FIELD FORTIFICATION.

Field fortification is the art of constructing all kinds of temporary works in the field. An army intrenched, or [7:9:784]fortified, in the field, produces, in many respects, the same effect as a fortress; for it covers a country, supplies the want of numbers, stops the advance of a superior enemy, or, if he chooses to risk a battle, obliges him to engage at a disadvantage. “In a war of march and manoeuvre,” says Napoleon, “if you would avoid a battle with a superior army, it is necessary to intrench every night, and to occupy a good defensive position. Those natural positions which are ordinarily met with are not sufficient to protect an army against superior numbers without recourse to art. Those who proscribe lines of circumval-Jation, and all the assistance which the science of the engineer can afford, deprive themselves gratuitously of an auxiliary which is never injurious, almost always useful, and often indispensable. It must be admitted at tłļe same time that the principles of field fortification require improvement. This important branch of the art of war has made no progress since the times of the ancients. It is even inferior to what it was two thousand years ago. Engineer officers should be encouraged in bringing this art to perfection, and in placing it on a level with the rest.”^[1. Military Marims of Napoleon. ]

Whenever Napoleon had time and occasion for strengthening his position by field-works, he acted upon the principles recommended in the above extract, as almost all his predecessors had done. In the wars which followed the revolution of 1688, in those of Queen Anne’s reign, and during the Seien Years’ War, we find the commanders of each period, William III. the Duke of Marlborough, Marshal Villars, Marshal Saxe, Frederick II. and Marshal Daun, practically exemplifying their conviction of the great utility of field-works. A few redoubts saved Peter the Great at Pultowa, and enabled him to gain a decisive victory over his formidable antagonist; and at Borodino, some slight open field-works, thrown up by the Russians, caused the French great loss, and rendered too costly to be of almost any avail the victory which, by incredible efforts of gallantry, they gained. It has been argued by some, against intrenchments and field-works, that they have offener been carried than successfully defended, and that hence incommensurate importance has been attached to them. But it should be remembered, on the other hand, that victory in such circumstances has generally been purchased at an expense which rendered it in effect equivalent to defeat; and that a practice which the greatest commanders of ancient and modem times have approved and followed, cannot be one of doubtful utility. At Austerlitz, where the contending armies were nearly equal, Napoleon was preparing to superintend the construction of intrenchments when he found himself called upon to receive battle; and in Portugal, the Duke of Wellington showed to what importance the art of the engineer might be turned for influencing, not merely the fortune of a campaign, but the fate of a cause. The lines of Torres Vedras, which formed the ne plus ultra of the powerful French army under Massena, and from which the tide of war was rolled back broken into Spain, were perhaps the most remarkable works of the kind ever constructed.

“Lisbon," says Sir John Jones, “being situated at the extremity of a peninsula formed by the sea and the Tagus, it is plain that if an arm}' be so posted as to extend across the peninsula, no enemy can penetrate into the city without a direct attack on the army so formed. It was on this principle that the lines covering Lisbon were planned by Lord Wellington. Nature drew the rude outline of a strong defensive position, and art rendered it perfect. A tract of country thirty miles, extending from the mouth of the Zizandra on the océan, to Alhandra on the Tagus, was modelled into a field of battle; mountains were scarped perpendicularly, rivers dammed, and inundations formed; all roads favourable to the enemy were destroyed, and others made to facilitate the communications of the defenders; formidable works were erected to strengthen and support the weak parts, whilst numerous cannon places, on inaccessible points, commanded the different approaches to them, and gave an equality of defence to the whole position."^[2. War in Spain, p. 124. The French army which invaded Portugal under Massena consisted of three corps, under Marshals Ney and Junot and General Regnier, amounting in all to 66,000 infantry and 6000 cavalry, besides a strong body of the imperial guard, which crossed the Pyrenees after the invading force had commenced its march from the neighbourhood of Salamanca. The force collected to oppose this threatened invasion did not exceed 48,000 infantry and 3000 cavalry, of which about a half was composed of Portuguese levies, yet untried in any general action, and of which a very unfavourable opinion still continued to be entertained. In point of numbers, and still more in the composition of their army, therefore, the French had a decided superiority; but all their advantages were neutralised by the defensive position of Torres vedras. ] These lines were not continuous and connected works; they consisted of independent forts, redoubts, flèches, redans, batteries, &c. so placed as to command and enfilade every approach, and to support each other by a cross or a flanking fire. The first line occupied a front of twenty-nine miles between the sea and the Tagus; and by means of telegraphs intelligence could be conveyed from one extremity to the other in a few minutes; whilst the troops were disposed in masses in the rear of the works, ready to move upon any point that might be attacked, by interior communications shorter than any by which the enemy could advance. “The aim and scope of these works,” says Colonel Napier, “was to bar the passes, and to strengthen the fighting positions between them, without impeding the movements of the army. These objects were attained; and it is certain that the loss of the first line would not have been injurious, save in reputation, because the retreat was secure upon the second and stronger line, and the guns of the first were all of inferior calibre, mounted on common truck carriages, and consequently immovable and useless to the enemy.”^[3. History of the War in the Peninsula, vol. iii. ] Both lines occupied a front of fifty miles, on which there were erected one hundred and fifty forts, mounting in all about six hundred pieces of artillery.

