Su Song (Chinese: 蘇頌; Pe̍h-ōe-jī: So͘ Siōng, 1020–1101), also known as Zirong (Chinese: 子容; Pe̍h-ōe-jī: Chú-iông), was a Chinese scientist and government official who lived during the Song dynasty (960–1279). He was an expert in many areas, such as mathematics, astronomy, mapmaking, geography, metalworking, mechanical engineering, water engineering, poetry, and politics.
Su Song designed and built a large clock tower in Kaifeng that used water and machines to track time. The clock tower included an early escapement mechanism, a part that controls the movement of gears. This mechanism was first created in 725 AD by a Buddhist monk named Yi Xing and a government official named Liang Lingzan to power a water-driven model of the stars. Su Song’s clock tower was the first to use a mechanical clock to move the model. The tower also had the oldest known endless chain drive, called the "celestial ladder" (天梯, tian ti), as described in Su’s written work about clocks. The clock tower had 133 small figures that rang bells to mark the hours. In 1127 AD, the tower was destroyed by invading Jurchen soldiers. Although people tried to rebuild it, the tower was never fully restored.
Su Song wrote a detailed book about the clock tower called Xinyi Xiangfayao in 1092. It was officially printed in 1094. This book was his most famous work, but he also wrote other books. These included a large map of the stars, maps of the Earth, and a book about medicine. The medicine book also covered topics such as the study of minerals, animals, plants, and metals.
Life and works
Su Song was born in modern-day Fujian, near the city of Quanzhou during the medieval period. He was of Hokkien ancestry and, like Shen Kuo (1031–1095), was an expert in many areas of study. A junior colleague and scholar named Ye Mengde (1077–1148) wrote that Su mastered the provincial exams in his youth and ranked first for writing the best essay on the structure of the Chinese calendar. From an early age, Su was interested in astronomy and calendar science, which led him to an important position in government. In his free time, he wrote poetry to praise the work of artists, such as the painter Li Gonglin (1049–1106). He also collected old artworks from earlier dynasties.
Su held the rank of Ambassador and led the Ministry of Personnel in the capital city of Kaifeng. He was known for his skills in administration and finance. After working in the Ministry of Personnel, he became a Minister of Justice in 1086. He was chosen to edit a classic work called Huainanzi for the Academy of Scholarly Worthies in 1063. Eventually, Su became Vice President of the Chancellery Secretariat. He also held the title of "Deputy Tutor of the Heir Apparent." At court, Su avoided taking sides in political conflicts between the Conservative group, led by Prime Minister Sima Guang (1019–1086), and the Reformist group, led by Prime Minister Wang Anshi (1021–1086), even though many of his friends were Conservatives.
In 1077, Su traveled north to the Liao dynasty of the Khitan people to share ideas about calendars. The Liao had created their own calendar in 994 AD, and Su admitted it was more accurate than the Song calendar, which led to fines for officials in the Bureau of Astronomy and Calendar. Su was supposed to arrive in Liao for a birthday celebration on the winter solstice, but the Liao calendar was one day ahead of the Song calendar. Historian Liu Heping noted that Emperor Zhezong of Song supported Su’s clocktower project in 1086 to show China’s scientific strength. In 1081, Su was ordered to write a detailed book about the diplomatic history between the Song and Liao dynasties, which filled 200 volumes. Using his knowledge of maps, Su helped resolve a border dispute between the two dynasties.
Su also created a star map in five parts. The maps showed the positions of stars using a special projection method and were based on the work of Shen Kuo, his contemporary and political rival. One of Su’s maps placed the pole star halfway between two points, a calculation that was more accurate than earlier estimates. These maps were the oldest printed star maps in China.
In 1070, Su and other scholars compiled a book called Bencao Tujing ("Illustrated Pharmacopoeia"), which was an important work on plants, animals, and minerals used in medicine. The book included information about the use of ephedrine as a drug and described methods for making steel and iron. Su organized details about minerals, such as mica, and their uses in medicine. He also described the appearance of cinnabar and provided information about copper carbonate, similar to later descriptions by German scientist Georgius Agricola. Su’s book was the first in China to describe certain plants, such as flax and Urtica thunbergiana. Edward H. Schafer noted that Su accurately described the translucent quality of realgar, its origin in rocky river gorges, and its deep red color.
