H. Wager Halleck , A. M. , Lieut. of Engineers, U. S. Army ., Elements of Military Art and Science; or, Course of Instruction in Strategy, Fortification, Tactis of Battles &c., Embracing the Duties of Staff, Infantry, Cavalry, Artillery and Engineers. Adapted to the Use of Volunteers and Militia.

Chapter 14: field-engineering.—Field Fortifications.—Military Communications.—Military Bridges.—Sapping, Mining, and the attack and defence of a fortified place

Military reconnaissance.--By this term is meant an examination of a portion of the theatre of war, to ascertain its military character and resources. If the examination be made of a large district of country, and for an entire campaign, the reconnaissance is general; if made for collecting detailed information respecting a proposed line of march, the passage of a river, the position of an enemy, &c., it is termed special.

In making a general reconnaissance, great care should be taken to collect accurate information respecting the general topography of the country; the character of the mountains, forests, and watercourses; the nature of the roads, canals, and railways; the quality of the soil, and the amount of provisions and forage it produces ; the population and character of the cities, towns, and villages; the commercial and manufacturing resources of every part of the country, and the means of transportation to be found in each district. The plan of military operations will be based on the information thus obtained, and any serious error in the reconnaissance may involve the results of the campaign, and even the fate of the war.

In a special reconnaissance, not only accurate but minute [343] information will be required: the character of the roads must be given in detail; the nature of the watercourses, their depth and velocity the position and character of bridges, and fords ;--in fine, a full description of all obstacles to be encountered, and the means that can be made available for overcoming these obstacles.

A reconnoitring officer may usually derive much valuable information from the published maps and descriptions of the country to be examined; additional matters of detail may be obtained from woodsmen, hunters, and fishermen; and also from the innkeepers and local authorities of the district. But the officer should always verify this information, so far as practical, by personal examination. In making a reconnaissance in the vicinity of an enemy, he must be supported by a strong escort of mounted troops, and in all his operations the greatest precaution will be requisite to ensure success.

Some simple instrument, such as a, pocket sextant, or compass, will be sufficient to enable the reconnoitring officer to measure, with considerable accuracy, the height of mountains, the width of streams. &c., and an ordinary scale and dividers will enable him to make a suitable military sketch.

Temporary Fortification.--It has been stated in the preceding chapter that temporary fortifications are properly confined to the operations of a single campaign, and are used to strengthen positions which are to be occupied only for a short period ; and that they are usually made of earthly thrown up by the troops in a single day. Temporary fortifications, as a part of field-engineering, may therefore be regarded rather as an arm than art. The principles of their construction are derived, of course, from the theory of permanent fortification, but in applying these principles to practice in the field, much greater latitude is allowed than in the exact scientific arrangement of permanent works. [344]

The purpose of field-works (or intrenchments, as they are commonly called) is to arrest, or at least to impede, the march of the attacking foe; to shelter the defensive troops from the missive weapons of the assailants, and to detain them in a position where they will be exposed to the fire of the defensive force. The numerical and positive strength of the assailed may be much less than that of the assailant, and yet an equilibrium exist; the material obstacles compensating for the difference in numbers. Intrenchments, though inert masses, must therefore be regarded as most valuable and important accessaries in the defence of a position.

Intrenchments consist either of lines of works made to cover extended positions, or of detached works designed simply to defend the ground they occupy. The former generally present a front against the enemy in but one direction, while the latter are usually closed on all their sides.

The following figures have been employed for the plan of simple intrenchments, viz.: the polygon, redan, lunette, mitre, star-fort, and bastion.

Square or polygonal redoubts are the most common forms given to field-works, on account of the ease of their construction. But they have many defects. There is a sector without fire in front of each salient, and the ditches are without protection. The latter objection also holds good against all circular works.

The redan (Fig. 45) is frequently used to cover a point in rear, as a bridge, a ford, or a defile. When used alone, its gorge should be closed by palisades. Its ditches are unprotected.

The lunette (Fig. 46) has nearly the same defects as the redan.

The mitre, or priest-cap, (Fig. 47,) may be employed with advantage when a cross-fire is required on the capital of the work [345]

The star-fort has all the defects, without the merit of simplicity, which belong to the polygonal redoubt.

The bastion-fort (Fig. 48) more fully satisfies the conditions of a good defence than any other plan; but it is less simple and easy of execution. It is usually composed of four or five fronts, but it may be applied to a polygon of any number of sides.

For the details of the construction of these several works, we must refer to the special treatises on field-fortification.

Lines of intrenchments may be made either continuous or with intervals. In adopting either plan, the engineer should avail himself of all the natural obstacles presented by the position, so as to diminish the labor of erecting artificial means of defence.

The simplest arrangement for a continuous intrenchment is the cremailliere, or indented line. When applied to an irregular site, or used to connect together distant and detached works, the indented line may be regarded as a good disposition. Mitres and redans, connected by straight curtains, are sometimes employed, as also a combination of large and small redans, forming alternate salient and re-entering angles. A continuous line of bastions is preferable to any other arrangement, when there is plenty of time for their construction.

Lines with intervals are frequently formed of alternate lunettes and square redoubts. Other detached works may be employed in the same way. This manner of intrenching a position has several advantages, with disciplined troops. The first shock of the assailant is sustained by the detached works, and when he attempts to penetrate in the intervals, his flanks become exposed to a deadly cross fire. These intervals also allow the assailed to act on the offensive, by charging the enemy at the opportune moment. But with raw and militia forces it will be safer to resort [346] to continuous lines. If cavalry form any part of the defensive force, it will be absolutely necessary to leave intervals through which these troops may charge.

A vertical section of all intrenchments is of the same general form; the dimensions will, of course, vary with the nature of the soil, and the time and means employed in their construction. The minimum dimensions that can be used with any considerable advantage are given in Fig. 49.

