Parapet
The parapet is a mound of earth, n b a m d i (fig. 1), raised in front of the position to be defended. Its thickness should be proportioned according to the arms by which it may be attacked. In ordinary earth, a musket ball, fired near at hand, penetrates 1 foot, the parapet is made 2 feet thick.
A 3-pound ball, ditto, 3 to 4 feet, the parapet is made 5 to 6 feet.
A 6-pound ball, ditto, 5 to 6 feet, the parapet is made 8 to 9 feet.
A 12-pound ball, ditto, 7 to 9 feet, the parapet is made 10 to 12 feet.
A 24-pound ball, ditto, 12 to 14 feet, the parapet is made 15 to 18 feet.
A hollow 24-pound ball, or 5½ inch howitzer shell, fired from a short piece, penetrates 4 to 5 feet.
The extra thickness which is given to the epaulement above the depth to which the bullet penetrates, is to prevent the interior face of the parapet from being destroyed, and forced in, by the percussion of the projectile. It is evident when cannon cannot be brought to bear on the entrenchment, except from a great distance, the thickness of the parapet may be diminished.
Note. – A 24-pound ball fired into masonry from a very short distance, will penetrate into it 3 feet; therefore, the wall should be built 4 feet thick. If fired in like manner into wood, it will penetrate 3 feet 10 inches, wooden parapets are, therefore, made 4½ feet thick. A 12-pound ball, fired from a distance of 700 yards, penetrates 1 foot 7 inches, and a 6-pound ball 8 or 9 inches.
Of the Height of the Parapet
The height, m h (fig. 1), should be such as to hide and protect the troops which occupy the position from the view and the fire of the enemy. It is usually elevated 8 feet above the ground, to cover the defenders from plunging or ricochet fires; some authors allege, that the reason of this height being given, is to shelter the interior of the work from the fire of a man on horseback, which is, however, not so important. When a serious attack is not apprehended, a height of only 6½ feet is sometimes given in horizontal ground, particularly when time presses.
Source: Lallemand, Henri Dominique: A Treatise on Artillery (New York 1820)