Su believed that ancient Zhou dynasty physicians used realgar to treat ulcers. At the time, the "five poisons" used for this purpose were thought to include cinnabar, realgar, chalcanthite, alum, and magnetite. Su also described different species of crabs and where they lived. For example, he noted that the freshwater crab Eriocher sinensis could be found in the Huai River and near the capital city. Su’s book was later copied into the Bencao Gangmu, a famous pharmacology text from the Ming dynasty.
Su compiled one of the most important Chinese books on timekeeping during the Middle Ages. He worked with engineers and astronomers to create Xinyi Xiangfayao ("Essentials of a New Method for Mechanizing the Rotation of an Armillary Sphere and a Celestial Globe"), written in 1092. The book included 47 detailed illustrations of the mechanical parts of his astronomical clock tower.
Su’s greatest project was a 40-foot-tall water-powered clock tower built in Kaifeng. A wooden model was completed in 1088, and the bronze parts were cast by 1090. The full tower was finished in 1094 during the reign of Emperor Zhezong of Song. The emperor had originally asked Han Gonglian to lead the project but later chose Su Song instead. Su worked with Han Gonglian, who used his math skills to help build the tower. A small wooden model was made first to test the design before
Su Song's escapement mechanism
Su Song's waterwheel linkwork device used gravity to control the movement of the escapement. This happened as water flowed into containers of limited size. Su Song's clock combined the ideas of a water clock (clepsydra) and a mechanical clock, using both mechanics and water power. In his writings, Su Song mentioned that his clock was inspired by Zhang Heng's hydraulic-powered armillary sphere, a device made by an earlier Chinese scientist. He was also influenced by Zhang Sixun's armillary sphere, which used liquid mercury instead of water in its waterwheel. Mercury was chosen because it does not freeze in cold weather and does not cause metal parts to rust over time. Su Song noted that after Zhang Sixun's death, no one could recreate his device, just as no one later replicated Su Song's own invention.
The mechanical parts of Su Song's clock included a large driving-wheel that was 11 feet wide. This wheel had 36 scoops, each filled with water at a steady rate from a "constant-level tank." The main iron shaft, supported by iron crescent-shaped bearings, ended in a pinion that connected to a gear on the vertical transmission shaft.
Joseph Needham described the clock tower in his writings. Figure 650 shows the tower, while Figure 656 displays the upper and lower norias with their tanks and a manual wheel for operation. Figure 657 shows a small illustration of the escapement mechanism from Su Song's book, labeled as the "celestial balance." This figure includes parts such as:
– right upper lock
– upper link
– left upper lock
– axle or pivot
– long chain
– upper counterweight
– sump
– checking fork of the lower balancing lever
– coupling tongue
– main (lower) counterweight
Figure 658 shows a detailed drawing of Su Song's escapement mechanism, with parts labeled as they interact:
– arrested spoke
– left upper lock
– scoop being filled by
– water jet from constant-level tank
– small counterweight
– checking fork tripped by a projection pin on the scoop, forming the near end of
– the lower balancing lever with
– its lower counterweight
– coupling tongue, connected by
– the long chain with
– the upper balancing lever, which has at its far end
– the upper counterweight, and at its near end
– a short length chain connecting it with the upper lock beneath it
– right upper lock
The endless chain drive
The oldest known picture of a chain that keeps moving without stopping to transfer power is found in Su Song's book about clocks. This chain was used in the clock's gears to connect the main power shaft to the gears that move the armillary sphere, as shown in Needham's figures 410 and 652. The chain was located at the top of the main vertical shaft, where right angle gears and slanted gears were linked by a short shaft that did not move the power itself. A toothed ring gear, called the diurnal motion gear ring, was placed around the armillary sphere near the southern pole. Although an ancient Greek inventor named Philo of Byzantium (3rd century BC) described a type of endless belt for a machine that did not transfer power continuously, the most likely influence on Su Song's chain drive was a continuously moving chain pump used in China since the Han dynasty (202 BC–220 AD). In his book, Su Song wrote:
Additionally, the motion gear rings and the upper drive wheel each had 600 teeth. Su Song carefully calculated these numbers to divide the day into units of 1/600. These gears, with 600 teeth, allowed the day to be measured in parts of 2 minutes and 24 seconds each.