In laying out field-works advantage should be taken of all available artificial obstacles, such as hedges, walls, houses, outbuildings, &c. A thickset hedge may be rendered defensible by throwing up against it a slight parapet of earth. Stone fences may be employed in the same way. Walls of masonry may be pierced with loopholes and arranged for one or two tiers of fire. The walls of houses are pierced in the same manner, and a projecting wooden structure, termed a machicoulis gallery, is sometimes made from the floor of the second story, to enable the assailed to fire down upon their opponents. This arrangement is frequently employed to advantage in wooden blockhouses against a savage foe; but it is of little avail when exposed to the fire of artillery. Some have proposed galleries of this description in permanent works of masonry, but the project is too obviously absurd to merit discussion.

In addition to the parapet of an intrenchment, a good engineer will always find time and means for constructing other artificial obstacles, such as trous-de-loup, abattis, palisades, stockades, fraises, chevaux-de-frise, crows'-feet, mines, &c.

Trous-de-loup are pits dug in the earth in the form of an inverted truncated cone, some six feet in diameter, and about the same number of feet in depth. They are usually placed a few yards in front of the ditch, and concealed by some slight covering.

Abattis are tops and large limbs of trees arranged along [347] the glacis of a work; the ends of the branches are lopped off and sharpened.

Palisades are stakes some eight or ten feet long, with one end fastened in the ground and the other made sharp. They are placed in juxtaposition and connected together by horizontal riband-pieces. This arrangement is frequently placed at the foot of the counterscarp. When the timbers are large and the work is intended as a part of a primary defence, it is called a stockade; when the stakes are placed at the foot of the scarp, either horizontally or inclined, they receive the name of fraises.

A cheval-de-frise consists of a horizontal piece of timber armed with wooden or iron lances, which project some eight or ten feet. It is much employed against cavalry, and on rocky soils serves as a substitute for palisades.

Crows'-feet are small wooden or iron forms filled with sharp spikes. They are thrown, with their points up-ward, on ground which is to be passed over by cavalry.

Mines are sometimes used in connection with intrenchments, but more commonly in the attack and defence of permanent works. They will be noticed further on.

Field-works which are to be occupied for a consider. able length of time will usually have their steeper slopes revetted, and be arranged with scarp and counterscarp, galleries, traverses, blindages, &c. Such works hold an intermediary rank between temporary and permanent fortification.

As examples of the importance of field fortifications and of the manner of organizing them, the reader is referred to the celebrated battle of Fontenoy, in 1745, where the carefully-arranged intrenchments of Marshal Sax e enabled the French to repel, with immense destruction, the attacks of greatly superior numbers; to the battle of Fleurus, in 1690, where the Prince of Waldeck exposed himself to a most disastrous defeat “by neglecting the resources [348] of fortification and other indispensable precautions;” to the battle of Malplaquet, in 1709, where Marshal Villars, by neglecting to occupy and intrench the farm that closed the passage between the woods of Sars and Laniere, exposed himself to a disastrous defeat; to the operations of 1792, where General Custine, by neglecting to intrench the heights that covered Bingen, as the engineers had recommended, exposed himself to those terrible disasters which forced him to a precipitate retreat; to the works of Wervike, which, by a vigorous resistance on the 10th of September, 1793, saved the Dutch army from total destruction; to the intrenched camp of Ulm, in 1800, which for six weeks held in check the victorious army of Moreau; to the intrenched lines of Torres Vedras, in 1810, which saved from destruction the English army of Wellington; to the field-defences of Hougomont, which contributed so much to the victory of Waterloo, &c.

Military communications.--The movements of armies are always much embarrassed by forests, marshes, and water-courses, and nothing contributes more to the dispatch of military operations than the means of opening practical and easy communication through these various obstacles.

It is not necessary here to enter into any detailed discussion of the manner of constructing military communications through forests or marshes. In a new country like ours, where almost every one has had some experience in road-making, no very great technical knowledge is required for the construction of temporary works of this character; but much professional skill and experience will be requisite for the engineers who make the preliminary reconnaissances, and fix the location of these roads.

Water-courses may be crossed by means of fords, on the ice, or by ferries and bridges. When temporary [349] bridges or ferries are constructed by the army in the field, they are classed under the general head of military bridges, or more properly, pontoniering.

Where the depth of the stream is not great, the current slight, and the bottom smooth and hard, the passage may be effected by fording. If the bottom be of mud, or large stones, the passage will be difficult and dangerous, even where the depth and current are favorable. Under favorable circumstances infantry can ford a stream where the depth is not greater than four feet; cavalry to a depth of four or five feet; but artillery, and engineer trains, cannot go to a depth of more than two and a half feet, without greatly exposing their ammunition and military stores. The fords should be accurately staked out before the passage is attempted, and ropes ought to be stretched across the stream, or cavalry and small boats stationed below, to prevent the loss of life.

Ice may be crossed by infantry, in small detachments. Its strength may be increased by covering it with boards, or straw, so as to distribute the weight over a greater surface. By sprinkling water over the straw, and allowing it to freeze, the mass may be made still more compact. But large bodies of cavalry, and heavy artillery, cannot venture on the ice unless it be of great thickness and strength. An army can never trust, for any length of time, to either fords or ice; if it did a freshet or a thaw would place it in a most critical state. Military bridges will, therefore, become its only safe reliance for keeping open its communications.

Military bridges are made with trestles, rafts, boats, and other floating bodies. Rope bridges are also sometimes resorted to by troops for passing rivers.

Trestle bridges are principally used for crossing small streams not more than seven or eight feet in depth: they also serve to connect floating bridges with the shore, in [350] shallow water. The form of the trestle is much the same as that of an ordinary carpenter's horse, i. e., a horizontal beam supported by four inclined legs. These trestles are placed in the stream, from twelve to twenty feet apart, and connected by string-pieces, (or balks as they are termed in technical language,) which are covered over with plank. The action of the current against the bridge may be counteracted by anchors and cables, or by means of boxes or baskets attached to the legs of the trestles, and filled with stones. A more substantial form may be given to the bridge by substituting for the trestles, piles, or the ordinary framed supports so much used in the newer parts of our country.