Before this formidable position, defended by a double line of works, and by an array massed and ready to move upon any point by interior communications, the French remained five months, wasting their numbers and resources; until at length, finding it utterly impracticable to force any part of even the exterior line, they were obliged to retire from Portugal, closely followed and harrassed by the army which they had previously driven out of Spain. Yet though the lines of Torres Vedras were thus perfect in themselves, and though one of the ablest of the French generals and a veteran French army were foiled before them, it is not meant to refer to this system of separate field-works as a model to be followed on all occasions; for whilst the old method of covering a considerable front by a continued line of regular bastions and curtains has been universally condemned by modern engineers, it is nevertheless certain that there are situations where a partial application of continued lines may be most judiciously made. In fact, it is not by any fixed rule, but from the [7:9:785] nature of the ground and of the position to be defended, that the species of works calculated to be most useful should, in every case, be determined.

It must be obvious indeed that even a limited knowledge of the art of war opens a wide field for the exercise of the talents and resources of engineers in field fortification; but the possession of a military coup-d’oeil, or of that intuitive judgment which comprehends at a glance the true bearing or character of objects as well as events, is necessary to enable them to convert theoretical stores of information to the best practical uses. In passing through a country, it requires an experienced eye to seize quickly on whatever it presents calculated to prove advantageous or disadvantageous to an army destined to attack or defend it; to appreciate the value of villages, stone-enclosures, and broken ground; to know where to dam up rivers, to scarp heights, to form abattis, trous-de-loup, and other obstacles; to select the best situations for field-forts and redoubts, and the best emplacements for batteries; and to arrange all the defensive means employed, with reference to the number of troops destined to act upon the different parts of the line, so that the movements of the defenders may not be obstructed or retarded, and the communications throughout may be short and easy. The variety of ground, however, upon which military operations are for the most part carried on, precludes the possibility of laying down fixed rules in regard to this subject; the accidents of ground, and the peculiar circumstances of each individual case, must, as already observed, determine the extent and description of the works to be constructed, as well as the obstacles most proper to be formed for retarding, if not obstructing, the advance of an enemy.^[4. ≡nΓ⅛Se 9, t e hVX J" t l ⅛ 8eful tπ * ct we have been indebted for assistance under hi s irl β p βc tin" P ’ ' pUteS connected "inch are engraved from drawings either executed by him or ]

At the same time, though the observance of fixed rules be impracticable, general principles are of universal ap∙ plication; and hence some of the received maxims in permanent fortification hold equally good in regard to the construction of field-works. These are, first, that the works to be flanked must never be beyond the range of the missiles discharged from the works flanking them, or in other words, never out of the effective range of musketry, that is, from 150 to 200 yards; secondly, that the angles of defence should be about right angles; thirdly, that the salient angles of all works should be as obtuse as possible; fourthly, that though ditches cannot always be as fully flanked as in permanent fortification, yet partial flanking must be carried as far as possible; and,jZ∕lΛZy, that in the construction of field-works, reference should not only be had to the direct and immediate obstacles which the work itself is calculated to present to the enemy, and the positive effect of the fire on the approaches to it, but the relative value of the work should likewise be considered, with respect to the support it can receive from or give to other works. These principles or maxims are of invariable application.

It is usual to divide field-works into three classes: First, works open at the gorge; secondly, works enclosed all round; and, thirdly, lines at intervals.
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The first class are of the simplest kind of field-works, and serve as a mere cover in front of avenues, bridges, causeways, and the like; but being quite open at the gorge, they are only suited for defence when their extremities rest on rivers, or obstacles which prevent their being turned, or when within the full sweeping fire of works in their rear. To increase the strength of a redan, its faces are sometimes broken into a kind of flank, as in fig. 1, Plate CCXLIX. The double redan, or queue d’hironde, fig. 2, is considerably stronger, as the re-entering faces defend each other; the tenanted heads in situations which require a greater extent of front; and the bastioned heads are also employed in similar circumstances. See fig. 3 and 4.

