桥梁的种类很多,功能也不尽相同。古代有一种仰开桥,桥面是可以开启的,将桥面放平,行人和车马可以顺利通过。如将桥面的一端用绳索高高吊起,则桥断路阻,无法通行。《简明大不列起颠百科全书》中记载,“仰开桥……起源于中世纪欧洲,用作城堡或城镇的一种防御设施,由平衡锤和绞盘操纵。”我国唐宋以来,在城门外的护城河上,就修建过不少这种仰开桥,当时俗称 Many types of bridges, functions are not the same. There is a kind of ancient open bridge, the bridge can be opened, the bridge flat, pedestrians and horses can pass smoothly. If one end of the deck with a rope lifted high, the bridge off the road block, unable to pass. “The Concise Bookcase” shows that “Yang Kai Bridge ... originated in the Middle Ages in Europe and was used as a defensive facility for castles or towns and was operated by counterbalance and winches.” Since the Tang and Song dynasties in our country, In the city outside the moat, built a lot of this kind of Yang Kaiqiao, was commonly known