For examples of the use of bridges of this description we would refer to Caesar's celebrated bridge across the Rhine; the passage of the Scheldt in 1588 by the Spaniards; the passage of the Lech in 1631 by Gustavus Adolphus; the passage of the Danube in 1740 by Marshal Saxe; the great bridge across the Var during Napoleon's Italian campaigns; the passage of the Lech in 1800 by Lecourbe; the bridges across the Piava, the Isonso, &c., in the subsequent operations of the army in Italy; the celebrated passage of the Danube at the island of Lobau in 1809; the passage of the Agueda in 1811 by the English; the passages of the Dwina, the Moscowa, the Dneiper, the Beresina, &c., in the campaign of 1812; the repairing of the bridge near Dresden, and the passage of the Elbe in 1813, &c.

Rafts formed of timbers, casks, barrels, &c., are frequently used as military bridges. They may be made to bear almost any weight, and will answer for the passage of rivers of any depth and width, provided the current be not rapid.

Where the bridge is to be supported by rafts made of solid timbers, these timbers should be first placed in the [351] water, to ascertain their natural position of stability, and then the larger ends cut away on the under side, so as to present the least possible resistance to the action of the current. They are afterwards lashed together by strong rope or withe lashing, or fastened by cross-pieces let into the timbers, and held firm by bolts, or wooden pins. These rafts are kept in place by anchors and cables placed up and down stream. The roadway is formed in nearly the same manner as for a bridge supported on trestles. Empty casks, and other floating bodies, may be substituted in place of logs in the construction of rafts.

For examples of the use of rafts in the construction of military bridges, we would refer to the passage of the Seine in 1465 by Count Charolais; the passage of the Meuse in 1579 by Alexander Farnese; the passage of the Vistula in 1704, the Borysthenese in 1709, and the Sound in 1718, by Charles XII.; the passage of the Adige in 1796; the passage of the Po in 1807; and the subsequent military operations in the Spanish Peninsula.

Military bridges are frequently made of boats, and the ordinary river-craft found in the vicinity of the intended passage. Flat-bottomed boats are the most suitable for this purpose, but if these cannot be obtained, keel boats will serve as a substitute. When these water-craft are of very unequal sizes, (as is frequently the case,) two smaller ones may be lashed together to form a single support; they can be brought to the same level by means of stone ballast. The gunwales must be suitably arranged for supporting the balks, or else frameworks should be erected for this purpose from the centre of the boat. The arrangement of the roadway, anchors, &c., is the same as before.

A bridge-equipage made to follow an army in its movements in the field, is generally composed of light skiffs or batteaux, and the necessary timbers, planks, anchors, &c., [352] for forming the roadway, and keeping the bridge in its position. All these articles are constructed especially for this purpose. All the wood-work should be of tough and well-seasoned timber, so as to impose no unnecessary weight on the wagon trains. The bateaux should also be made of strong and light materials. For convenience in transportation, these boats are sometimes made with hinges so as to fold up. The ribs are usually of oak, and the sides and bottom of pine. Instead of plank, a covering of tin, copper, India-rubber, &c., has sometimes been substituted. Floating supports of this character are often made in compartments, so as to prevent their sinking when injured by the enemy's projectiles. Indian-rubber pontons may be folded up into a small space, and their slight weight renders them convenient for transportation.

On navigable streams a part of the bridge resting on one or two bateaux should be so arranged that it can be shipped out of its place, forming a draw for the passage of river-craft. Indeed, it would be well, even where the river is not navigable, to form a draw for the passage of trees, and other floating bodies, sent down by the enemy against the bridge.

An ordinary bridge-equipage of bateaux, or light pontons, for crossing a river of from three to four hundred yards in width, and of moderate current, will require a train of from sixty to eighty wagons.¹ Under favorable circumstances, and with a well-instructed corps of pontoniers, the bridge may be thrown across the river, and prepared for the passage of an army in a few hours at most.² After the troops [353] have passed over, the bridge may be taken up, and replaced on the wagons in from a quarter to half an hour.

The following examples will serve to illustrate the use of different kinds of boat-bridges in military operations :--the passage of the Rhine, in 1702, by Villars; the passage of the Dnieper and the Bog, in 1739, by the Russians; the passage of the Danube, in 1740, by Marshal Saxe; the passage of the Rhine, near Cologne, in 1758, by the Prince of Clermont; the passage of the Rhine, in 1795, by Jourdan; the passage of the Rhine, at Kehl, in 1796, by Moreau; and again the same year, at Weissenthurn, and at Neuwied, by Jourdan; the bridges across the Rhine, at the sieges of Kehl and Huninguen, in 1797; the passage of the Limmat, in 1799, by Massena; the passages of the Mincio, the Adige, the Brenta, the Piava, &c., in 1800 ; the passages of these rivers again in 1805; the passages of the Narew, in 1807, by the Russians; the several passages of the Danube, in 1709, by the French and Austrian armies; the passages of the Ta.. gus and Douro, in 1810; by the English ; the passages of the Niemen, the Dwina, the Moskwa, and the Beresina, in 1812, by the French; and of the great rivers of Germany and France, in 1813 and 1814.

A floating body, propelled from one bank to the other by the current of the stream, is termed a flying-bridge. The usual mode of establishing a ferry of this kind, is to attach [354] the head of the boat by means of a cable and anchor, to some point near the middle of the stream. By steering obliquely to the current, the boat may be made to cross and recross at the same point. A single passage may be made in the same way, by the action of the current without the cable and anchor, but the boat in this case will be carried some distance down the stream. Rowboats are employed for crossing over infantry by successive debarkations; but this process is too slow for the passage of a large force; it may very well be resorted to as auxiliary to other means.

