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.
When within range of the enemy's grape, the _flying sap_ is resorted to in order to place the workmen speedily under cover.

In this operation, gabions are placed in juxtaposition on the side towards the besieged work, and filled with all possible speed by the workmen.