used are kept and displayed in the Zigong Salt Industry Museum. Many of the wells were sunk deeper than 450 m and at least one well was more than 1000 meters deep. The medieval Venetian traveler to China Marco Polo reported an annual production in a single province of more than 30,000 tonnes of brine during his time there. According to Salt: A World History, a Qing Dynasty well, also located in Zigong, "continued down to 3,300 feet [1,000 m] making it at the time the deepest drilled well in the world." Seismometer: The Chinese polymath and inventor Zhang Heng (78–139) of the Han Dynasty (202 BC – 220 AD) invented the first seismometer in 132, a large metal urn-shaped instrument which employed either a suspended pendulum or inverted pendulum acting on inertia (i.e. ground tremors from earthquakes) to dislodge a metal ball by a lever trip device; this ball would fall out of dragon-shaped metal mouth into the corresponding metal toad mouth indicating the exact cardinal direction of where a distant earthquake had occurred in order for the state to send swift aid and relief to the affected regions; several subsequent recreations of his device were employed by Chinese states up until the Tang Dynasty (618–907), when use of the device fell into obscurity, a fact noted even by the writer Zhou Mi around 1290, during the Yuan Dynasty (1271–1368). Sky lantern: The Chinese military strategist, politician, writer, and inventor Zhuge Liang of the Three Kingdoms era is credited with its invention, and reportedly used it during military campaigns. According to the British scientist, historian, and sinologist Joseph Needham, sky lanterns have been used in China since the 3rd century BC. In 1783, the French entrepreneurs and inventors Joseph-Michel and Jacques-Etienne Montgolfier took part in the first modern manned hot air balloon flight. Snow gauge: The first use of snow gauges were precipitation gauges that was widely used in 1247 during the Southern Song dynasty to gather meteorological data. The Song Chinese mathematician and inventor Qin Jiushao records the use of gathering rain and snowfall measurements in the Song mathematical treatise Mathematical Treatise in Nine Sections. The book discusses the use of large conical or barrel-shaped snow gauges made from bamboo situated in mountain passes and uplands which are speculated to be first referenced to snow measurement. Solid-propellant rocket: The medieval Song dynasty Chinese invented the solid-propellant rocket at