Steam craft are so common at the present day on all navigable streams, that an army in the field will frequently be able to avail itself of this means of passing the larger rivers. But, in a hostile country, or in one already passed over by the enemy, it will not be safe to rely with confidence upon obtaining craft of this character. A well-organized army will always carry in its train the means of effecting a certain and speedy passage of all water-courses that may intercept its line of march.

Flying-bridges or rowboats were employed in the passage of the Dwina, in 1701, by the Swedes; the passage of the Po, in 1701, by Prince Eugene; the passage of the Rhine, at Huninguen, in 1704; Jourdan's passage of the Rhine in 1795; Moreau's passage in 1796; the sieges of Kehl and Huninguen in 1797; Massena's passage of the Limmat, and Soult's passage of the Linth, in 1799; the passage of the Rhine, at Lucisteig, in 1800; the passage of the Po, by the French, just before the battle of Marengo; and others in Italy, Germany, and Spain, in the subsequent campaigns of Napoleon.

Military bridges have sometimes been formed of ropes, cables stretched across the stream, and firmly attached at each end to trees, or posts let into the earth. If the shore is of rock, rings with staples let into the stone form the [355] best means for securing the ends of the main ropes. Plank are laid on these cables to form the road-way. The ropes forming the “side-rail” of the bridge are passed over trestles at each shore, and then fastened as before. Short vertical ropes attach the main supports to these side ropes, in order thai they may sustain a part of the weight passing over the bridge. Constructions of this character are fully described in Douglas's Essay on Military Bridges. For example, see the passage of the Po, near Casal, in 1515, by the Swiss; the bridge thrown over the Clain by Admiral Coligni, at the siege of Poitiers, in 1569; the operations of the Prince of Orange against Ghent and Bruges, in 1631. ; the passage of the Tagus, at Alcantara, in. 1810, by the English; the bridge constructed across the Zezere, by the French, in 1810; the bridge thrown across the Scarpe, near Douai, in 1820; the experiments made at Fere in 1823, &c.

The passage of a river in the presence of an enemy, whether acting offensively or in retreat, is an operation of great delicacy and danger. In either case the army is called upon to show the coolest and most determined courage, for its success will depend on its maintaining the strictest discipline and good order.

In the case of a retreat the bridge should be covered by field intrenchments, called a tete de pont, and defended by a strong guard. If the river be of moderate width, the enemy may be kept at a distance by heavy batteries on the opposite shore. As soon as the passage is effected by the main body, the bridge, if permanent, will be blown up, or otherwise destroyed by the miners, and if floating, will be swung round to the other shore. The rear-guard will pass over in rowboats, or the end pontons detached for that purpose. An army retreating in the face of an enemy should never rely upon one single bridge, no matter what may be its character; for the slightest accident [356] happening to it might expose the whole army to inevitable destruction.

The passage of a river by main force, against an enterprising and active enemy on the opposite shore, is always an operation of the greatest difficulty, and not unfrequently accompanied with the most bloody results.

The most effectual method of accomplishing this object is by stratagem. Demonstrations are made at several points at the same time: bodies of troops are thrown across, after nightfall, in rowboats or by flying-bridges, to get possession of the opposite bank. The vanguard of light cavalry may cross by swimming. The pontoniers should have their bridge equipage in readiness near the intended point of passage, so that it can be thrown across with the greatest possible rapidity, while the advanced guards are still able to keep the enemy at a distance. Under favorable circumstances the pontoniers will have the bridge in readiness for the passage of the army before the enemy can collect his troops upon the threatened point.

Cannon-balls and hollow shot are the most effectual means for destroying an enemy's bridge when our batteries can be planted within reach. When this cannot be done, we must resort to fire-boats, floating rafts, &c., to accomplish our object. Operations of this kind carried on in the night, are most likely to succeed.

To protect bridges from the action of these floating bodies, stockades, or floating chevaux-de-frise are constructed across the stream at some distance above the bridge; strong cables, or chains stretched directly across the river, or with an angle up stream, may be used in place of stockades, or in conjunction with them. Guards should be stationed above the bridge, with boats, ropes, grapnels, &c., for the purpose of arresting all floating bodies and drawing them ashore, or directing them safely through the draw in the bridge arrangement. [357]

The troops especially charged with the construction and management of the various kinds of military bridges, are denominated pontoniers. The duties of these troops are arduous and important, and, in a country like ours, intersected by numerous water-courses, the success of a campaign will often depend upon their skill and efficiency.

Sapping.--This is a general term applied to the operations of forming trenches, along which troops may approach a work without being exposed to the fire of the besieged.

In addition to the ordinary sapping-tools, such as shovels, picks, gabion-forks, &c., used in constructing trenches, there will also be required a considerable amount of sapping materials, such as gabions, fascines, sap-fagots, sandbags, &c.

The gabion is a cylindrical basket of twigs, about two feet in diameter, and some three feet in length, and without a bottom. It is made by driving into the ground, in a circular form, a number of small pickets about an inch in diameter, and of the length required for the gabion. Twigs are wattled between the pickets like ordinary basket-work, and fastened at the ends by withs or packthread. Gabions are used in forming saps, batteries, blindages, powder-magazines, and in revetting the steep slopes of field-works.

The fascine is a bundle of twigs closely bound up, from nine to twelve inches in diameter, and from ten to fifteen or twenty feet in length. The largest are sometimes called saucissons. In making a fascine, straight twigs about the thickness of a man's finger are laid side by side, and firmly compressed together by a strong rope or chain attached to the extremities of two levers. While held in this position the twigs are firmly bound together by withs or cords. Fascines are used in constructing trenches, batteries, &c., and for filling up wet ditches. [358]

The sap-fagot is a strong fascine about ten inches in diameter and two feet in length, with a picket inserted through the middle. It is used in the double sap in connection with gabions.

Sand-bags are usually made of coarse canvass. When filled with earth they are some six or eight inches in diameter, and from eighteen inches to two feet in length. From their perishable nature, they are used only when other materials cannot be procured, and where it is important to place the troops speedily under cover from the enemy's fire.

