Ballista, a military engine of the Romans, used in the siege and defence of fortified places. Neither from the description of authors nor from any carved or painted representation extant - although Trajan's column presents several specimens of these machines - can any distinct understanding be had of the principle or process of working these primitive substitutes for artillery. They were all included under one general term of torment um, which, as is shown by its root torquere, to twist, would imply that the propulsion was given by means of the torsion of ropes or fibres. Yet the use of the term is not decisive, since torquere came in time to signify simply to hurl a missile by any means. Whatever may have been the method of its operation, the ballista was originally an engine for hurling stones with a parabolic ascent, in order to destroy the battlements of walls and the roofs of buildings in their fall. The ordinary ballista threw stones of three various weights, according to which standard the power of the engines was rated, as our cannon are by their calibre; these were, half a hundredweight, a hundredweight, and three hundredweight - which last appears to have been the maximum.

Josephus mentions ballistoe, the destructive power of which he records as very formidable, capable of throwing their missiles with execution to the distance of a quarter of a mile. Vitruvius also mentions smaller ballistse, which threw stones not exceeding two pounds in weight, and which seem to have been used as field artillery, and to have been plied from the rear, over the heads of the front ranks, into the enemy's lines. - In the middle ages, ballista wa3 the term applied to the crossbow, and in the reign of Henry III. of England there was an officer named ballistarim, the keeper of the crossbows, whose pay was a shilling a day, and an attiliator ballistarum, whose duty it was to provide the harness and accoutrements of the crossbowmen. In the classics, however, the catapulta, not the ballista, is the large wall-crossbow, used in the place of cannon.