Redoubts are works closed on all sides; they are constructed of a square or polygonal figure, but most commonly square, as, when of this form, each front can furnish a strong perpendicular fire. Provision should be made for defending the ground before the angles, which, however, are sometimes rounded or cut en crémaillière, so that a fire may be delivered from them. See fig. 5.

Star-forts were proposed in order to remedy the defects of redoubts having the ground before them undefended by a flanking fire, so that a cross fire might be delivered from the adjacent sides. But, according to Jomini, “starforts are the very worst description of fortification; they cannot have flanks, and the re-entering angles take so much from the interior space that it is impossible to place troops and artillery in them sufficient for their defence:” an opinion confirmed by the practice of Sir Richard rletcher and Sir John Jones in the construction of the lines of Torres Vedras, where the trace of the redoubts was made subservient to the conformation of the ground, to the object in view, and to the protecting them as much as possible from the fire of the enemy’s position.

In bastioned forts, fig. 7, the principles of construction correspond with those detailed in permanent fortification; and the flanking defence thus obtained for the ditch is nearly perfect. As bastioned forts are only constructed m cases of great importance, no labour or expense should be spared in the formation of such works.

Forts with demi-bastions, fig. 12, are objectionable, as the ditches are only defended by an oblique fire from the sides. The parapets of these works should be of sufficient thickness to resist the fire of the heaviest guns that can be brought against them. In some cases, however, the parapets need only be strong enough to resist the fire [7:9:786]of light field-guns, whilst in others it will be sufficient if they serve as a cover to the men within them against musketry. The latter kind is generally that which is thrown up in an evening after taking up a position, and which, if the army does not move next day, it may be considered as necessary to strengthen in some parts, according to circumstances.

Continued lines, or connected works, are resorted to in order to enclose the front, or to connect important works or forts. The most simple tracing is that of redans joined together by curtains, fig. 9; but as the ditches of these curtains can only be defended by an oblique fire from the faces of the redans, this defect may be remedied by breaking the curtains so as to form nearly right angles with the faces of the redans, in which case they are called lines of tenailles. Lines en crémailliere have long faces, with flanks perpendicular to these, in order to defend their ditches. When these faces can be directed towards ground upon which it is impracticable to establish enfilading batteries, the construction is considered as good.

Bastioned lines are the strongest trace which can be given to continued lines, when the ground will admit of such. A perfectly regular trace is only suited for level ground. The ditches in field-works are often sloped en rampe towards the adjoining flanks, in order that the déblai, or quantity of earth excavated, may not exceed the remblai, or quantity contained in the mass of the rampart or parapet, a circumstance which often occurs in fieldworks, where there is seldom any rampart, and only cover sufficient for the defenders.

Lines with intervals, fig. 14, shows the general trace of lines of this kind. The salient works should never be beyond the range of musketry from the re-entering works, and the angles of defence between the two lines should be as near as possible to right angles.

Tĉtes-de-pont, or bridge-heads, are works generally open at the gorge, and whose flanks rest upon a river, in order to cover one or more bridges. The best situation for these works is the re-entering sinuosity of a river. As têtes-de-pont, fig. 13, are usually constructed for the purpose of enabling a retiring army to cross a river in order, and to check an enemy pressing upon it, the tracing and profile should be such as to secure a double advantage to the greatest extent possible. In Sir Howard Douglas’s able work on the Construction of Military Bridges will be found much valuable scientific information upon this important subject.

