Glacis

Glacis

“When the work is intended to last for some time, and to make a long resistance, a prismatic mound is usually established upon the outer edge of the ditch as c q t, (fig. 4) which is called the glacis. It adds to the force of the entrenchment, inasmuch as it covers by its height the slope d i, and renders the descent of the ditch more difficult. The height which may be given to this glacis is not indifferent; this height, added to the breast height of the parapet, should be less, or not more than equal to the whole height m h of the parapet; so that the assailant may not be able to have a plunging fire from its crest into the entrenchment. In order to trace it, the base c t is sometimes made from 30 to 36 feet, and the line of fire n o t drawn, or at others the height q r is taken, such that it may be commanded by the magistral of the parapet any given number of feet.”

“A species of berm, or covered way c a q (fig. 5) is sometimes made from 6 to 9 feet wide between the counterscarp and the foot of the glacis.”

“In order to procure the earth necessary to make the glacis, the ditch is widened or deepened, or else an advanced ditch is dug t s w, (fig. 4). It is proper to take care that the bottom S of this advanced ditch will be nearly in the prolongation of the line m q. This advanced ditch serves to hold a row of abatis, (fig. 35). By the term abatis, is meant a range of trees or strong branches, laid side by side, and interwoven. Their roots being placed towards the parapet and well fastened down with pickets. Because these abatis are not out of the reach of cannon, the earth must be raised before them, forming thus what may be called a second glacis w v z, (fig. 4) with soil taken towards the country still more in advance. It is in like manner proper to be attentive, that plunging fire cannot be made from the top of this glacis into the entrenchment.” (Savart)

Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)

Military Glossary