CIRCLE, in mathematics, is a plane figure, comprehended under one line only, to which all right lines drawn from a point in the middle of it are equal to one another.
CIRCUMFERENTER, an instrument used by engineers for measuring angles.
CIRCUMVALLATION, or line of circumvallation, in military affairs, implies a fortification of earth, consisting of a parapet and trench, made round the town intended to be besieged, when any molestation is apprehended from parties of the enemy, which may march to relieve the place.
Before the attack of a place is begun, care is to be taken to have the most exact plan of it possible; and upon this the line of circumvallation and the attack are projected. This line, being a fortification opposed to an enemy that may come from the open country to relieve the besieged, ought to have its defence directed against them; that is, so as to fire from the town: and the besiegers are to be encamped behind this line, and between it and the place. The camp should be as much as possible out of the reach of the shot of the place; and the line of circumvallation, which is to be farther distant from the place than the camp, ought still more to be out of the reach of its artillery.
As cannon are never to be fired from the rear of the camp, this line should be upwards of 1200 fathoms from the place: we will suppose its distance fixed at 1400 fathoms from the covert way. The depth of the camp may be computed at about 30 fathom, and from the head of the camp to the line of circumvallation 120 fathoms, that the army may have room to draw up in order of battle at the head of the camp, behind the line. This distance added to the 30 fathoms, makes 150 fathoms, which being added to the 1400, makes 1550 fathoms constitute the distance of the line of circumvallation from the covert-way. The top of this line is generally 12 feet broad, and 7 feet deep: the parapet runs quite round the top of it; and at certain distances is frequently strengthened with redoubts and small forts; the base 18 feet wide, the height within 6, and on the outside 5 feet, with a banquette of 3 feet wide, and 1¹⁄₂ high. See [Contravallation], or [Countervallation].
CIRCUS, in military antiquity, a very capacious building, of a round or oval form, erected by the ancients for exhibiting shews to the people.
CISEAUX, Fr. chissels made use of by miners, to loosen earth from the sides of the excavation, without making a noise, which the miner effects by striking the handle.
CITADEL, is a fort with 4, 5, or 6 bastions, raised on the most advantageous ground about a city, the better to command it; and commonly divided from it by an esplanade, the better to hinder the approach of an enemy; so that the citadel defends the inhabitants if they continue in their duty, and punishes them if they revolt. Besiegers always attack the city first, that, being masters of it, they may cover themselves the better against the fire of the citadel. Its having bastions distinguishes it from a castle. Sometimes the citadel stands half within, and half without the ramparts of the place.
CIVIC-CROWN, among the ancient Romans, was a crown given to any soldier who had saved the life of a citizen. It was composed only of oaken boughs, but accounted more honorable than any other.
CIVIERE, Fr. a small hand-barrow, which is carried by 2 men, and is much used by the artillery.