A field mill, also known as a camp mill, was a premodern vehicle which acted as a mobile mill used for grinding grains, which had the very practical use of feeding a moving army.
In the Yezhongji (鄴中記) ('Record of Affairs at the Capital of the Later Zhao Dynasty') by Lu Hui, covering the history of the Later Zhao (319 - 351 AD) court in China, the text describes various mechanical devices used, including the wheeled odometer for measuring distance and the south-pointing chariot for indicating cardinal direction.[1] Two engineers in particular, the Palace Officer Xie Fei and Director of Imperial Workshops Wei Mengbian,[2] were known for their designs and worked at the court of Shi Hu (r. 334 - 349). The two had crafted a four-wheeled carriage about 6 m (20 ft) long with water-spouting dragons hanging over a large golden Buddhist statue that had a mechanical wooden statue of a Daoist continually rubbing his front.[3] Other mechanical figures included ten Daoists dressed in monastic robes who continually rotated around the Buddha while periodically bowing, saluting, and throwing incense into a censer.[3] All of these mechanical figures were driven only by the movement of the carriage; once the carriage halted, the figures stopped moving and the water stopped spouting from the artificial dragons.[4]
Xie and Wei created a similar device operated by wheel motion called the field mill, although it served a more practical purpose than the theatrical display of moving statues and water-spouting dragons.[5] The Yezhongji states that the two devised a "pounding cart" or "pounding wagon" which had figurine statues armed with real tilt hammers who pounded and hulled rice only when the cart moved.[1] In addition to this they had a "mill cart" (field mill or camp mill) which had rotating millstones mounted on their frames, which would rotate and grind wheat as the cart moved forward.[1] Just like the carriage with mechanical figures mentioned above, when the carriage stopped, the devices associated with them halted.[2]
Use of the field mill in China seems to have died out in use after the Later Zhao, since it was no longer mentioned in Chinese texts until Ming Dynasty.[6]
The Italian military engineer Pompeo Targone, who was most notably involved in the Siege of La Rochelle (1627-1628) in western France, invented the field mill in Europe by 1580.[7] As shown in the Italian Vittorio Zonca's engineering treatise of 1607, two mills mounted to a wagon are rotated by a horse whim and gearing while in a stationary position at military camp or near billets.[7]
In the Yuanxi Qiqi Tushuo Luzui ('Collected Diagrams and Explanations of the Wonderful Machines of the Far West') compiled and translated in 1627 by German Jesuit Johann Schreck (1576 - 1630) and Ming Dynasty Chinese author Wang Zheng (王徵 1571 - 1644), a field mill is shown amongst other devices.[7] In this picture, two mills are operated by the gearing of a rotating bar and a whippletree harnessed to a single horse, unlike the two horses seen in Zonca's illustration.[8]