The predecessor of the metal barrel was made of bamboo, which was recorded in use by a Chinese garrison commander at Anlu, Hubei province, in the year 1132. One of the earliest references to the destructive force of a cannon in China was made by Zhang Xian in 1341, with his verse known as "The Iron Cannon Affair". Zhang wrote that its cannonball could "pierce the heart or belly when it strikes a man or horse, and can even transfix several persons at once". Jiao Yu describes the cannon, called the "eruptor", as a cast bronze device which had an average length of . He wrote that some cannons were simply filled with about 100 lead balls, but others, called the "flying-cloud thunderclap eruptor" (飞云霹雳炮; feiyun pili pao) had large rounds that produced a bursting charge upon impact. The ammunition consisted of hollow cast iron shells packed with gunpowder to create an explosive effect. Also mentioned is a "poison-fog divine smoke eruptor," in which "blinding gunpowder" and "poisonous gunpowder" were packed into hollow shells used in burning the faces and eyes of enemies, along with choking them with a formidable spray of poisonous smoke. Cannons were mounted on frames or on wheeled carriages so that they could be rotated to change directions.