The surface of the embankment of the covered way is made, from the interior crest to the exterior, with a gentle slope. The principle to be attended to in arranging these glacis planes is, that they should all be swept by the artillery fire of the works in their rear, and by the musketry fire at least of the bastion face. From what has been said, it will be seen that in this system, when the relief and plan are suitably arranged, the fortification possesses the advantage of having its ditches thoroughly swept from within the main work itself, of bringing a cross and flank fire to bear upon the approaches on the salients, and furnishing a strong direct and cross fire upon the ground in advance of the curtains and the faces of the bastions.-The bastioned system came into existence after the application of gunpowder to military purposes. The precise date and name of its author are not known. The best authorities give as the date of its origin the close of the loth or the beginning of the 16th century. The system as it appeared in Italy, and as practised by the Italian engineers of that date, was soon adopted throughout Europe. In its application in the different states, it was varied and modified in different degrees.
These variations and modifications were due to the discussions among the profession as to the best method of combining the parts, of adapting it to the natural features of the country where applied, and to the natural characteristics of the people. From these arose the schools known as the Italian, Dutch, French, Spanish, German, and Swedish. The Italian school was characterized by very small bastions connected by very long curtains, with the flanks perpendicular to the curtains, and no outworks. In some cases, a small and very obtuse bastion was placed at the middle of the curtain. The ditches were about 33 yards wide and 24 ft. deep. The defects of this plan were soon felt, and an improvement was made by shortening the curtain, enlarging the bastions, and introducing a covered way, with a tete de pont to cover the communication across the main ditch at the middle of the curtain. In the Netherlands, the low country, want of time and money, and presence of water on or near the site, led to decided changes in the system. From the nature of their struggle, their defensive works were based upon a strictly passive defence.
The characteristic features of the Dutch school were wide ditches filled with water, low ramparts without revetment walls, an enceinte often within one, and numerous outworks. The French school was characterized by a combination of the best features of the Dutch and Italian schools. Retaining the profile of the latter, the outworks of the former were added. It is to this school that we are indebted for the rules and principles of the bastioned system. Scientific and systematic fortification may be said to date from Vauban, so perfect are his works in comparison with those of his predecessors throughout Europe, in the arrangement of the details, the proportions of the lines, and the adaptation of the system to the requirements of every locality. The characteristics of the Spanish school are the increase in dimensions of profile and height of scarp, with interior intrenchments, and often a bomb-proof keep, the object being to render the defence more obstinate. They made free use of detached works, but, like the main work, they were generally organized for a strictly passive defence, depending upon their own resources rather than cooperation from the main or other works. They frequently omitted the covered way.
The works of the German school differ but slightly from those of the French and Italian. In some fortresses, the adoption of casemated batteries, which in recent years have formed so important a part in their works, was the distinguishing feature. This school reckons a number of original writers on fortifications, among the most noted of whom are the celebrated Albert Durer, Speckle, and Rimpler. Many of the characteristic features of the French school were suggested by Speckle many years before they were adopted in France. Swedish engineers paid special attention to covering the faces of their works from enfilading fire. They made free use of casemated batteries, having them often in several tiers. They also arranged their interior parts so that each should contribute to the defence of the others and be capable of an independent resistance.-Polygonal or German System. This system has been proposed by several engineers of distinction, but its most ardent advocate has been the French engineer Montalembert. The leading features of this system are as follows: 1. To occupy the principal points of the position to be fortified that are liable to be attacked by works which shall contain within themselves all the resources necessary for a vigorous defence; these works to be placed in reciprocal defensive relations with each other, but so arranged that the falling of one of them into the hands, of the enemy will not compel the loss of the others, nor the surrender of the place.
These are called independent works. 2. To enclose the space in rear of these by a continuous enceinte; or connect them by long curtains; or employ them as a system of detached works in advance of a main work, for the purpose of forming capacious intrenched camps. The enceinte, when used, to be polygonal in plan with a revetted scarp, and so arranged with the independent works as to sweep in the most effective manner by their fire the approaches of the enemy, both near and distant. 3. To provide the most ample means for an active defence by covered ways strengthened by case-mated redoubts, and ample communications between them and the main work for sorties in large bodies. 4. To shelter the artillery from the enemy's fire, and so arrange it that it shall be superior to that of the besiegers at any period of their attack. In this system the plan of the enceinte and of any independent work when detached is polygonal, the ditches of which are flanked by caponnieres, which are casemated structures of two and sometimes three tiers of fire; or the front may be either slightly tenailled or of a bastion form, with short casemated flanks to flank the main caponniere; the main flanking arrangement for the ditch being the caponniere, a work exterior to the enceinte.