Discoveries and Inventions of the Nineteenth Century

by Robert Routledge

So long as muzzle-loading was in use, guns were necessarily made short, for had they not to be run in from the port-holes and embrasures of forts in order to be loaded? Now there was an obvious disadvantage in this, for the projectile left the gun before the expansive force of the gases had been spent that could have imparted additional velocity. When however muzzle-loading was abandoned, and especially when strong and trustworthy steel became available for the construction of the gun throughout, there was no reason to waste in this way the power of the charge, so that barrels were made lighter, much longer in proportion to the calibre, and every part accurately adapted in strength to the strain to be resisted. For instances of increasing length, take the 38–ton 12–inch guns built up at Woolwich (of only seven pieces) for H.M.S. Thunderer (see Fig. [93]), on Mr. Fraser’s plan. These had a bore equal to only 16 times their calibre, while in the Armstrong 100–ton guns the bore is 21 calibres long; and in the 110–ton guns the total length of the chase is 31 times the diameter of the rifled part. It has since been the practice to make the bore of guns from 30 to 40 calibres in length.

The effect of a longer chase used with an appropriate charge is very clearly and instructively shown by the diagram Fig. [98], which is by permission copied from the very comprehensive work by Messrs. E. W. Lloyd and A. G. Hadcock, entitled Artillery: its Progress and Present Position. The reader should not pass over this diagram until he has thoroughly understood it, for it is an excellent example of the graphic method of presenting the results of scientific investigations. At the lower part of the diagram there are drawn to scale half-sections of a long and of a short gun. The horizontal line above is marked in equal parts representing feet numbered from the base of the projectiles. The upright line on the left numbered at every fourth division is the scale for the pressures in tons per square inch on the base of the projectile, and these are represented by the height of the plain curves above the horizontal line at each point in the travel of the shot. The dotted lines represent in the same way, but not on the same scale as the former, the velocity with which the base of the projectile passes every point in the chase. The figures 2, 4, and 6 on the upright line at the right-hand side refer only to pressures: the velocities scale is such that the point where the dotted meets the right-hand one is 2,680 units above the horizontal line, as the middle upright in the same way is 1,561 high, and the heights of the dotted lines represent each on the same scale the velocities of the bases of the projectiles at the corresponding parts of the chases. The shorter gun has the rifled part of the chase 15·4 calibres long; the corresponding part of the longer is nearly 33 calibres. The short 7–inch gun has a charge of 30 lbs. of gunpowder, and its projectile weighed 115 lbs. The longer 6–inch gun was not charged with gunpowder, but with the more powerful modern explosive cordite (see Index), of which there was 19·5 lbs., and its projectile weighed 100 lbs. The charges were so adjusted that the shots had the same initial maximum pressure of 20 tons per square inch applied to them. Now the cordite, though much more powerful than gunpowder (that is, a given weight will produce far more gas), is slower in its ignition, continuing longer to supply gas. The maximum pressure, 20 tons in both cases, is suddenly attained by the gunpowder gases, when the shot has hardly moved 6 inches onward, and the pressure declines rapidly as the moving shot leaves more space for the gas; while the cordite gases produce their greatest pressure more gradually at a part where the shot is already about 20 inches on its way, and not only do their highest pressures continue for a greater distance,—but the decline is far less rapid than in the other case. It will be observed by the intersection of the dotted lines, that when the shots in each case have moved about 2 ft. their velocities are equal. They finally leave the muzzles with the velocities marked on the diagram, and if the reader will apply the formula given on page [174] he will obtain their respective energies in foot-lbs.; but for large amounts like these it is more usual to state the energy in foot-tons, which of course will be arrived at by dividing the foot-lbs. numbers by 2,240, and these will work out in the one case to 4978·9 ft.-tons, and in the other to 1942·5 ft.-tons. The shot from the long gun will therefore have more than 2½ times the destructive power of the other.

Fig. 98.—Diagram of Velocities and Pressure.

The operations required in constructing guns are multiform, and have to be very carefully conducted so that the workmanship shall be of the best quality. The finest ores are selected for reduction, and the steel is obtained by the Siemens-Martin process already described. It must be free from sulphur and phosphorus, and contain such proportions of carbon, silicon, and manganese as experience has shown to be best, and its composition is ascertained by careful chemical analysis before it is used. The fluid steel is run into large ladles lined with fire-brick, and provided with an opening in the bottom from which the metal can be allowed to run out into the ingot moulds, the size and proportions of these being in accordance with the object required; some admitting of as much as 80 tons at one operation. When a barrel or hoop is required of not less than 6 inches internal diameter the ingot is cut to the required length and roughly bored. The ingot is then heated, a long cylindrical steel bar is put through the hole, and under a hydraulic press the hot metal is squeezed into greater length and less diameter. The hole first bored through the ingot is of somewhat greater, and the steel bar (called a mandril) of less, diameter than required in the finished piece. Portions are cut from each end of what is now called the forging and subjected to mechanical tests: if these are satisfactory, the forging is rough bored and turned on the outside. It is then annealed, by being heated and allowed to cool very slowly. The next operation is to harden the metal by raising it to a certain temperature, at which it is immersed in rape oil until cold. Then the piece is again annealed, and fine-turned and bored. All these operations have to be performed not only on the barrel, but also on each hoop, before the hoops are shrunk on, and the greatest nicety of measurement is required in each piece. Then the gun has to be turned on the outside, the screw for the breech piece cut, the bore rifled, etc. The object of the annealing is to relieve the metal from internal strains. It will not be wondered at that months are required for the construction of the larger kind of guns. Thus at Elswick a 6 in. quick firing gun, upon which men are employed night and day, cannot be completed in less than five months, and sixteen months are required for making a 67–ton gun.

