Elements of Military Art and Science - Henry Wager Halleck (best free ebook reader .TXT) 📗

The general principles of permanent fortification may be best learned from the writings of Cormontaigne, St. Paul de Noizet, and Laurillard-Fallot. A list of valuable books of reference on the several branches of military engineering will be given at the close of the next chapter.

FIELD-ENGINEERING.

Field-Engineering includes the making of military reconnaissances, temporary fortifications, and military roads; the planning and construction of military bridges; the attack and defence of military works;—in fine, all the various duties of engineer troops, either in the operations of a campaign, or in the dispositions on the battle-field.

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 water-courses; 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 information will be required: the character of the roads must be given in detail; the nature of the water-courses, 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 earth, 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 an 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.

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. 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 cremaillière 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 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 loop-holes 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 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 upward, 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 considerable 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