly, and has a small hopper for the meal or grain, and a self-feeding apparatus, the mould plate sliding backwards and forwards, so as to ba alternately underneath the hopper and punches. Self-feeding machines of this nature are found to get clogged when used with powder-meal, and this was doubtless the chief reason why granulated powder was first used; the size of grain is about that shown in fig. 10, b. It is probable that a more uniform density could be given to the prisms by hydraulic 329 pressure than by the cam arrangement ; the latter is said to exert a maximum pressure of 2000 Ib on the square inch. The prismatic powder only needs careful drying at a moderate heat to finish it. It has been found that all powders thus made possess less explosiveriess than those granulated or cut up from press-cake ; the smooth surfaces of the pieces apparently afford little hold for the flame, and thus they ignite slowly; by some this is considered a defect, but by others an ad vantage. For this reason, as well as those already detailed, prismatic powder strains the metal of the gun less, in pro portion to the velocity obtained, than pebble or cubical, but, to give the projectile an equal velocity, the charge of prismatic must be considerably larger. The cost would probably be also greater, weight for weight. . PBOOF OF GUNPOWDEK. The tests to which powder is subjected are as follows : 1. For proper Colour, amount of Glaze, sufficiently hard and crisp Texture, and Freedom from Dust. Thesepoints can be judged by the hand and eye alone, and require a certain amount of experience in the examiner. The cleanness of the powder is tested by pouring a quantity from a bowl held 2 or 3 feet above the barrel ; if there be any dust it will be thus easily detected. If it is injured by damp there will be little or no dust, but the grain will be " rotten," and may be broken between the fingers; minute crystals of salt petre may be also detected on the surface in a good light. 2. For proper Incorporation. By "flashing," that is, burning a small quantity on a glass, porcelain, or copper plate. The powder is put in a small copper cylinder, which is then inverted on the flashing plate ; this provides for the particles being arranged in nearly the same way each test, which is very important. If the powder be very large, it must be broken up and sifted to a certain size through a small hand-sieve. Properly made gunpowder should "flash," or puff off, when touched by a hot-iron, with few " lights" or sparks, leaving only some smoke marks on the plate. A badly incorporated powder will give out a quantity of sparks, and leave specks of uncombined salt petre and sulphur, forming a dirty residue. Powder made from very slack-burnt charcoal, or which has been injured by damp, will always flash badly. 3. For Shape, Size, and Proportion of the Grains. Shape can be judged by the eye only, and the size of large powders can be measured, or the number of pieces to 1 Ib counted ; granulated powder may readily be sifted upon the two sieves which determine its higher and lower limits of size. The proportion of different sized grains is ascer tained by using three or more sieves. For example, the Government small-arm powder is sifted with 12-mesh, 16- mesh, and 20-mesh sieves ; all must pass the first, not less than three quarters be retained upon the second, and only one-sixteenth part is allowed to pass the last-named sieve. 4. Density. Formerly this was ascertained by "cubing," or finding the exact weight of a carefully constructed box of known contents, when filled with powder in a particular manner. But as this plan gave only an approximate result, a mercurial densimeter has been substituted, by means of which the density can be ascertained to three places of decimals. Briefly, the machine determines with great accuracy the weight of a globe when it is (a) filled with mercury alone under a certain pressure, and (I) filled with a known weight (say 100 grammes) of powder and mercury under precisely similar conditions ; then, if S be the specific gravity of mercury at the time of experiment, W the weight of globe filled with mercury alone, and W the weight when filled with powder and mercury, Density Sxl W-W +100

XL