Mauritius militari Architecture totum se primis ab annis impenderat. Cujus usu oppida Belgarum passim munita: et exercitationes scripto vulgatas velut normam artis et legem, Reges plerique Principesque etiam hostes sequuntur."M. Z. Boxhorn, Hist. Obsidionis Bredse.

CHAPTER VIII.

"Beneath, each bold and promontory mound
With embrasure embossed and armour crowned.
And arrowy trine and wedged Ravelin
Wove kes diadem their tracery round

The lofty summit of that mountain green."

GERTRUDE OF WYOMING.

THE points in which the construction of permanent fortifications differs from that of field-works, are chiefly dependent on

1. The vastly greater excavation practicable; 2. The greater length of exterior sides; and, 3. The use of masonry.

We have said before (see page 15) that the distance of flanks from the salient angle which they defend should be considerably within the range of their fire; and in our examples of field-traces this is adhered to, by making the exterior sides 200 yards and under, so as to bring the salients within an easy musket-range of the flanks.

But, in permanent works, one of the advantages which we expect from free expenditure and deliberate execution, is by greatly increasing the relief over what is attainable in a fieldwork, to increase the command of the work, and to render its capture by surprise or vive force improbable, to a degree which cannot be looked for in field fortification.

Now, it has been shown (see page 48) that, with the ordinary form of parapet, in order that every part of the ditch before the curtain of a bastioned front may be seen and defended from the flank, the length of the curtain and the height of the flank must be to one another in the proportion of 12 to 1. This may be somewhat modified, inasmuch as it is not essential to the defence of the ditch that lines of fire from the flanks should strike the ground in every part. If the lines from the opposite flanks intersect at 4 feet or less above ground, they may be held completely to defend the ditch. This consideration reduces the necessary length of curtain about 16 yards.

If, then, the exterior side be 200 yards, and the bastions be constructed of the same proportions as in fig. 72,* the length of the curtain will be but 75 yards, and the whole relief only 22 or 23 feet, which would be trifling and insufficient for the objects of a permanent work.

It is therefore absolutely necessary to increase the length of the exterior side, and to increase it, in truth, beyond a good musketry range; making the limit of our line of defence. a fair range for grape, and the large pieces called rampart

* Plate V.

muskets. This will be 250 to 300 yards, giving an exterior side of 350 to 400, which allows a relief of between 40 and 50 feet.

Masonry in permanent works is used to revet the escarp and counterscarp. Its use enables the engineer to form these parts of the work perpendicular, or nearly so, so that the work cannot be entered till they are destroyed, unless by the bold and dangerous enterprise of escalade. These walls, in fact, are intended to serve the purpose of high walls in civil life-to keep out trespassers. The scarp wall does not by any means add to the strength of a rampart-i.e, to its strength to resist battering guns. On the contrary, the latter is rendered by it more liable to destruction, as the hard masonry broken by shot, and falling in masses, brings down with it the earthen parapet which it supports. It is necessary, therefore, to cover the masonry as much as possible from external fire.

In such works, having deep and wide ditches, with sides of nearly perpendicular masonry, it is manifest that egress from the fortress to the exterior must be maintained by occasional narrow communications, gateways, bridges, staircases, &c.; and that by these the sorties of the garrison must be made, if made at all. If, then, there is no cover beyond the ditch, an effectual sortie is scarcely practicable, as the troops attempting it would have to defile before the enemy; nor, indeed, could even a sentry be safely posted outside the ditch.

The work called the glacis serves both these purposes―viz., of protecting that part of the escarp masonry which is above ground from external observation, and of providing cover for the free circulation of troops, &c., on the outside of the ditch. The glacis is a parapet of the usual height, but having its upper surface extended in one long slope to meet the natural level of the ground. Were it terminated in an abrupt exterior slope like ordinary parapets, it is plain that it would give ready cover to an assailant.

That external communication round the counterscarp, of which the glacis forms the parapet, is termed the Covered Way. In addition to its use for the protection of guards and the formation of troops for sorties, it affords a close musketry fire on an advancing assailant.

It is desirable not to make it wider than its purposes absolutely require, in order that it may not form a too convenient site for the enemy to construct his batteries on; but occasional enlarged spaces are provided for the assembly of troops, either for sorties or for the defence of the covered way. These are called Places of Arms.

The width of the ditch must be adjusted to the relative levels of the rampart and the covered way, with the glacis before it. The general conditions are, that the covered way shall be so near the enceinte that the fire of the latter on the foot of the glacis shall not be masqued by defenders ranged on the banquette of the former, but that it shall not be too close to the enceinte to be fully seen and swept by the fire of the parapets. To consider these adjustments so nicely as is sometimes done, can only be useful as an exercise of the attention and understanding. A glance at a plan of one of the ordinary systems will show that the width of the ditch and covered way, and consequently the distance of the foot of the glacis from the enceinte, varies greatly on every half-front of a fortress. Hence, to preserve these adjustments accurately, the relief of the works also would require to be varied to a proportionate extent.

Besides the covered way, permanent fortifications are generally provided with other auxiliary works. Where these are within the glacis, and have ditches communicating immediately with the main ditch, they are termed outworks. Their names will be best learned in the construction of a particular system. Where such auxiliary works are beyond the glacis, but within musketry range of the enceinte or its immediate outworks, they are termed advanced works. Detached works are also intended to retard the advance of an enemy on the main place of strength, but are so far thrown forward as not to be dependent for defence on the parapets of the latter.

It is of the highest importance that all auxiliary works should have ready communication with the body of the place; that their defenders should feel that there is free access for succour to them from the rear, and free means of retreating

on their interior defences when they have done their best to maintain the outworks.

The difficulty of keeping up communication across wet ditches has been usually considered a preponderating objection to their use in sites where a choice exists between wet and dry, in spite of the undoubtedly greater obstacle which the wet ditch throws in the way of the assault of a well-provided fortress. In fact, a dry ditch not only affords free communication, but may often be a most safe and useful locality for posting large bodies of the garrison intended as reserves, or for the defence of outworks.* Where the main ditch is dry, it may be possible to obtain, in some degree, the advantage of a wet one by excavating a cunette or small ditch in the bottom, where means exist of filling it with water. Such a work the French had prepared at Badajos, at the foot of the counterscarp. It "took from the space calculated on for the formation and movement of the troops in the main ditch, besides causing the loss of many men who were drowned by jumping into the water."†

The masonry revetment generally terminates at the foot of the exterior slope. This is technically called a demi-revetment.‡ When the exterior slope is omitted altogether, the wall rising to the superior surface of the parapet, it is called a full revetment.§ Sometimes the upper part of the full revetment is a wall breast-high, with a berm behind it, the parapet being thrown back to a distance equal to the breadth of the berm.|| This berm or path is called by the French chemin des rondes, because used for going the rounds. Whilst presenting nearly the height of a full revetment against surprise, it affords greater facility for observing the ditches than a banquette behind the parapet; and the distance to which the mass of

*

66

During the Venetian defence of Corfu against the Turks, in 1716, Marshal Schulembourg seems to have kept a large portion of his troops, when on duty, in the main ditch, where they were probably more secure than within the walls."—Account of the Siege, in R. Engineer Corps Papers, i. p. 272. In Carnot's and also in Coehorn's Systems of Fortification, the assembly of troops in the ditches is especially regarded.

+ Jones's Sieges, i. 232.

Fig. 119. § Fig. 117

|| Fig. 118.