To field-works there are usually added a variety of obstacles, in order to render the approaches more difficult to the enemy; such as palisades, barriers, abbatis, trous-de-loup, chevaux-de-srise, harrows, and crows' -feet. Palisades are strong stakes fixe*d in the ground, and well sharpened at the upper extremities. They are sometimes placed horizontally in the face of the parapet, in which case they are called fraises. Barrier-gates, for closing the entrances of field-works, should be made strong enough to resist a sudden attack; they are always placed on the most inaccessible side. Abattis are lines of felled trees of considerable size, strongly bound together and picketed down, with the smaller branches cut off, and the remainder sharpened and pointed towards the enemy. They are interlaced as much as possible. Abattis, well disposed, present a formidable obstacle. Trous-de-loup are rows of pits, made before a work, in the form of inverted cones, having each a strong palisade or stake in the middle. Chevaux-de-srise are formed by inserting iron-pointed stakes, five or six feet in length, into a heavy beam of wood, which will thus, if it be square, present four projecting spears, and if hexagonal, six. Chevaux-de-srise are generally chained together in a line, in order to cover the ground required; and sometimes they are so constructed as to revolve on an axis. Harrows, such as those ordinarily used in husbandry, are recommended to be spread, with the teeth uppermost, over the ground by which an enemy, particularly cavalry, is likely to advance to the attack. Crows'-feet are pieces of iron, with four points, each four or five inches long, projecting in different directions, and also intended to impede the advance of cavalry.

For the defence of open towns and villages, the following methods, recommended by the French minister of war in 1814, are considered as the best that have yet been suggested: “To admit of a town being advantageously intrenched, it is necessary that it should not be commanded within any short distance, that the houses should not be of a construction easily set on fire, and that its extent should not be out of proportion to the means and time at the disposal of the defenders. The first thing to be done is to clear the approaches to the town, by levelling houses, hedges, shrubberies, and whatever may favour the assailants. Wood ought to be cut two feet from the ground, that it may serve to impede the advance of the enemy without masking the fire of the defenders. The next object is to form or complete the enclosure round the town. For this purpose advantage is taken of buildings, walls, and fences applicable to the defence. The openings which remain must be closed by palisades, stockades, or ditches strengthened by abattis. All streets leading out of town must be barricaded. The barricades must be sufficient to resist field artillery, and high enough not to∙be easily got over; and they ought to be flanked by loopholing the neighbouring houses. When pressed for time, carts filled with dung and the wheels taken off, sand-bags, bales of wool or cotton, and furniture taken from the neighbouring houses, all form good barricades. If there should be any old castle, church, or large substantial building, it should be converted into a keep, by blocking up useless entrances, loopholing walls, and surrounding them by a ditch or abattis. If a town is situated near a stream or river, by which part of it may be covered by inundations, this should never be neglected.”

Villages are intrenched on similar principles, and being generally surrounded by gardens with live hedges, the latter may be made use of in forming the lines of defence. If there should only be sufficient troops to defend part of a village or town, a part only should be intrenched and separated from the rest by means of carts and barricades. If there are very few houses, it may be necessary to confine the defence to the church or churchyard, which may in all cases serve as a sort of keep.

The destruction of bridges. Nothing is of greater consequence to a retiring army than to be able to destroy the bridges in its rear, in order to retard the advance of the enemy. Its safety, nay even its existence, may depend upon the success with which this operation is performed. In order to destroy a stone-bridge, a trench in the form of a cross is made in the crown of the arch, the branches of which are about ten feet in length, and sunk to the top of the arch-stones. A hundred and sixty pounds of powder are placed in each cut or trench for an arch three feet thick, strong planks are then laid over the powder, and the whole being well covered with rubbish, the fire is communicated by means of saucisson or long powder-hose. Stone bridges are also destroyed by simply cuttinga trench about eighteen inches deep across the crown of the arch, and placing in it 345 pounds of powder covered in the manner just described. This quantity has been found sufficient to destroy semicircular arches of twenty-five feet in span, and three feet in thickness at the key. Wooden bridges may be destroyed in different ways; [7:9:787]they may be pulled to pieces, burned, or blown up. When there is time to take them to pieces, they are unspiked, and the timbers so separated that they may be speedily removed. The best method of burning such bridges is to tar them, and to cover and surround them with fascines or tarred brushwood. When it is necessary to blow up wooden bridges, this may be effected by means of 220 pounds of powder suspended under the superstructure, and fired in the manner above described.

A ford is rendered impassable by throwing in large stones, by sinking boards with spikes standing upright in them, by scattering in it crows’-feet, or by placing harrows taken from the neighbouring farms. A low wall may also be built across, so as not to be perceptible above the water; strong stakes may be driven into the bottom,and trees fastened to them; waggons loaded with stones, and the wheels taken off, may in like manner be employed; not to mention a number of other things which may be easily found, and which will answer the purpose equally well. The rendering a ford impassable by such means is only second in importance to the destruction of a bridge, when the enemy whose progress it is desired to retard has either no pontoon-train at all, or has outstripped it by the rapidity of his advance. In this way much valuable time may be gained on the one hand and lost on the other. (a.)