We may take as an illustration of the progress of modern artillery one of the products of the Elswick factory which has just been referred to, and for which the demand from all quarters has been unprecedentedly great, namely, the 4·7 inch gun. This weapon is mounted in various manners according to the position it has to occupy, whether for a land defence, or on ship-board between decks, or on the upper deck. The arrangement shown in Fig. [99], which is reproduced from Messrs. Lloyd & Hadcock’s work, is known as the centre pivot mounting, and is suitable for such a position as the upper deck of a ship. The reader should compare the proportions and mounting of this weapon with those of the old 32–pounder sketched in Fig. [90], observing the very much greater comparative length of the modern weapon, and the mechanism for elevating and training it (which, however, the scale of the drawing crowds into too small a space to show as it deserves). C is a projection from the breech, to which is attached the piston of the recoil press; at T is the handle for training, which actuates a worm at V; the elevation is regulated by the turning of the four-armed wheel. The long chase of the gun projects in front; but the mounting and the breech machinery are protected by shields of thick steel, of which the sections of two plates are denoted by the dark upright parts in front. These are fixed; but a movable plate above the gun can be raised or lowered into an inclined position, for better taking sights. In the figure this is shown as open and in a horizontal position. This gun is provided with sights by which it can be aimed at night; that is, the sights can be illuminated by small electric lamps suitably placed; the wires connecting these with voltaic battery cells carried on the mounting are indicated. The figure represents the gun as constructed about 1893, but the improvements that are continually being made have brought about some modifications in the details.

Fig. 99.—Elswick 4·7 inch Q. F. Gun on Pivot Mounting.

Very notable among the productions of the great Elswick factory are the quick firing guns. These were at first confined to guns of small calibre, such as the 6–pounders. They are, of course, all breech-loaders, and the powder and shot are both contained in a single metallic cartridge case. A more formidable weapon of the same class is the 45–pounder rapid firing gun, which, like the rest, is constructed entirely of steel, with a total length of 16 ft. 2½ in., a calibre of 4·724 in., and a length of bore equal to 40 diameters. The weight of this gun is 41 cwt., and it throws a shell of 45 lbs. weight with a 12–lb. charge of gunpowder. Quick firing guns having a calibre of 6 in. are now also made in great numbers for arming our ironclads. The breech block in the quick firing guns turns aside on a hinge, and after the introduction of the cartridge it is closed and screwed up to its place by a slight turn of a handle. The piece is then pointed and trained by aid of mechanical gearing as in the case of the heavier guns. But Mr. Hotchkiss has introduced a simpler method of elevating and training his 3–pounder and 6–pounder quick firing guns, by attaching to the rear, and unaffected by the recoil, a shoulder piece against which the marksman can lean, and move the weapon as he takes his aim. Though these guns weigh respectively 4½ cwt. and 7 cwt., they can thus be pointed with the greatest ease. The firing is done by pulling a trigger in what seems like the stock of a pistol. The empty cartridge case is automatically extracted from the firing chamber by the act of opening the breech, and it drops to the ground. Ten or twelve rounds per minute can be fired from these guns, and Lord Armstrong has advocated the use of a number of them for naval armament in preference to that of a few ordinary breech-loaders of more unwieldy dimensions. He has calculated that in a given time a far greater weight of metal can be projected from a vessel armed with quick firing guns than from one provided only with the heavier class of cannons.

The breech pieces in the Elswick guns are closed on the “interrupted screw” system—that is, a very large screw thread of V-shaped section is cut in the barrel at the breech end, and a corresponding thread on the principal part of the breech block, which is, of course, capable of rotating about the axis. The screw threads, however, are not continuous, segments parallel to the axis being cut away, the spaces in the outer thread corresponding with the projecting parts in the inner, and vice versâ, so that when the block is pushed home, one very small part of a turn suffices to engage all the threads. The screw is also made conical, and is so cut into steps, as it were, that great resisting power is brought into play. The Elswick guns are provided with hydraulic buffers for checking the recoil, and the principle is applied in various modified forms. In some cases the pistons allow for the water a passage, which towards the end gradually diminishes. This is the arrangement for the 3–pounder rapid firing Hotchkiss gun, and the force of the recoil is made at the same time to compress two springs, which serve to return the gun to the firing position. This very handy gun is said to be able to fire twenty rounds per minute. In Mr. Vavasseur’s plan of mounting, the recoil is checked by ports, or openings, in the piston of a hydraulic cylinder being gradually closed, which is easily arranged by making a spiral groove within the cylinder, which gives a small axial motion to part of the piston.