Bales of wool, cotton, hay, straw, &c., may be employed in sapping for the same purposes as the above materials, when they can be procured in sufficient quantity. Pork and flour barrels, which are usually in abundance in a camp, are frequently filled with sand and used for forming magazines, blindages, &c., in field-works.

A trench constructed in ordinary soil beyond the range of the enemy's grape, is called a simple sap, or ordinary trench. The earth is thrown up on the side towards the place besieged, so as to form a kind of parapet to cover the men in the trench. The labor is here executed under the supervision of engineer soldiers, by working parties detached from the other arms. Fig. 50 represents a vertical section of a simple sap.

When within range of the enemy's grape, the flying sap is resorted to in order to place the workmen speedily under cover. In this operation, gabions are placed in juxtaposition on the side towards the besieged work, and filled with all possible speed by the workmen. Three rows of fascines are usually placed on the top of the gabions to increase the height. The most difficult part of the flying sap is executed by engineer troops, and tie trench is completed by the ordinary working parties. Fig. 51 represents a section of this sap. [359]

The full-sap is employed when the works of the besiegers are within range of musketry, or when the grape fire of the besieged is so deadly that the flying sap can no longer be used. This is a difficult operation, and unless executed with great care and by well-instructed engineer troops, the construction of the trench will be attended with an immense loss of life. The work must be executed under cover of a sap-roller, which is a cylindrical mass of fascines, wool, or cotton, some two feet in diameter. On very smooth ground a ball-proof shelter on wheels might be used as a substitute. The sap-roller being placed along the line of the trench so as to cover the sapper in front, who is armed with a musket-proof headpiece and cuirass, this sapper commences the sap by placing a gabion on the line of the proposed trench and fills it with earth, working on his hands and knees. Having filled the first gabion, he pushes forward the sap-roller and places a second one next the first, stopping the open joint between the two with a stop-fagot. The second gabion being filled in the same manner as the first, others are successively established. When the first sapper has advanced a few feet, he is followed by a second, also in defensive armor, who increases the excavation and embankment; this sapper is then followed in the same way by a third and a fourth, after which the trench will be sufficiently advanced to be turned over to the ordinary work-men. The sap-fagots may be removed when the embankment becomes thick enough to resist grape. Fig. 52 represents a plan and section of a full-sap.

When the direction of the trench is such that the men are exposed on both sides, it will be necessary to throw up an embankment both to the right and left. This operation is called the double sap, and is executed by two parties of sappers, working side by side. In this sap it will be necessary to frequently change the direction of [360] the trench, or to throw up traverses, in order to cover the men at a distance from the sap-roller. Wing-traverses, on the side of the trench which is least exposed, sometimes serve the same purpose as a double sap.

Mines.--By mining, as a military term, we understand the operations resorted to for the demolition, with powder, of a military structure of any description. The term mine is applied both to the excavation charged with powder for the purpose of producing an explosion, and to the communications which lead to this excavation.

The place in which the charge of powder is lodged is called the chamber, the communication by which this place is reached the gallery, and the excavation made by the explosion is termed the crater.

The form of the crater caused by an explosion in ordinary soils is assumed to be a truncated cone, the diameter, c d, (Fig. 53,) of the lower circle being one-half the diameter, a b, of the upper circle. This form has never been ascertained to be exactly correct, but the theoretical results deduced from a mathematical discussion of this figure have been fully verified in practice. The radius, p b, of the upper circle is termed the crater radius; the line o p, drawn from the centre of the charge perpendicular to the surface where the explosion takes place, is termed the line of least resistance; the line o b, drawn from the centre of the powder to any point in the circumference of the upper circle, is termed the radius of explosion.

When the crater radius is equal to the line of least resistance, the mine is termed common; when this radius is greater than the line of least resistance, the mine is termed overcharged; and when the radius is less, undercharged. A mine of small dimensions, formed by sinking a shaft in the ground, is termed a fougasse. The term camouflet is applied to a mine used to suffocate the enemy's miner, without producing an explosion. Small mines [361] made in rock or masonry, merely for the purpose of excavation, without any considerable external explosion, are called blasts.

From experiments made on common mines, whose line of least resistance did not exceed fifteen feet, it has been ascertained that the tenacity of the earth is completely destroyed around the crater to a distance equal to the crater radius, and that empty galleries would be broken in at once and a half that distance. It has also been proved by experiment, that the crater radius in overcharged mines may be increased to six times the line of least resistance, but not much beyond. this; that within this limit the diameter of the crater increases nearly in the ratio of the square roots of the charge; and that empty galleries may be destroyed by overcharged mines at the distance of four times the line of least resistance.

By means of the deductions of physico-mathematical theory, and the results of experiments, rules have been determined by which the miner can calculate, with much accuracy, the charge necessary to produce a required result in any given soil.

In the earlier stages of the history of this art, mines were only used to open breaches and demolish masses of masonry; but in later times they have been employed as important elements in the attack and defence of places.

An isolated wall, only two or three feet thick, may readily be demolished by exploding one or two casks of powder placed in contact with its base. If the wall be five or six feet thick, the charges should be placed under the foundation. For walls of still greater thickness it will be best to open a gallery to the centre of the wall, a foot or two above its base, and place the powder in chambers thus excavated. Revetment walls may be overturned by placing the charges at the back of the wall, about one-third or one-quarter of the way up from the [362] base. If placed too near the base, a breach will be made in the wall without overturning it.

To demolish a bridge of masonry the powder should be lodged in chambers excavated in the centre of the piers. When there is not time for excavating these chambers in the piers, a trench may be cut over the key of the arch, in which the powder is placed and exploded; or, the casks of powder may be suspended immediately under the arch, with the same results. Where a saving of powder is of consequence, small chambers may be excavated in the haunches of the arch, and the mine carefully tamped before firing it.

Bridges of wood may be destroyed by suspending casks of powder under the principal timbers, or attaching them to the supports.

