A wet ditch adds considerably to the difficulties at this stage of the operations, but we cannot enter here into the modes of effecting a passage. Many have been proposed, and a few have been practised. Probably, after the flanks have been effectually counter-battered, Blanshard's light pontoon bridge might be boomed out, from a breach made by mining in the counterscarp, to the breach in the ravelin or bastion.

Fortresses are very frequently provided with a system of subterraneous galleries beneath the covered-way and glacis, by which the garrison are enabled to spring counter-mines under the works of the besiegers. These must be met and destroyed by mining on the part of the assailants. This mining and countermining, of course, greatly varies the progress of the latter stages of the attack, but we cannot here enter into details of this underground warfare.

It was long supposed that the destruction under ground produced by a mine could not extend to a distance much greater than the line of least resistance, or depth of the charge from the surface nearest vent. But Belidor proved by experiment that with large charges craters could be produced, the radius of which on the surface was nearly three times the line of least resistance. This discovery simplifies the attack of countermined systems, and gives a decided advantage to the besieger in such warfare. For the defender cannot resort to similar overcharges, as they would overwhelm his own galleries; so that the contest is between a long sword and a short one.

Instead of using artillery to open a gap for a storming party, breaches have often been made by what is called "attaching the miner to the wall." Miners, in this case, are sent across the ditch secretly, or under the protection of an overwhelming fire from all the trenches, to excavate a gallery and lodge a charge of powder under the rampart of the place. By this means Bhurtpoor was breached and captured in 1826.

Near the flanks of the first parallel, epaulements † are constructed to cover bodies of cavalry, kept in readiness to act against sorties, if there are no natural irregularities sufficient to afford them cover within a convenient distance.

* See NOTE V, Belidor.

+k, k, Fig. 107.

"The Plataans

beginning at either end, where the wall was of its original height, built another in the form of a crescent running inwards into the city; that if the great wall were taken this might hold out, and their opponents might have to throw up a second mound against it, and, as they advanced within, might have double trouble, and be more exposed to missiles on both their flanks.”— THUCYDIDES, ii. 76, (Dale.)

1. In the construction of Vauban's first system, which we have laid down, we have described no retrenchments or interior defences requiring to be captured, in addition to the main works of ravelin, bastion, and covered-way. Though these do not form any essential part of the system as it is taught, there

is nothing in the trace which precludes their being grafted on it, either as permanent works, or as field-works thrown up during the progress of a siege, to cut off breaches or other weak points from the body of the fortress.

In outworks and empty bastions, the simplest form of retrenchment is a ditch cut at right angles to the face of the work, through the parapet and terreplein of the rampart on either side of the attacked salient. This is most easily applicable where a perpendicular wall occupies the place of the interior slope of the rampart. Otherwise, when the retrenchment is called for, that slope must be scarped and revetted with timber.

A lofty work called a Cavalier is sometimes constructed in the interior of full bastions, having a command of fire over the rampart of the bastion. Its object is to have full command of ground which would otherwise be imperfectly seen from the ramparts, or to aid the defilade of the fortress from adjoining heights. Cormontaingne, speaking from his own experience, says that, in the attack of places provided with cavaliers, they always caused him great difficulty in defilading his approaches, and annoyed the troops in the trenches much by their plunging fire. In old places, these works were frequently constructed in the middle of a long curtain, as well as on the bastions. Where the cavalier in a bastion is provided with a ditch before its rampart, cuts similar to those just described may be made from its ditch across the ramparts of the bastion, thus isolating the two faces and salient under the fire of the cavalier. This is called a Cavalier Retrenchment.

In a full bastion without a cavalier, a tenaille retrenchment is sometimes constructed by running two parapets inwards from the shoulders, so as to form a re-entering angle in the middle of the bastion. Or, in a large bastion, this may be broken into the form of a small bastioned front. In all these retrenchments, though the cut extends through the parapet of the bastion, the escarp of the latter is left of the full height. The objection to them all is the extent to which they cut up

* Fortification Permanente, chap. xi.

and cramp the space available for the movements of troops and guns in the earlier stages of a defence.

The bastioned retrenchment may, however, if there is interior space, be drawn across the gorge instead of between the shoulder angles. Or, the retrenchment may be a straight line across the gorge, or two faces in the alinement of the adjoining curtains, meeting in an obtuse salient, so that their ditches may receive a flanking fire from the flanks of the collateral bastions. As we have shown, however, that in Vauban's system a part of the curtain is liable to be breached through the trouée of the tenaille, it will be better to embrace so much of the curtain in any proposed retrenchment, or to complete the isolation of the breaches by cuts across the rampart of the curtain on either side, scarping the interior slope of the rampart between them.

The retrenchment often consists of a stockade, or loopholed wall, instead of a parapet. So at the siege of Maestricht by the Prince of Orange, (William III.,) in 1676, "when we were in possession of the Dauphin Bastion, the enemy fired most furiously upon us with their small cannon through a thin brick wall, by which, and their hand-grenadoes, we lost more men than we did in the attack itself."*

A retrenchment sometimes cuts off a much greater portion of a work than a single bastion. Thus at Turin, during the French attack of 1706, the citadel was cut right across the middle by a retrenchment separating the attacked bastions from those on the side of the town.†

Of retrenchments generally, Vauban says, that they ought to be made, not on the spur of the moment, but with deliberate foresight; that they should be well revetted, with spacious ramparts, capable of mounting guns of the same weight as the enceinte, with shot-proof parapets, revetted counterscarps, and counter-mines, with all necessary communications; and that they should be so traced as not to interfere with the free use of the flanks and faces before them.‡

*Memoirs of Captain Carleton.

Mil. Hist. of Eugene and Marlborough, i. 205.

+ Defence of Fortresses, quoted by Cormontaingne.

2. Most important places are furnished with casemates. These are bomb-proof vaults, constructed under the rampart of the bastions, or elsewhere, to afford barrack accommodation for the garrison, and an additional fire upon the ditch, through embrasures pierced in the escarp. Good casemated flanks, being protected from enfilade and vertical fire, are of great importance to the defence of the ditch; and it is a disadvantage of Vauban's tenaille that it precludes the use of casemated artillery on the flanks of the bastions. General Haxo, an eminent French engineer, who directed the attack of Antwerp citadel in 1832, has proposed a species of casemate for the protection of guns on the terreplein of the rampart. Guns in battery, hospitals, &c., are frequently protected from vertical fire by timber bomb-proofs as a substitute for casemates. These are called blindages.

In many of Vauban's works, and in older fortresses, bastions were constructed with orillons and retired flanks.

The orillon was a round tower or curved projection at the shoulder of the bastion, intended to conceal and cover the flank, which is usually concave. Cut 6 shows one form of this

construction. The oril

Cut 6.

lon a b occupies of the length of the flank, and its curve is tangent to the face at the shoulder angle. The chord e d of the retired flank is 10 yards in rear of the originally traced flank. Its length is determined by two lines, be and 3 d, the first drawn from the salient of the collateral bastion through the extremity of the orillon, the other being the line of defence produced beyond the curtain angle. The segment e d is one-sixth of a circle, be is called the reverse of the orillon, and there is generally a passage through this to the bottom of the ditch; d 3 is called the brisure of the curtain.

When Vauban employed the orillon, the faces of his ravelin were directed on the shoulder angles of the bastion, and the

G