bal·lis·ta /bəˈlɪstə/
  發射石頭的古代武器
  Bal·lis·ta n.; pl. Ballistæ   An ancient military engine, in the form of a crossbow, used for hurling large missiles.
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  ballista
       n : medieval artillery used during sieges; a heavy war engine
           for hurling large stones and other missiles [syn: catapult,
            arbalest, arbalist, bricole, mangonel, onager,
            trebuchet, trebucket]
       [also: ballistae (pl)]