Part 147

Practical FORTIFICATION, consists in forming a project of a fortification, according to the nature of the ground, and other necessary circumstances, to trace it on the ground, and to execute the project, together with all the military buildings, such as magazines, store houses, barracks, bridges, &c.

The names of every part of a FORTIFICATION; and first of lines, which are divided into right lines, and curve lines.

Line of defence, is the distance between the saliant angle of the bastion, and the opposite flank; that is, it is the face produced to the flank. Common experience, together with some of the greatest artists in fortification, unanimously agree, that the lines of defence may extend (though not exceed) 150 fathom. Some indeed will affirm, that as a musquet does not carry more than 130 fathom point blank, the angle of the bastion should be no further removed from its opposite flank. We agree that a musquet carries no farther point blank; but we are sure it will do execution, and kill, at 180 fathom. The enemy generally makes his breaches near the middle of the face; which if granted, the line of fire from the flank to the breach, scarcely exceeds 130 fathom; besides, the cannon of the flank does less execution upon a short line of defence than on a long one.

Line of defence fichant, is a line drawn from the angle of the curtain, to the point of the opposite bastion, which is not to exceed 120 fathom; and from the point of the curtain, and flank, to the face of the opposite bastion, which is to be defended. This line may not improperly be called in good English the butting flank, since it partly sees the opposite faces in reverse; and the shot from it, especially near the orillon, strike against the faces. Authors are numerous both for and against the fichant and rasant lines; we can only set down as a fixed rule, that the more powerful the active quality is, the more the passive must suffer; that in fortification the active quality is the fire, which discovers the assailants (who are the passive) going to attack the face of the opposite bastion; consequently, the more this active quality is augmented, by so much the more must the passive subjects suffer; and from thence we argue for the fichant flank, since it augments this active quality, by all the fire of the curtain added to the flank, which is the principal action in the art of defence.

Line of defence rasant, is a line drawn from the point of the bastion along the face, ’till it comes to the curtain, which shews how much of the curtain will clear, or defend the face. This line may very justly in our language be called the sweeping flank; because the shot as it were sweeps along the opposite faces. This line, as well as the fichant, has many supporters, and as many opponents. In our humble opinion, the line fichant is preferable to the line rasant.

Line of circumvallation. See SIEGE. See CIRCUMVALLATION.

Line of contravallation. See CONTRAVALLATION.

Line of counter-approach. See APPROACHES.

Capital line, is an imaginary line which divides the work into two equal and similar parts, or a line drawn from the point of the bastion to the point where the two demi-gorges meet, &c.

Line of defence prolonged. In the square, and most polygons of the lesser fortification, you prolong the line of defence; but in the polygons of the greater and meaner, you draw a line from the angle of the opposite shoulder to the angle of the curtain, upon which you raise a perpendicular, which serves for the first line of the flank.

Names of the angles in a FORTIFICATION.

Angle of the centre, in a polygon, is formed by two radii drawn to the extremities of the same side, or from the centre, terminating at the two nearest angles of the figure.

Angle of a bastion, Flanked angle, that which is made by the two faces, being the outermost part of the bastion, most exposed to the enemy’s batteries, frequently called the saliant angle, or point of the bastion.

Angle of the polygon, is made by the concourse of two adjacent sides of a polygon, in the centre of the bastion.

Angle of the triangle, is half the angle of the polygon.

Angle of the shoulder, Angle of the epaule, is made by the face and flank of the bastion.

Angle of the flank, Angle of the curtain that which is made by, and contained between the curtain and the flank.

Angle of the tenaille, Flanking angle, made by two lines fichant, that is, the face of the two bastions extended till they meet in an angle towards the curtain, and is that which always carries its point towards the work.

Dead-angle. Every angle is so called, that points inwards, or is not well defended.

Angle of the ditch, is formed before the centre of the curtain, by the outward line of the ditch.

Angle rentrant, Re-entering angle, is any angle whose point turns inwards, or towards the place; that is, whose legs open towards the field.

Saliant angle, is that which points outwards or whose legs open towards the place.

Angle of the complement of the line of defence, is the angle formed by the intersection of the two complements with each other.

Inward flanking angle, that which is made by the flanking-line and the curtain. See ANGLE.

Names of the solid works of a FORTIFICATION.

Advanced-foss, Avant-fossé, or ditch, made at the foot of the glacis: it is but very seldom made, because it is easily taken, and serves for a trench to the besiegers.

Appareille, is that slope or easy ascent which leads to the platform of the bastion, or to any other work, where the artillery, &c. are brought up and carried down.

Approaches, are a kind of roads or passages sunk in the ground by the besiegers, whereby they approach the place under cover of the fire from the garrison.