Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/436

422 to the parapet, forms by its depth an additional obstruction to an advancing enemy. Ordinary intrenchments are formed of a simple parapet and ditch, but in more important out works and in fortresses the height is further augmented by elevating the parapet on another mound of earth called the Rampart (riparo in Italian), and as this additional height requires a greater quantity of earth, the ditch is made both wider and deeper. By these means the difficulties of attack are increased, additional cover is given to the magazines or other buildings within the fortress, and the command over the country is raised and improved in efficiency, by elevating the soldier so that he can see over the many minor obstacles which would otherwise restrict his field of view. Some writers, regarding the presence or absence of the rampart as the main distinction between permanent and temporary works, have proposed to call the former Ram part or Town Fortification, the latter Parapet or Field Fortification ; but this distinction is not of universal appli cation, and the more ordinary divisions of Permanent and Field Fortification better indicate the nature of the works and the objects for which they are intended. In order to study efficiently the combinations of these simple elements, with a view to form from them works of b c d <* s ff ff FIG. 1. Simple Parapet. Plan above, profile below. defence, it is necessary that we should know the manner in which such works are represented on paper. As in architecture, of which in earlier times fortification was only a military branch, this is effected by the plan, the section, and the elevation, of which the first two are the most important. The plan of a work is the orthographic pro jection of the lines of intersection of the planes of its slopes on the plane of con struction. The elevation is a similar projection on a vertical plane. The section or profile is made on a plane perpendi cular to the lines of intersec tion of the planes or slopes, and therefore represents the traces of these planes on the sectional plane. Fig. 1 represents a small portion of a simple parapet in plan and profile, and leads to the follow ing explanation of terms : In the plan, cc represents the crest of the parapet or highest ridge line of the work. In delineating the outline of a work, it is this line which is always drawn ; it is called the "trace." Between cc and ee is the superior slope ; between ce and a line parallel to it through s is the exterior slope, prolonged in this case to the bot tom of the ditch del, being continuous with the escarp sd; gg is the crest of the glacis, or ridge of a slightly elevated mound of earth raised on the exterior edge of the ditch, or counterscarp, and slop ing gently outward, so that the assailants passing over its surface may be in the prolongation of the superior slope of the parapet, and therefore in the line of fire from its crest ; by its elevation the glacis increases the amount of descent into the ditch. Within the crest of the parapet are seen the interior slope of the parapet, bounded by b b , the banquette, between b b and Ib, and the interior slope of the banquette, bounded by aa. Relief. This term is adopted to indicate the height of any point of the work either above the plane of construc tion, which is sometimes the plane of site, when it is called constructive relief, or above the bottom of the ditch, when it is called absolute relief. The absolute relief is a very important datum, as it expresses the total obstruction offered by the parapet and ditch to the ascent of the assailant, and it has a bearing on the regulation of the length of lines, which mutually defend each other. The Relief of a work is the relief of the crest of its parapet. The Command of a work is the height of the crest of its parapet, either above the plane of site if horizontal, or above any point of that plane specially referred to, or above the crest of the parapet of any other work in front of it ; the difference of height, therefore, between the crest of the parapet in fig. 1 and the crest of the glacis is the command of the parapet over the glacis ; in the one case the com mand is absolute, in the other relative. But after all a simple parapet and ditch afford only tem porary protection, and it is necessary to stop an enemy as he advances to them by placing obstacles in his path which shall retain him as long as possible under the fire of the works opposed to him either in front or in flank. In the profile in fig. 1, the simplest form of such an obstacle, viz., a Palisade, is shown. When, however, in combination with direct fire only, as in the figure, palisades check an enemy but for a short time, and are principally useful as affording more time to the defenders. Indeed in any combination of defence palisades are now of much less value than formerly, as they are readily destroyed by indirect fire, and by the new explosives which are more portable and far stronger than gunpowder ; they are mainly used in closing the gorges of temporary works. Many other obstacles may, however, be so arranged as to assist materially in rendering simple direct fire more effective. Abattis are formed of trees cut down, and arranged side by side with the branches interlaced outwards, and the stems inwards the branches should be freed from foliage, and their ends cut sharp. They may be arranged in one or more rows, so that the fire from the parapet shall sweep along their summits, their stems being firmly fastened by pickets to the ground and partly buried in it ; an enemy would suffer great loss whilst attempting to remove them under fire. The best abattis are formed of good-sized growing trees. The trunks are partially cut through a little above the ground, and the trees are bent down and FIG. 2. Arrangement of Abattis. securely fastened to the ground and to one another by pickets and iron wire. Fig. 2 exhibits an arrangement of Abattis ; and it will be observed that in this profile the exterior slopes of the parapet and the escarp are formed into one gentle slope, whilst the counterscarp retains its ordinary slope. By this modification the difficulty of descending into the ditch remains as before, and the sloping pickets in front of the abattis prevent the assailants from clearing it away. In