Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/306

292 GUN MAKING [OFO>:S T ANCE. entirely converted into gas than a small one will ; lience ! the effect of enlarging the grains is to render the action of j the charge less violent, the composition of the powder being | the same in all gun charges. The projectile is driven out of ressure the bore by the pressure of the gas on its base, that is, on an area which varies with the square of the calibre. The weight of projectiles of similar form varies with the cube of th3 calibra. Hence the larger the gun the heavier will be the column of metal or projectile driven by each square inch of its base; and the greater must be either the pressure applied, or the tima of its application, if a given velocity is to be attained. The great object of the gunmaker is to obtain t!i3 highest possible ratio of muzzle velocity to breech prassure. His ideal would be a charge so arranged that a pressure equal to the amount the gun is constructed to bear should be uniformly maintained till the shot has left the nvjzzb. Science is still a long way from this, but has done a good deal towards it iu the last few years. A charge of ganpowder, composed of service ingredients in service pro portions, exploded in a closed vessel at a density of I OO (equal to that of water), sets up a pressure of 43 tons per square inch; at a density of 75, of 23 2 tons; at 50, of 1 1 8 tons. Supposing a gun cartridge to be rammed home to the density of water, and entirely converted into gas bofore the projectile began to move, the pressure in the bore would rise to 43 tons per square inch at the breech, an I fall towards the muzzle, as the travel of the shot afforded increasing room for expansion behind it. The column of metal to be moved, even in the heaviest pro jectiles yet known, is only a few pounds to the square inch of base, while the maximum pressure of the powder gas is measured in tons ; it is clear therefore that the shot must get under way at some period antecedent to the setting up of the maximum pressure. In a breech-loader, where the projectile has to be forced through a bore slightly less than its greatest diameter, it will be detained longer than in a muzzle-loader, where it moves freely away, but the difference is insignificant as regards the present argument. The result of tli3 shot s early motion is that space is at once given for expansion, and the normal 43 tons is never reached. Before these matters were fully understood, badly-proportioned charges of violent powder were found sometimes to set up what are known to artillerists as " wave pressures," which were dynamical in character, being caused by rushes of gas from one end of the charge to the other, so that the gauges indicated far higher pressures at the ends of the powder chamber than in the centre. This has now been overcome, and a great increase of both power and safety has been ob tained. Several important improvements have been made of late years ; the principal ones are three in number : (1) a great stride was made in the manufacture of powder when pebbles, prisms, and H-inch cubes were introduced; (2) the discovery of the beneficial effect of "chambering," that is, of boring out the powder chamber to a greater diameter than that of the rest of the bore; (3) tho method of "air spacing" the cartridge, so that a certain Air sj weight of powder should have a certain definitespace allotted in - to it, irrespective of the actual volume of the powder grains. Thus in the 80-ton gun powder cubes of 1 ?> in. edge are used, having an absolute density a little over T75, or about 15 7 cubic inches to the pound. If these grains were rammed tightly home in a silk-cloth bag, the space occupied behind the shot would be 24 - G cubic inches per pound ; as actually used, an air-space over and within the cartridge is left, so that the space behind the shot amounts to 34 cubic inches per pound. This density would set up a pressure in a closed vessel of 26 6 tons per square inch, but the relief afforded by the shot s motion reduces it to about 19 tons per square inch. The effect of chambering out the end of Chan: the bore where the powder lies is practically to permit a in - small gun to consume effectively the charge of a larger one. The cartridge is shortened, and the mechanical conditions of burning are greatly improved, so that, with large charges, higher velocity with lower pressure is obtained from a chambered than from an unchambered gun. The above information is derived from the indications of the crusher gauge, which registers the pressure of the gas at various pnrts of the bore. The chronoscope measures the rate at which the projectile acquires velocity during its travel from the breech to the muzzle. Knowing the increment of velocity at any point, we can calculate the amount of pressure required to produce this increment, and thus con firmation is obtained of the accuracy of the records obtained by the crusher gauge. The following table gives the increase obtained at successive stages in the development of the power of the 80-ton gun, which was first under-bored for experiment, and gradually brought to its present dimensions : Table showing Experiments with 80-ton Gun. Powder, Service P 2 (-incli cubes). Calibre. Diam. of Chamber. Weight of Charge. Density of Charge. Weight of Projectile. Mean Pressure in Chamber. Muzzle Velocity. Muzzle Energy. Muzzle Energy per Ton of Mean Pressure. 1 enetrating Power into Armour at 1000 yds. Thickness of Armour which would be pene trated at 1000 yds. ins. ins. ri>. cubic ins. per 11). It). tons per sq. in. f.s. ft. -ions. E Y ft.-tons per in. of shot s circumf. Ins. 14-5 14-5 220 25-6 1259 22-9 1502 19,637 857-5 373 22-7 15-0 15-0 2-20 25-6 1466 23-8 1423 20,577 864-6 383 23-1 15-0 15-0 290 30-0 1466 22-4 1511 23,205 10o6 433 25-1 15-0 16-0 310 30-0 1466 22-5 1553 24,493 1089-0 457 25-8 16-0 16-0 350 32 1703 20-4 1505 26,740 1311-0 466 261 16-0 18-0 425 34-0 1703 19-3 1588 29,745 1543-0 523 28-1 16-0 13-0 460 1 31-4 1703 191 1626 31,527 1651-0 545 30-2 Mild prismatic powder (German). It will be observed that each improvement has tended to facility of consumption of increased charges, so that, while the pressures are diminished, the penetrating power of the projectile is augmented, a heavier and more destruc tive shell being driven through thicker armour. The manner in which the various principles, of which an explanation has been above attempted, are practically applied is laborious and complicated ; the conditions are often con flicting, and the ultimate dimensions of a piece of ordnance H commonly a compromise. A couple of simple examples will illustrate the modus operandi. Suppose that it is required to design a gun which shall not exceed a given length, but shall throw a projectile capable of piercing a given thickness of iron at a given range. There are several formulae of a more or less empirical nature for calculating the perforating Perfo power of a projectile moving with a known velocity. Pene- t< tration is by some regarded as a punching action, by some as a wedging action ; probably it is a compound of the two. Recent experiments carried on with the very high velocity of about 2000 f. s. have thrown some doubt on the sound ness of any of the formulae. That generally used iu

England is as follows :