This new method is known as the polygonal system, and as many recent fortifications in Germany have been constructed according to it, it is often called the German system. Whatever be the system used, the object is the same, that is, to make the place so strong that to gain possession of it the enemy will be compelled to resort to the operations of a siege or blockade. Whatever be the diversities of opinion on the best mode of effecting this object, they all agree on certain general conditions as necessary. These may be summed up as follows: 1. They should be strong enough to resist with success an open assault. 2. They should have secure and easy communications for the troops, both within and to the exterior. 3. They should be so planned that every exterior point within cannon range shall be swept by the fire from the work. 4. They should be provided with bomb-proof shelters for the troops, and magazines of provisions and munitions of war. 5. They should be provided with all the accessory means of defence that the natural features of the position may afford.-The most convenient mode of fortifying a position in a simple manner consists in enclosing it with a rampart surmounted by a parapet, with a ditch. The latter when dry has its sides revetted with masonry.

The accompanying profile (fig. 1), which is a section made by a vertical plane perpendicular to the general direction of the intrenchment, will show the form of parapet and ditch generally used. When the place fortified is expected to contain the defenders only, called the garrison, it is termed a fort. If it surrounds a town, or is expected to contain other persons than those designed to defend it, it is called a fortress. For both cases the character of the fortification is the same. The rampart is an earthen mound raised above the natural surface of the ground upon which the parapet is placed, and serves to give the troops a commanding view over the ground exterior to the work, while it increases the obstacles to an open assault by the additional height it gives to the scarp. The top surface in rear of the parapet, called the terre-plein, affords a convenient and secure communication for the troops. The form and dimensions of the rampart are so arranged that it shall afford cover to the troops and to the armament, and facility for firing over it by the defence both with artillery and small arms. The ditch serves the double purpose of increasing the obstacles to be overcome by the enemy and furnishing the earth to form the rampart and parapet.

To give strength and durability, the faces are revetted with walls of mason-ry, called respectively scarp and counterscarp walls. When dry, the ditch is made from 20 to 30 yards wide, and receives a slight slope toward the middle, where a small drain called a cunette is dug to receive the drainage and keep it dry. When wet, the ditches are wider. Scarp walls are of three kinds: 1, the ordinary retaining wall, strengthened by counter forts; 2, the same with relieving arches; 3, detached in part or wholly from the rampart. They are usually made not less than 30 ft. high, which is sufficient to prevent an escalade if the defence offer an ordinary resistance. Counterscarps are ordinarily of the first and second class, and are generally from 18 to 24 ft. high. The height of the interior crest of the parapet above the exterior ground is called the command, and its height above the bottom of the ditch the relief. The covered way is an open passage bordering the ditches, forming a continuous communication around the work, sheltered from the enemy by an embankment high enough to cover the troops using it. This embankment is arranged like an ordinary parapet, having on the exterior a gentle slope or glacis.

Slopes and dimensions of profile are as follows: scarp and counterscarp slopes, 24/1- (or 1 base to 24 altitude); exterior slope, 1/1 (45°); superior slope, 1/6; interior slope, -3/1; banquette slope, 1/2; rampart slope, §; terre-pleins, 8 ft. below interior crest; berm, 2 ft.; thickness of parapet, 25 ft.; height of interior crest above banquette tread, 4 1/2 ft.; width of banquette treads, from 2 ft. to 6 ft.; general width of terreplein, 48 ft. The continuous line enclosing the place is called the enceinte or main enclosure. Although a great diversity of figures may thus be presented by the outline of the work enclosing the place to be fortified, they may all be classed under four heads, to each of which engineers have applied the term system of fortification. These four classes are: 1, circular; 2, polygonal; 3, tenailled; 4, bastioned. The circular system consists of a work the plan of which is circular or curved. The polygonal is when this plan is a polygon with salient angles only, or where the reenter-ings are very slight.

The tenailled is where the plan consists of a tenailled line, the reentering angles being between 90° and 100°, and the salient angles not less than 600. The hastened consists generally of two faces and two Hanks, the extremities of the flanks being connected by curtains. A work consisting of an enceinte alone would restrict the garrison to a passive defence, and would be more or less exposed to surprise. To provide against the latter, and to enable the garrison to make a more active defence by operating on the exterior of the place, engineers have devised certain exterior defences called outworks without the enceinte. Others have been placed within the enceinte, called interior works, more particularly for the purpose of defending any breach that may be made in the main work. When an interior work is detached from the enceinte and is organized to receive the garrison and rely on its own resources after the main work has fallen, it is called a citadel. Owing to the form and height of the parapet, its tire can take effect only at some distance beyond it. The enemy having gained the ditch will not be exposed to the fire from the works unless some arrangement has been made for this emergency. Such points where the enemy can find shelter are called dead angles or spaces.