Palisading, gates, doors, &c., may be destroyed in the same way, by suspending casks or bags of powder against their sides; or still more effectually, by burying the charges just beneath their base.

To demolish a tower, magazine, or house, of masonry, place charges of powder under the piers and principal walls of the building. In wooden structures the powder should be placed under, or attached to the principal supports. Where time is wanting to effect these arrangements, a building may be blown down by placing a large mass of powder in the interior. The powder may be economized, in this case, by putting it in a strong case, which should be connected with the walls of the building on all sides by wooden props.

Special treatises on military mining contain full instructions for regulating the size and position of the charge for the various cases that may be met with in the practical operations of field-engineering.

As applied to the attack and defence of a fortified place, mines are divided into two general classes--offensive and [363] defensive mines. The former are employed by the besiegers to overthrow the scarps and counterscarps of the place, to demolish barriers, palisades, walls, and other temporary means of defence, and to destroy the mines of the besieged. The latter are employed by the opposite party to blow up the besiegers' works of attack, and to defend the passage of ditches against an assault. Small mines called fougasses may be employed for the last named object. The shell-fougasse is composed of a wooden box filled with one or more tiers of shells, and buried just below the surface of the earth. Sometimes a quantity of powder is placed under the shells, so as to project them into the air previous to their explosion. The stone fougasse is formed by making a funnel-shaped excavation, some five or six feet deep, and placing at the bottom a charge of powder enclosed in a box, and covered with a strong wooden shield; several cubic yards of pebbles, broken stone, or brickbats, are placed against the shield, and earth well rammed round, to prevent he explosion from talking place in the wrong direction. These mines are fired by means of powder hose, or by wires connected with a galvanic battery.

The defensive mines employed to blow up the besiegers' works, are generally common mines with the lines of least resistance seldom greater than fifteen feet. All the main galleries and principal branches of mines for a permanent fortification are constructed at the same time with the other portions of the work, leaving only the secondary branches, chambers, &c., to be made during the siege, For the general arrangement of these galleries, and the precautions necessary for their protection from the operations of the besiegers, reference must be made to treatises specially devoted to the discussion of this subject.

Mines can seldom be employed with advantage in works of slight relief, and liable to an assault. But if judiciously [364] arranged in the plan of their construction, and well managed during the operations of the siege, they contribute very materially to the length of the defence.

Attack and defence.--This subject admits of two natural divisions: 1st, of intrenchments, and 2d, of permanent works.

I. Intrenchments may be attacked either by surprise, or by open force. In either case the operations should be based on exact information of the strength of the works and the number and character of the garrison-information that can be obtained from spies, deserters, and prisoners, and confirmed by examinations or reconnaissances made by officers of engineers. By these means a pretty accurate knowledge may be obtained of the natural features of the ground exterior to the works; their weak and strong points; and their interior arrangements for defence.

In an attack by surprise, the troops should consist of a storming party and a reserve of picked men. The attacking column is preceded by a company of sappers armed with axes, shovels, picks, crowbars, &c.; bags of powder are also used for blowing down gates, palisades, &c. All the operations must be carried on with the utmost dispatch. The time most favorable for a surprise is an hour or two before day, as at this moment the sentinels are generally less vigilant, and the garrison in a profound sleep; moreover, the subsequent operations, after the first surprise, will be facilitated by the approach of day. Under certain circumstances, it may be advisable to make false attacks at the same time with the true one, in order to distract the attention of the garrison from the true point of danger. But false attacks have, in general, the objection of dividing the forces of the assailants as well as of the assailed. In all attacks by surprise, secrecy is the soul of the enterprise.

In an open assault, if artillery be employed, the troops [365] should be drawn up in a sheltered position, until the fire of the works is silenced, and breaches effected in the parapet. But if the bayonet alone be resorted to, the troops are immediately brought forward at the beginning of the assault. The attack is begun by a storming party of picked men: they are preceded, as before, by a body of sappers, provided with necessary means for removing obstacles, and followed by a second detachment of engineers, who will widen the passages, and render them more accessible to the main body of troops who now advance to the assistance of the storming party. If the assailants should be arrested at the counterscarp by obstacles which must be removed before any farther progress can be made, the infantry troops of the detachment display and open a fire upon the assailed, in order to divert their fire from the sappers. A few pieces of light artillery, on the flanks of the column, may sometimes be employed for this purpose with great advantage.

The storming party should always be provided with scaling-ladders, planks, fascines, &c., for crossing the ditch, and mounting the scarp. If the counterscarp be revetted with masonry, the troops must either descend by ladders, or fill up the ditch with fascines, bales of straw, bundles of wool, &c.: if not revetted, a passage for the troops into the ditch will soon be formed by the shovels of the sappers. When the ditch is gained, shelter is sought in a dead angle till the means are prepared for mounting the scarp, and storming the work. If the scarp be of earth only, the sappers will soon prepare a passage for the escalade; but if revetted with masonry, the walls must be breached with hollow shot, or scaled by means of ladders.

In the defence, the strictest vigilance should be at all times exerted to guard against a surprise : sentinels are posted on all the most commanding points of the work; all the avenues of approach are most thoroughly guarded, [366] and patroles are constantly scouring the ground in all directions. At night all these precautions are redoubled. Light and fire-balls are thrown out in front of the work to light up the ground, and discover the movements and approach of the enemy. Each man should have his particular post assigned to him, and be thoroughly instructed in the duties he will have to perform. All auxiliary arrangements, such as palisades, abattis, &c., should be defended with the utmost obstinacy; the longer the enemy is held in check by these obstacles, the longer will he be exposed to the grape and musketry of the main work. When he assaults the parapet, he will be opposed by the bayonet in front and a well-aimed fire in flank. While in the ditch, or as he mounts the scarp, hollow projectiles, incendiary preparations, stones, logs, &c., will be rolled down upon his head. But when the assaulting column has gained the top of the scarp, the bayonet forms the most effective means of resistance.

