An _abatis_ consists of obstructions placed in front of a work to form obstacles to a storming party. The most convenient method of forming it is to cut down trees and allow them to lie helterskelter. When there is time, the trees are laid with the butts toward the work, and the branches outward--the small limbs being removed, and the ends of the remainder sharpened. [Illustration] A _redan_ is a letter V, with the point toward the enemy, and is used generally to cover the heads of bridges, etc. [Illustration] A _lunette_ is the redan with flanking wings. [Illustration] A _redoubt_ is an enclosed parallelogram. [Illustration] These works are very imperfect, because they have exposed points. The angles are not protected by the fire from the sides. To remedy this difficulty, the next most usual work is the _star fort_, made in the form of a regular or irregular star. It will be perceived that the fire from the sides covers the angles. The next and still more improved form of work is the bastioned fort, which consists of projecting bastions at the corners, the fire from which enfilades the ditches. The following is a diagram of a vertical section of the parapet and ditch used in all fully constructed field works: [Illustration: A B is the slope of the banquette. B C head of the banquette, or place where the men stand to deliver their fire. C D the interior slope of the parapet. D E superior slope of the same. F G the berme, or place left to prevent the parapet from washing down into the ditch. G H the scarp or interior wall of the ditch. H I the bottom of the ditch. I K the counterscarp. L M N the glacis, which, except the abatis near the ditch, is left free and open, so as to expose the assailants to the fire from the parapet. The proportions and angles of all the lines given are fixed according to mathematical rules.] The operations of a siege present many incidents of great interest; but we can do nothing more in this article than illustrate the methods in which the approaches are made to the works the capture of which is designed. When reconnoissances have established the conclusion that the works of an enemy cannot be carried by assault, the lines of the investing army are advanced as near to them as is compatible with safety; advantage is then taken of the opportunities afforded by the ground to cover working parties, which are thrown forward to the place fixed for the first parallel; sometimes these parties can commence their work only at night. The parallel is only a deep trench with the dirt thrown toward the enemy; and after the excavation has progressed, the trench is occupied by parties of troops to resist any sorties of the enemy, and to prevent attempts against the batteries established behind the parallel.