Page:The American Cyclopædia (1879) Volume VII.djvu/342

334: FORTIFICATION would be to discourage the nation and to cause a large portion of the people to give up all hope of a successful defence against the enemy. A position that is the key of several important communications, like Atlanta in Georgia, where several railroads centre, or on some river, as Cairo at the junction of the Ohio and Missis- sippi, would be an example. In mountainous regions, the entrance of defiles, at points where several valleys branch, or at the junction of roads, are examples. Points like these are the ones to be fortified when the intention is to put the country in a state of defence against an in- vading force ; but as in the United States there is little probability of invasion in any case ex- cept along the seacoast, the attention of Amer- ican engineers has been directed to securing the principal harbors, naval stations, and com- mercial cities. The works for this purpose have been arranged and armed with guns and mortars of the heaviest calibre, with the design of excluding the enemy's fleet from the use of the harbor or roadstead in their vicinity. Hence these works exhibit some peculiarities of con- struction at variance with the general rules already laid down for permanent works. The cardinal maxim adopted by engineers is that all masonry should be masked from the distant batteries of the enemy ; that no masonry should be exposed. When this rule is violated there are reasons for it which an examination of the site will explain. Earth or sand, or a mixture of them, is the material used for parapets and for the masks. An examination of fig. 1 will show that the glacis of the covered way com- pletely masks the scarp wall of the main work from the enemy's fire. In addition to this other works are often used. In cases where, in order to get the necessary amount of fire, great- ly exposed masonry has heretofore been resort- ed to, it is probable that wrought iron will hereafter have to be used. Experiments are in progress to ascertain the best method of sub- stituting it for stone in those parts ; and it was used to some extent in casemates by Gen. Tot- ten, late chief engineer of the United States army. Its expense is the present chief objection. The casemates and embrasures used to protect the artillery and the men serving it weaken the walls and limit the field of fire. Various ex- periments have been proposed to secure such protection without using embrasures. One of these is the project of having the gun exposed above the parapet only at the moment of firing, the recoil of the piece causing it and the carriage to descend by inclined rails or other devices to a level below the interior crest, and the piece when reloaded being raised to its former position in battery by the aid of a coun- terpoise attached to the gun or carriage, which has been lifted to a certain height by the gun in its descent to the lower level. Several in- genious plans have been proposed to carry out this principle. Among the first was that of Gen. De Hussy, colonel of the United States engineers, who made a model showing this principle about 1840. The best known plans, however, are the gun carriage devised by Ma- jor Moncrieff of the English militia, and that of Major King of the United States engineers. These carriages have been tested by actual ex- periments, and show that the principle is capa- ble of practical application. Maj. Moncrieff's project has been suggested for sunken batte- ries, the gun and carriage sinking into a pit prepared for the purpose. Maj. King's is in- tended to apply to existing works. To com- plete the fortification for the defence of har- bors or rivers, the fifth general condition must be extended to include a good system of tor- pedoes. (See TORPEDOES.) All arrangements made for the defence with musketry and artille- ry belong to what is known as the armament. That for small arms is complete when the slopes of the parapet and the position of the ban- quette are arranged. The final defence of the work depends upon the effective use of these arms. For artillery the arrangements may be barbette, embrasure, or casemate. The first is a construction by means of which the piece fires over the parapet ; the second, an opening for a gun in the parapet, exposed to vertical fire ; and the third, an opening protected from ver- tical fire. The calibre, the kind of guns or mortars, and their positions in the work, will depend upon the object of the fortification, and the kind of attack that it is exposed to. II. TEMPORARY OK FIELD FORTIFICATIONS. These are of two kinds, those used in the de- fence of a position, and those used in the at- tack of a position or place. They might be classed as defensive and offensive works. The main objects of these works are to afford a shelter from the enemy's fire, an obstacle to the enemy's progress, and means for the as- sailed to use their arms with effect. The gene- ral term intrenchments is applied to all field works, and a position strengthened by them is said to be intrenched. The general principles involved in permanent fortification are ap- plicable to these defences, the only difference being in degree. The parapet differs from the parapet represented in fig. 1 only in thickness and in resting on the natural surface of the ground instead of on a rampart. The thick- ness is regulated by the material used, the kind of attack, its probable duration, and the length of time at the disposal of the assailed to throw up the work. As a general rule, its thickness is one half greater than the depth of penetra- tion of the projectile into the material used. If the projectile from a field gun at a distance of 440 yards, firing against the work, can pene- trate 6 ft., the thickness must not be less than 9 ft., measured horizontally between the inte- rior and exterior crests. The height of the interior crest above the ground within the work must not be less than 6 ft. 6 in. We assume it ordinarily to be 8 ft., and limit its greatest height to 12 ft., owing to the difficulty of throwing up a work with this relief in a rea- sonable length of time, with the ordinary means