Su Song's armillary sphere
In Joseph Needham's third volume of Science and Civilization in China, Figure 159 shows a drawing of Su Song's armillary sphere, as described in his 11th-century work. The drawing includes three "nests," or layers, of rings that move mechanically. Earlier, in 633 AD, the Chinese astronomer Li Chun-feng of the Tang dynasty built the first armillary sphere with three layers to help measure different aspects of the sky. Zhang's armillary sphere is often compared to the one made by Alfonso X, a 13th-century ruler in Islamic Spain. The main difference was that Alfonso's device had tools for measuring the direction (azimuth) and height (altitude) of objects, a method used in Arabic astronomy. Su Song's armillary sphere, however, had precise markings. The parts of Su's armillary sphere are listed as:
- Outer Nest: meridian circle, horizon circle, outer equator circle
- Middle Nest: solstitial colure circle, ecliptic circle, diurnal motion gear-ring connected to the power source
- Inner Nest: polar-mounted declination ring or hour-angle circle, with a sighting tube attached and supported by a crossbar
- Other Parts: vertical column hiding the transmission shaft, supporting columns shaped like dragons, cross-piece at the base with water-levels, south polar pivot, and north polar pivot.
Transmission of Su's text and his legacy
In 1092, Su Song completed his book called Xinyi Xiangfayao, and in 1094, he finished a detailed work about timekeeping devices. This work was printed and shared widely in the northern regions of China using woodblock printing and Bi Sheng’s movable type. However, in the southern regions, the book was not printed or widely shared until 1172, when a man named Shi Yuanzhi from Jiangsu printed it there.
When Su Song presented his clocktower design to Emperor Zhezong, he compared the steady flow of water to the unending motion of the heavens. This symbolized the emperor’s unending power and authority, which impressed the emperor. As a result, artwork showing the clocktower was included on vehicles used during major imperial processions, as seen in a drawing called Illustration of the Imperial Grand Carriage Procession of 1053.
In the late Ming and Qing dynasties, a scholar named Qian Zeng (1629–1699) carefully copied an old version of Su’s work and published a new edition. He made sure to copy the text exactly without changing any words or details. Later, another scholar named Zhang Xizu (1799–1844) also published a new version of the work.
Su Song’s treatise on astronomical clocks was not the only one from his time. The Song Shi, a historical record written in 1345, mentions a treatise by Juan Taifa called Shuiyunhun Tianjiyao, which described water-powered astronomical devices. However, this work no longer exists today.
European Jesuit missionaries, such as Matteo Ricci and Nicolas Trigault, briefly wrote about Chinese clocks with wheel-driven mechanisms. However, some Europeans incorrectly believed that Chinese clocks had not advanced beyond simple devices like water clocks, incense clocks, and sundials. They thought mechanical clocks were new to China and saw them as European inventions. Chinese texts from the Ming dynasty (1368–1644) describe a continuous history of mechanical clocks in China from the 13th to the 16th century. However, Su Song’s clocktower relied on a waterwheel and chain drive, not weights or gears like medieval European clocks, which operated without constant external power.
In modern times, Joseph Needham, a British scientist and historian, studied Su Song’s work in his Science and Civilization in China series. He included many details from historical Chinese sources about Su’s life and achievements. In 1956, John Christiansen created a model of Su Song’s clocktower based on a famous drawing, which helped Western scholars learn about 11th-century Chinese engineering. A smaller model was built by John Cambridge and is now displayed at the National Science Museum in London. In China, a one-fifth-scale model of the clocktower was built by Wang Zhenduo in the 1950s for the Chinese Historical Museum in Beijing.