後漢書·張衡列傳（節選）

Zhang Heng, courtesy name Pingzi, was a man of Xi’e in Nanyang. His family had been eminent for generations; his grandfather Kan was governor of Shu commandery. As a youth Heng was good at composition. He travelled through the three districts around the old capital, went on to the capital itself and visited the Imperial Academy, and so came to master the Five Classics and to understand the Six Arts. Though his talent stood above his generation, he had no trace of arrogance. He was always calm and unhurried, and did not care to keep company with common people. In the Yongyuan years he was recommended as filial and incorruptible but did not take it up, and he declined one summons after another to the offices of the high ministers. At that time the empire had long been at peace, and from the princes and marquises down, everyone lived in excess. Heng therefore took Ban Gu’s Two Capitals as his model and wrote the Rhapsody on the Two Metropolises as a quiet reproof. He worked over its thought and its fittings for ten years before it was finished. The text is long and so is not given here. The grand general Deng Zhi admired his talent and summoned him several times, but he did not respond.

Heng had a gift for ingenious mechanisms, and gave particular thought to astronomy, yin and yang, and calendrical calculation. He was devoted to the Taixuan jing, and said to Cui Yuan: “Only now, reading the Taixuan, do I see how Ziyun reached the height of the Way and its numbers. It stands comparison with the Five Classics and is no mere commentary; it leaves others little to say about yin and yang. It is a book for the two hundred years since the Han house won the empire. In another two hundred years, will its time be over? A maker’s work must come to light within an age: that is the constant sign. When the Han has lasted four hundred years, the Xuan will flourish.” Emperor An had heard that Heng was skilled in learning and calculation, and by a special summons through the office of the Public Carriages appointed him a gentleman of the palace; promoted again, he became Grand Astrologer. There he examined yin and yang thoroughly and mastered the true working of the armillary instruments; he made an armillary sphere and wrote the Lingxian and the Suanwang lun, whose words are most detailed and clear.

In the first year of Yangjia he made, in addition, the instrument for watching the winds and the movements of the earth. It was cast of fine bronze, eight feet across, with a domed lid, shaped like a wine jar, and decorated with seal characters and figures of mountains, tortoises, birds and beasts. Inside was a central column, with eight tracks running out from it, fitted with catches and triggers. Outside were eight dragons, each holding a bronze ball in its mouth, and below them toads with open mouths to receive the balls. The teeth, triggers and clever workings were all hidden inside the jar, and the cover fitted so closely that no seam could be seen. When there was an earthquake, the jar shook, a dragon’s trigger was released and it spat out its ball, and a toad caught it. The sound rang out sharply, and so the watchman knew. Though one dragon’s trigger had gone, the other seven heads did not move; by finding its direction one knew where the quake had been. Tested against events, it matched like the two halves of a tally, as if by spirit. Nothing like it had been recorded in any book. Once a dragon’s trigger was released and no movement of the earth was felt, and the scholars of the capital all thought it strange that there was no sign. Some days later a courier arrived: there had indeed been an earthquake in Longxi, and from then on all acknowledged how fine it was. After this the astrologers were ordered to record the direction from which each earthquake came.