Sapping.—­This is a general term applied to the operations of forming trenches, along which troops may approach a work without being exposed to the fire of the besieged.

In addition to the ordinary sapping-tools, such as shovels, picks, gabion-forks, &c., used in constructing trenches, there will also be required a considerable amount of sapping materials, such as gabions, fascines, sap-fagots, sandbags, &c.

The gabion is a cylindrical basket of twigs, about two feet in diameter, and some three feet in length, and without a bottom.  It is made by driving into the ground, in a circular form, a number of small pickets about an inch in diameter, and of the length required for the gabion.  Twigs are wattled between the pickets like ordinary basket-work, and fastened at the ends by withs or packthread.  Gabions are used in forming saps, batteries, blindages, powder-magazines, and in revetting the steep slopes of field-works.

The fascine is a bundle of twigs closely bound up, from nine to twelve inches in diameter, and from ten to fifteen or twenty feet in length.  The largest are sometimes called saucissons.  In making a fascine, straight twigs about the thickness of a man’s finger are laid side by side, and firmly compressed together by a strong rope or chain attached to the extremities of two levers.  While held in this position the twigs are firmly bound together by withs or cords.  Fascines are used in constructing trenches, batteries, &c., and for filling up wet ditches.

The sap-fagot is a strong fascine about ten inches in diameter and two feet in length, with a picket inserted through the middle.  It is used in the double sap in connection with gabions.

Sand-bags are usually made of coarse canvass.  When filled with earth they are some six or eight inches in diameter, and from eighteen inches to two feet in length.  From their perishable nature, they are used only when other materials cannot be procured, and where it is important to place the troops speedily under cover from the enemy’s fire.

Bales of wool, cotton, hay, straw, &c., may be employed in sapping for the same purposes as the above materials, when they can be procured in sufficient quantity.  Pork and flour barrels, which are usually in abundance in a camp, are frequently filled with sand and used for forming magazines, blindages, &c., in field-works.

A trench constructed in ordinary soil beyond the range of the enemy’s grape, is called a simple sap, or ordinary trench.  The earth is thrown up on the side towards the place besieged, so as to form a kind of parapet to cover the men in the trench.  The labor is here executed under the supervision of engineer soldiers, by working parties detached from the other arms.  Fig. 50 represents a vertical section of a simple sap.