If the inner work be separated from the other it is called a retrenchment, and if in addition it has a commanding fire, it is termed, as was just remarked, a cavalier.

The capital of a bastion is a line bisecting its salient angle. All the works comprehended between the capitals of two adjacent bastions is termed a front: it is taken as the unit in permanent fortification.

Fig. 39 represents the ground plan of a modern bastioned front, of a regular and simple form, on a horizontal site.

A, A, A-Is the enceinte, or body M-The redoubt of the re-entering

Fig. 40 represents a section through the line mn' of the

Sometimes half embrasures are cut in the earthen parapet of a fort, so as to sink the gun below the crest, and thus more effectually cover the men from the enemy's fire.

* The term retrenchment implies an interior work, which is constructed within or in rear of another, for the purpose of strengthening it; the term intrenchment, on the contrary, implies an independent work, constructed in the open field, without reference to any other adjoining work.

But guns in embrasure have a far less extended field of fire than when mounted in barbette; moreover, the embrasures present openings through which an enemy may penetrate in an assault. Owing to these objections, they are employed only for the protection of particular points; that is, where it is important to cover the artillerists from the enemy's fire, or where the guns are to be used merely to protect a ditch, or to enfilade a road, &c. The bottom of the embrasure is called the sole, the sides are called cheeks, and the mass of earth between two embrasures, the merlon. Embrasures may be made either direct or oblique, according as the fire is required to be perpendicular or oblique to the parapet.

A coverport is a small outwork of any convenient form, erected immediately in front of a gateway, to screen it from the enemy's fire.

A counterguard is a more extensive work, constructed in front of a part of the fortress itself, or of some other outwork of greater importance, which it is intended to cover. These are sometimes called coverfaces, from their situation and object; but the former term is most commonly used.

Sometimes outworks, called tenaillons, consisting of one long and one short face, are placed on each side of the demi-lune of a front of fortification, for the purpose of prolonging the siege. (Fig. 41.)

Small, or demi-tenaillons, are frequently so arranged as to cover only one-half of the demi-lune, and then a bonnet constructed in front of the salient of the demi-lune. (Fig. 42.) In this case the bonnet is flanked by the short faces of the demi-tenaillons; these short faces are themselves flanked by the demi-lune, while the bastions flank the long faces.

A hornwork consists of a front of fortification, and two wings resting on the faces of bastions of a front of the

fortress. It sometimes has also a demi-lune or bonnet, as in the case of demi-tenaillons. (Fig. 43.)

A crownwork consists of two fronts of fortification, and two wings. (Fig. 44.) It is sometimes made double, and even triple.

These works are also employed as advanced works, and placed entirely in front of the glacis. They have generally been added to a fortress for the purpose of occupying some important piece of ground not included within the limits of the main work. They may be constructed with covered ways, and sometimes it may be found advantageous to secure them by retrenchments.

A detached work may be made in any form deemed best suited to the site. Being but remotely connected with the fortress, the latter will exercise but slight influence on the character of its plan or construction. They are usually of limited extent and slight relief, partaking much of the nature of field-works.*

*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.

29*

CHAPTER XIV.

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 defeat 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 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 mi

nute 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 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.