Zhang Heng Seismoscope

The Eastern Han court treated earthquakes as something that had to be recorded. The earthquake section of the Treatise on the Five Phases in the Book of the Later Han records many "earthquakes at the capital" under the years in which they fell, and some entries add a note on court affairs of the time: after the capital earthquake of the fourth month of Yangjia 2, for example, it notes that "at this time Song E was given the title Lady of Shanyang". Such records were the charge of the Grand Astrologer: the Treatise on the Hundred Offices in the Book of the Later Han says the Grand Astrologer "is in charge of the seasons of Heaven and of the stars and the calendar. At the end of each year he presents the calendar for the new year", and "whenever the state has auspicious signs or calamities and anomalies, he is in charge of recording them". Zhang Heng twice served as Grand Astrologer; he made an armillary sphere and wrote the Ling Xian and the Suan Wang Lun (Book of the Later Han, Biography of Zhang Heng). The Encyclopaedia Britannica gives his dates as 78 to 139.

In Yangjia 1 (132) Zhang Heng made the wind-and-earth seismoscope, "cast of fine bronze, eight chi across, with a domed cover, shaped like a wine vessel"; "inside is a central pillar, with eight tracks running to the sides, and catches and triggers fitted. Outside are eight dragons, each holding a bronze ball in its mouth, and below are toads with open mouths to catch them." When the earth moved, one dragon's catch was released and it dropped its ball into a toad's mouth with a loud sound, so that the watchers noticed; the other seven heads did not move, and by following its direction one knew from which quarter the earthquake had come. The Biography also records that once a dragon's catch was released though in the capital "the earth was not felt to move", and the scholars all wondered that it had no proof, but "a few days later a courier arrived: there had indeed been an earthquake in Longxi". This test is not dated. Some place it in Yonghe 3 (138) on the strength of the Annals of Emperor Shun, which record "earthquakes at the capital, Jincheng and Longxi", but the same entry says the capital had an earthquake, which does not fit "the earth was not felt to move", so the date is only an inference and remains unsettled.

The change the instrument brought is recorded concretely: "From then on, the historians were ordered to record the direction from which an earthquake came." That is, from then on the court required the historians to note the direction from which an earthquake came. As the Biography describes it, the instrument showed the direction after an earthquake had happened, letting the capital know that an earthquake had happened in some distant direction before the courier arrived; and its credibility did not rest on a word of praise but was established before the scholars of the capital by checking it against a courier's report days later. The Book of the Later Han says that "tested against events, it matched like the two halves of a tally, as if by spirit" and that "nothing like it is recorded in the books", which is the historian's own judgement. In a memorial Zhang Heng also wrote of calamities such as "the earthquake at the capital and the cracking of the ground the year before last" (a note dates that earthquake to Yongjian 3, 128, before the seismoscope), and, holding the prophetic charts to be false, proposed that "the prophetic charts should be collected and banned altogether". The Grand Astrologer who built an instrument to find directions also urged at court that the prophetic charts be banned, yet when he spoke of the earthquake and the cracked ground he still treated them as calamities.

Where earthquake knowledge sat in the old catalogues also shows how it was understood: the Treatise on Books in the Book of Sui lists a Chart of Earthquakes in one scroll in the five-phases class of the masters section. What people know of the seismoscope today comes mainly from these passages of the Book of the Later Han and from later reconstructions based on them. In 1951 Wang Zhenduo designed and built a 1:10 wooden model, which was later put into primary and secondary school textbooks across the country; some scholars hold that the model rests on a mistaken principle and could not really detect earthquakes. In 2018 the People's Education Press replied that the content on Zhang Heng and the seismoscope had not disappeared from the national textbooks but had only been rearranged (an article by the Guangdong Provincial Committee of the Communist Youth League carried by The Paper). The Encyclopaedia Britannica entry shows a picture of a model, and says only that the original was apparently cylindrical, with eight dragon heads above and eight frogs below. From a few lines in a history, to a model in the textbooks, to the dispute over that model, the seismoscope has remained something people keep checking against when they talk about early science.

The seismoscope's ties to three other canons mostly run through the office of the Grand Astrologer. With Armillary Spheres: Zhang Heng made an armillary sphere while he was Grand Astrologer, so the seismoscope and the armillary sphere came from the hands of the same official in charge of the seasons of Heaven, one observing the sky, the other sensing the movement of the earth. With Lunisolar Calendar: at the end of each year the Grand Astrologer presented the new calendar, and he also recorded auspicious signs and calamities; calendar-making and the recording of calamities belonged to the same office, so the recording of an earthquake's direction entered this system for recording the seasons and anomalies. With Yin-Yang & Five Elements: the Treatise on the Five Phases records earthquakes together with court affairs, and its entry for the earthquake of Yangjia 2 (133) still adds a note on a grant of title; the Treatise on Books in the Book of Sui also places a book on earthquakes in the five-phases class. Zhang Heng's instrument added a direction that could be checked to these records, but it did not change the practice of reading earthquakes as signs.

The weight of the seismoscope lies not in the object but in the change it left in the histories: after it, the court required the historians to record the direction from which an earthquake came, and its accuracy was established before the scholars of the capital by a courier's report days later. It came from the Grand Astrologer, the office that presented the new calendar and recorded calamities and from which the armillary sphere also came; it added a direction that could be checked to the record of calamities, yet it did not change the reading of earthquakes as signs. From 1951 it also became an image of early science in the textbooks, and, together with the dispute over whether the reconstruction can be trusted, it is still a point of reference when early science is discussed.