Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/483

FORTIFICATION 453 of radius, an iron-walled fort, which admits of the guns being placed in a comparatively very small area, is the most economical method of construction, and that for constructive purposes a circle is the best form of such fort. In all forts designed for very heavy guns special provision becomes necessary for the ammunition service of the guns. The cartridge and shell stores are kept separate, and are pro vided in the proportion of one of each for each gun, or each pair of guns. As a rule, these stores are securely placed in the basement under the gun they supply, the cartridges and projectiles being raised by mechanical appliances through " lifts" in the heart of the building to the rear of the gun. The ammunition stores are lighted by lamps placed in light chambers," separated from the stores by thick plate glass ; these " chambers " are entered from passages entirely distinct from the passages used for the service of the am munition. The proper organization of the ammunition service is of the utmost importance for the efficiency of the fort ; by it the labour of the gun detachment is lessened, and the rapid service of the gun facilitated. Each gun chamber is in speaking communication, by means of tubes, with its own ammunition stores, so that the men in the latter, though absolutely shut in below during action, still form part of their own gun detachment. Finally, the guns of the fort are collected into groups for purposes of command, and the guns of each group are directed from a " look-out " place constructed on the top of the fort, which is in speakiug communication with each gun of the group. It Ins been found that when armour is struck by a shot, lining of some kind is necessary to deaden the effect of vibration and sound, and to act as a target to receive, and protect the gun detachment from, splinters thrown off from the interior. This lining should be flexible, so that it may be drawn close round the gun while it is being discharged, thereby keeping the smoke out of the gun chamber, may reduce the effect of blast and concussion, and may prevent splinters and balls entering through the port. No material has been found to answer so well as rope, and " mantlets," as such appliances are called, are now invariably made of this material. The only drawback to rope is its liability to catch fire, but this is entirely obviated by washing it with a saturated solution of chloride of calcium. There are two other forms in which iron has been applied which should not be passed by without notice, viz., " curved- fronted shields" and "turrets." In the former the case mate is of the ordinary character, but the shield frame, which is constructed with a sharp curve of 13 feet or 14 feet internal radius, is more difficult to make, and more costly. Two ports of the usual dimensions are formed in the shield, and the gun is placed behind it upon a turn-table, on which racers are laid so that the gun will fire through a lateral arc of training of 60 at each port. The gun is trans ferred at pleasure from one port to the other by the move ment of the turn-table, and commands an arc of 120. The weight of iron is 150 tons per gun, the cost 7500 per gun ; but in considering the cost of this arrangement it should be observed that by it one gun does the work of two. In the turret one or two guns are placed in the interior of a cylindrical wall of iron armour closed overhead with ordinary bomb-proof covering. The turret with its con tents is revolved by steam or other motive power, so that each gun commands a whole circle. Various arrangements have been designed for the service of the gun, in one the loading is done inside the turret, in another under the glacis, in a third while the turret is being turned to the rear, but whatever the arrangement, no time need be thereby lost, as the guns may be so placed that one revolves into action, while the other revolves out of action for load ing. It would seem that a breech-bading arrangement for heavy guns would lend itself more readily to the turret form of construction. With an armament of breech-loaders, the turret might be reduced in internal diameter; the port opening need be no larger than the chace of the gun, which would not have to be withdrawn for loading, and which is laid by means of a small aperture contrived in the roof of the turret the man who lays the gun having the machinery under his hand, and " training " the turret and gun himself in any direction. In turrets provision is made in the basement in the usual way for the accommodation of the garrison, and for the storage and service of the ammunition, as well as for the motive power and machinery. Special arrangements are made for ventilation and for the escape of smoke. The surface of the exterior to a certain distance in advance of the turret is plated with iron and sloped up wards " en glacis," so as to cover the base of the turret, and protect the turning gear from dislocation under the blows of heavy projectiles. See Plate X., fig. 1. Before quitting this subject it may be well to offer a few remarks on the employment of cupolas or turrets on land defences. Either of these forms of iron construction may be advantageously employed in advanced positions where it is necessary to retain guns until the very last such as the salients of ravelins, and in the angles of fronts of fortifica tions ; also in points which are commanded from heights in their vicinity. As these constructions are indestructible by the direct fire of siege guns, and are absolutely safe from enfilade and reverse fire, they confer very great advantages upon the defence. They should mount heavier guns than can be brought against them ; they cannot be silenced by the concentration of any number of light guns, and, judici ously used, they should place the artillery defence above the artillery attack. They render the construction of the first batteries of attack far more difficult, and force them to open at a greater distance from the place ; and, at any period of the attack, they can meet its artillery with heavier metal The guns of the attack are practically limited to about the calibre of the G4-pounders, and comparatively light armour will withstand these, and will preserve in security and render more easy the service of guns at least two calibres higher. Such constructions have been made by the Belgians in the detached forts round Antwerp and in Antwerp; by the Germans in the detached forts round Met* and in Metz, on the Hhine, and on the AVeser ; and by the Russians. In England there has as yet been no occasion to resort to them. In some at least of the places mentioned above, a system of construction and a material advocated by Grusen have been employed ; a type is shown in Plate X., fig. 2. The plates which are made of chilled cast iron manufactured by the Grusen process, are fitted together, without screws or bolts, in a simple manner. Each plate is a segment of the cupola, and is grooved at the edges; the segments are placed in position, and melted zinc is poured into the grooves, form ing a close and solid joint. A glacis similarly constructed protects the base, which is set in motion by hand machinery. The interior arrangements do not differ in principle from those described above for turrets. It is said that such a turret weighing 133 tons can be made to revolve round the whole circle by four men in three and a half minutes. Its cost is about 16,000. The Grusen metal is reported to have a very hard exterior surface, with great interior toughness and tenacity, and may probably be used with advantage in cupolas for land defence ; but the experience in England does not lead to the conclusion that it is a suitable material for resisting the extremely heavy projectiles carried by the war ships of the present day. Particular attention has been directed to the nature and manufacture of armour plates in Britain, and, up to the present time, the results afforded have been most satisfactory. The iron employed is best described in