The measures resorted to in the attack and defence of the larger class of field-works, will necessarily partake much of the nature of the operations employed in the attack and defence of permanent fortifications.

II. The attack and defence of a fortress may be carried on either by a regular siege, or by irregular operations and an assault. The latter plan has sometimes been adopted when the works of the place were weak and improperly defended; where the time and means were wanting for conducting a regular siege; or where the assailants were ignorant of the means proper to be resorted to for the reduction of the fortress. Such operations, however, are usually attended by an immense sacrifice of human life, and the general who neglects to employ all the resources of the engineer's art in carrying on a siege, is justly chargeable with the lives of his men. In the siege of Cambrai, Louis XIV., on the solicitation of Du Metz, [367] but contrary to the advice of Vauban, ordered the demilune to be taken by assault, instead of waiting for the result of a regular siege. The assault was made, but it was unsuccessful, and the French sustained great losses. The king now directed Vauban to take the demi-lune by regular approaches, which was done in a very short time, and with a loss of only five men! Again, at the siege of Ypres, the generals advised an assault before the breaches were ready. “You will gain a day by the assault,” said Vauban, “but you will lose a thousand men.” The king directed the regular works to be continued, and the next day the place was taken with but little loss to the besiegers.

But a work may be of such a character as to render it unnecessary to resort to all the works of attack which would be required for the reduction of a regular bastioned fort, on a horizontal site. For example : the nature of the ground may be such as to enable the troops to approach to the foot of the glacis, without erecting any works whatever; of course, all the works up to the third parallel may in this case be dispensed with without any violation of the rules of a siege. Again, the point of attack may be such that the other parts of the place will not flank the works, of approach; here a single line of boyaux, and short parallels may be all-sufficient.

But for the purpose of discussion, we will here suppose the place besieged to be a regular bastioned work on a horizontal site, (Fig. 54.)

The operations of the siege may be divided into three distinct periods.

1st. The preliminary operations of the attack and defence previous to the opening of the trenches.

2d. The operations of the two parties from the opening of the trenches to the establishment of the third parallel.

3d. From the completion of the third parallel to the reduction of the place. [368]

First period. The object of the investment of the place is to cut off all communication between the work and the exterior, thus preventing it from receiving succors, provisions, and military munitions, and also to facilitate a close reconnoissance of the place by the engineers, who should always accompany the investing corps, and pursue their labors under its protection. This corps should be composed chiefly of light troops — cavalry, light infantry, horse artillery, “brigades of engineers and mounted sappers,” --who march in advance of the besieging army, and, by a sudden movement, surround the work, seize upon all the avenues of approach, and carry off every thing without the world that can be of service either to the garrison or to the besiegers. To effect this object, the enterprise must be conducted with secrecy and dispatch.

The investing corps is now distributed around the work in the most favorable positions for cutting off all access to it, and also to prevent any communication with the exterior by detachments from the garrison, and even single individuals are sent out to give intelligence to a succoring army or to reconnoitre the operations of the besieging corps. These posts and sentinels, called the daily cordon, are placed some mile or mile and a half from the work, and beyond the reach of the guns. But in the night-time these posts are insufficient to accomplish their object, and consequently as soon as it is dark the troops move up as close to the work as possible without being exposed to the fire of musketry. This arrangement constitutes the nightly cordon.

By the time the main army arrives the reconnoissance will be sufficiently complete to enable the chief engineer to lay before the general the outline of his plan of attack, so as to establish the position of his depots and camp. These will be placed some two miles from the work, according to the nature of the ground. As they occupy a [369] considerable extent of ground around the work, it will generally be necessary to form intrenchments strong enough to prevent succors of troops, provisions, &c., from being thrown into the place, and also to restrain the excursions of the garrison. The works thrown up between the camp and besieged place are termed the line of countervallation, and those on the exterior side of the camp form the line of circumvallation. These lines are generally about six hundred yards apart. It is not unusual in modern warfare to dispense with lines of circumvallation, (except a few detached works for covering the parks of the engineers and artillery,) and to hold the succoring army in check by means of an opposing force, called the army of observation.

The measures of defence resorted to by the garrison will, of course, be subordinate, in some degree, to those of attack. As soon as any danger of an investment is apprehended, the commanding general should collect into the place all the necessary provisions, forage, military munitions, &c., to be found in the surrounding country; all useless persons should be expelled from the garrison; a supply of timber for the works of the engineers and artillery, fascines, gabions, palisades, &c., prepared; all ground within cannon range around the work levelled; hedges and trees cut down; holes filled up ; temporary buildings demolished or burnt; and all obstacles capable of covering an enemy and interrupting the fire of the work, removed.

During this period the engineer troops and working parties detached from the other arms will be most actively employed. As soon as the investing corps makes its appearance, bodies of light troops are thrown out to cut off reconnoitring parties, and, if possible, to draw the enemy into ambush. To facilitate these exterior operations, and to prevent a surprise, several guns of long range are [370] placed on the salients of the bastions and demi-lunes, and others, loaded with grape, in the embrasures of the flanks so as to sweep the ditches. About one-third of the garrison may be employed in exterior operations, and the other two-thirds in arranging the means of defence in the interior.

Second period.--As soon as the engineers have completed their reconnaissances and determined on the front of attack, and all the other preparations are made, the general will direct the opening of the trenches. The ground being previously marked out, battalions of light troops, termed guards of the trenches, as soon as it is dark, are placed about thirty yards in front of the first parallel, (A. Fig. 54,) with smaller sections, and sentinels about the same distance further in advance. These guards lie down, or otherwise conceal themselves from the fire of the work. The engineer troops and detachments of workmen being first marched to the depots and supplied with all the necessary tools for carrying on the work, now commence their labors under the protection of these guards. By daybreak the construction of the first parallel, and the trenches connecting it with the depots, will be sufficiently advanced to cover the men from the fire of the place; the guards will therefore be withdrawn, and the workmen continue their labors during the day to give the trenches the proper size and form.

