SANTONIN. [Sasro.xle Aviv.] SAP is a mode of executing the trenches at the siege of a fortress, when the besiegers arrive within such a distance from the covered way that the fire from thence becomes too dangerous to allow the men to work on the ground without being protected by some covering objects, as gabions, placed between themselves and the enemy.
The process of sapping varies with the distance from the works of the fortress and the degree,of activity with which the fire of the defenders is kept np. It is, therefore, divided into what is called the flying sap and the complete or full sap. In ordinary circumstances, in sieges, before the introduction of rifled ordnance or rifles to any great extent, the flying sap began to be used in forming the second parallel trench, which may be about 320 yards from the covered-way ; and it is executed in the following manner.
If the distance from the depot of siege-materials to the place of the intended trench is not too great, every man carries two gabions, one on each side of him, or both slung at his back ; he carries also a pickaxe and a spade, and in the first case these are fixed in the gabions, but in the latter he carries them in his hands. If the distance which the working party has to march is considerable, this burden would be too fatiguing, and then each man carries on his shoulder one gabion toge ther with a pickaxe or a spade. Carrying the gabions one on each side has the advantage of permitting the man to carry his musket slung. The work is begun at night, and when the sappers have arrived at the ground where the tracing-line for the intended trench has been laid down, they set up their gabions a few inches in front of that tracing line, the officers observing that the row of gabions in its whole length is correctly placed. The portion of trench to be executed by each sapper; or workman, is equal in length to the space covered by two gabions (about 4 feet) : the men sit down or otherwise keep themselves covered till the order is given to commence digging, and when the number of men who constitute the working party is greater than suffices to allow one man to every two gabions, those who have not room to work retire to a little distance till they are required to relieve the others. A man may fill his two gabions with earth in about a quarter of an horn-, and then they will be proof against a musket bullet, except at the place where they touch each other ; after this the earth obtained in executing the trench is thrown beyond the gabions towards the fortress. During the progress of the work the gahions aro pushed a little way outwards at the top, in order that they may effeCtually resist the pressure of the earth which they are to retain ; and they are sometimes crowned by two or three rows of fascines which are laid upon them In a direction parallel to the trench.
If the work proceeds by day and night, the parties are relieved every eight hours ; and a trench executed by flying sap may, in soil of medium tenacity, be completed by three reliefs of men.
When the approaches of the besiegers have advanced so near the covered-way that the fire of the defenders will no longer permit the men to bring the gabions openly to tho ground, the full sap is prac tised. For this purpose the mappers are divided into brigades of eight men each; and of these a demi-brigade only of four men is employed in the formation of a single line of trench. The party is provided with
a mantelet (a plate of tron thick enough to be musket-proof, and capable of being moved forward by being mounted on small wheels) by which the men may be covered In front ; or, instead of this machine, there is provided for the same purpose a great gabion called a sap roller. This is a cylinder of basket-work, 0 feet long, and about 4 feet diameter, and having within it a gabion of equal length, but about 2 feet six inches diameter ; the axes of the two gabions are coincident, and the space between the exterior of tho one and the interior of the other is stuffed with fascines, by which means it is rendered musket proof. The sap-roller turning on its convex surface is found to be more manageable than the mantelet ; and when it is intended to form a trench in any proposed direction, by breaking out from one which has been already executed, it is raised over tho parapet of the Latter trench, and gradually lowered on the exterior side, being guided by means of a hook, so that it may have a position perpendicular to the line of the intended trench. The leading sapper of the demi-brigade then cuts through tho parapet, and pushing the :sap-roller forward about 2 feet, ho hastily places an empty gabion in rear of that extre mity of the sap-roller which is nearest to the enemy's work, in order that be may be covered on his flank : then kneeling behind tho gabion, he excavates a portion of a trench, 18 inches wide, and as many in depth, leaving a berme about 12 inches broad between the gabion and the nearest edge of the excavation, and throwing the earth into the gabion. When this portion is dug, the sap-roller is advanced about 2 feet farther, and another gabion is set up in its rear, adjacent to the former, and in the line of the intended trench ; a short fascine or two sand-bags (bags full of earth), are placed one above another in the hollow between the two gabions, in order that a musket-ball may not be able to penetrate through the screen in that part. The sapper then excavates as before, and having filled the second gabion, a third I. handed to him, which he places and fills as he continues to advance. The second sapper of the squad follows the first, keeping a little way in his rear, and increases the width only of the trench by 20 inches on the side which is farthest from the line of gahions, and he also throws the earth into and beyond tho gabions. The third sapper follows, and increases the depth only by 18 inches on a breadth of 20 inches measured from the rear side of the trench towards the gabions; and the fourth sapper excavates a portion 3 feet deep, increasing the breadth of the trench towards the rear by 10 inches. By this arrange ment the tasks of the different men are rendered nearly equal, and complete cover is obtained when the work of the third man is executed. The four men thus form a trench 4 feet wide and 3 feet deep, and a step is left on the side nearest to the gabions for convenience in standing to fire over the parapet. The working parties from the infantry of the line afterwards complete the trench by increasing the width to its usual extent, about 12 feet.