The parallels are the long lines of trench which envelop the besieged work, and serve both as covered ways for the circulation of the besiegers, and as means of defence against sorties from the garrison; they are therefore arranged with banquettes for musketry fire. The boyaux are trenches run in a zigzag direction along the capitals of the front of attack, and are intended exclusively for the circulation of the troops; they have no banquettes. The first parallel is about six hundred yards from the place, [371] and consequently beyond the reach of grape. It is constructed by the simple sap. After the first night, the guards, instead of advancing in front of the work, are placed in the trenches.

The second parallel (B) is made some three hundred or three hundred and fifty yards from the place, and being much exposed to grape, the flying-sap is employed in its construction. Batteries (H) are established between the first and second parallels to silence the fire of the demilunes of the collateral bastions, and others (I) near the second parallel, to enfilade the faces of the front of attack. These are armed in part with mortars and in part with heavy siege-pieces.

The works are now gradually pushed forward to the third parallel, (C,) which is constructed about sixty yards from the salients of the place. As the operations of the besiegers are here greatly exposed to musketry fire, the trenches are constructed by the full-sap. The third parallel, having to contain the guards of the trenches, and being of less development than the two preceding, is made much wider. The second parallel now contains the reserve, and the first parallel becomes the depot of materials Demi-parallels (G) are frequently established between the second and third, to be occupied by detachments of guards.

The operations of defence during this period are so directed as to harass the workmen in the trenches and retard the advance of the works of attack. Garrison pieces of long range and large howitzers are brought for-ward on the salients of the bastions and demi-lunes of attack, so as to fire in ricochet along the capitals on which the boyaux must be pushed: light and fire-balls are thrown out as soon as it becomes dark, to light up the ground occupied by the besiegers, thus exposing them to the fire of the work and to the attacks of the sortie parties. These parties are composed of light troops who charge the guards [372] and compel the workmen to abandon their sapping tools and stand upon the defence. They are most effective when the besiegers commence the second parallel, as the guards in the first parallel are not so immediately at hand to protect the workmen. When the sortie detachment has driven these workmen from the trenches, instead of pursuing them into the first parallel, it will display itself in battle order to cover the engineer troops, (who should always accompany the detachment in this enterprise,) while they fill up the trenches and destroy the implements of the besiegers. When the guards of the trenches appear in force, the detachment will retire in such a way, if possible, as to draw the enemy within range of the grape and musketry of the collateral works. These sorties, if successful, may be frequently repeated, for they tend very much to prolong the siege. The best time for making them is an hour or two before day, when the workmen and guards are fatigued with the labors of the night. While the besiegers are establishing their enfilading batteries, a strong fire of solid shot and shells will be concentrated on the points selected for their construction. The garrison will also labor during this period to put the work into a complete state of defence: constructing all necessary palisadings, traverses, blindages, barriers; and strengthening, if necessary, the covering of the magazines.

Third period.--After the completion of the third parallel, the crowning of the covered way may be effected by storm, by regular approaches, or (if the work is secured by defensive mines) by a subterranean warfare.

In the first case stone mortar-batteries are established in front of the third parallel, which, on a given signal, will open their fire in concert with all the enfilading and mortar batteries. When this fire has produced its effect in. clearing the outworks, picked troops will sally forth and carry the covered way with the bayonet, sheltering themselves [373] behind the traverses until the sappers throw up a trench some four or five yards from the crest of the glacis, high enough to protect the troops from the fire of the besieged. It may afterwards be connected with the third parallel by boyaux.

When the covered way is to be crowned by regular approaches, a double sap is pushed forward from the third parallel to within thirty yards of the salient of the covered way; the trench is then extended some fifteen or twenty yards to the right or left, and the earth thrown. ap high enough to enable the besiegers to obtain a plunging fire into the covered way, and thus prevent the enemy from occupying it. This mound of earth is termed a trench cavalier, (0.) Boyaux are now pushed forward to the crowning of the covered way and the establishing of breach batteries, (J.) Descents are then constructed into the ditches, and as soon as these batteries have made a breach into the walls of the bastions and outworks, the boyaux are pushed across the ditches and lodgments effected in the breaches. The demi-lune is first carried; next the demi-lune redoubt and bastion; and lastly, the interior retrenchments and citadel. In some cases the breaches are carried by assault, but the same objection is applicable here as in the storming of the covered way; time is gained, but at an immense expense of human life.

If the place is defended by mines it will be necessary for the besiegers to counteract the effects of these works by resorting to the slow and tedious operations of a subterranean warfare. In this case a fourth trench is formed in front of the third parallel; shafts are sunk in this, about six yards apart, for establishing overcharged mines; as soon. as the galleries of the besieged are destroyed by the explosion of these mines, the covered way is attacked by storm; other mines are established on the terre-plain of the covered. way to destroy the entrance to the galleries, [374] and thus deprive the besieged of the use of their entire system of mines.

The measures of defence during this period must embrace every thing calculated to retard the works of the besiegers. This may be most effectually accomplished by maintaining a constant fire of grape and musketry on the heads of the sap, and throwing grenades, shells, &c., into the trenches, to harass and destroy the workmen. As the musketry fire of the besiegers now becomes very destructive to the artillerists at the guns, strong musket-proof blinds are arranged to mask the mouths of the embrasures when the guns are not in battery, and also sloping blindages to cover the men when serving at the pieces. The possession of the outworks should be disputed inch by inch, and when the besiegers have reached the ditch of the body of the place, sorties, and every species of projectile, should be employed to drive off the sappers, and to retard the construction of their works. In fine, all the resources of the engineer's art should be put in requisition for the defence of the breach, and the final assault should be vigorously resisted by the bayonet, and by a well-sustained fire from all the collateral works.

With respect to the relative strength of the opposing forces it may be well to remark, that if the fortress is properly constructed the garrison will be able to resist a besieging army six times as numerous as itself. Such is the estimate of the best engineers.³ [375] [376] [377]