Zhang Zunyan and Vassilios Christides, there is literary evidence to suggest that the axial stern rudder existed in China since the 1st century BC, while Gang Deng asserts the first reference was made in the Huainanzi of the 2nd century BC, and K.S. Tom says the first clear reference dates to the 5th century AD. However, K.S. Tom points to the fact that all Chinese pottery models of ships before this Guangzhou tomb model show steering oars instead of a rudder, which he states is strong evidence for the rudder's invention only by the 1st century AD. Jacques Gernet states that while the Chinese had invented the rudder in the 1st century AD, it was not completely fixed to the sternpost of Chinese ships until the end of the 4th century. The bulkhead ship design of the junk, which appeared roughly the same time as the rudder, provided the essential vertical components for the hinged axial rudder. Deng points out that an Eastern Han (25–220) model distinctly shows a rudder located in its own separate cabin, suggesting that helmsmanship had already become an established profession. Following the invention of the balanced rudder pivoted on an axis, Tom and Deng state that the Chinese then innovated the fenestrated rudder by the Song Dynasty (960–1279), with deliberate puncturing and boring out of holes in shapes such as diamonds, which, according to Tom, made the rudder "easier to steer, reduced turbulence drag, did not affect efficiency and was hydrodynamically sound." S Salt well: The Chinese have been using brine wells and a form of salt solution mining as part of their civilization for more than 2000 years. The first recorded salt well in China was dug in the Sichuan province around 2,250 years ago. This was the first time that ancient water well technology was applied successfully for the exploitation of salt, and marked the beginning of Sichuan's salt drilling industry. Shaft wells were sunk as early as 220 BC in the Sichuan and Yunnan provinces. By 1035 AD, Chinese in the Sichuan area were using percussion drilling to recover deep brines, a technique that would not be introduced to Europe and the Western World for another 600 to 800 years. Medieval and modern European travelers to China between 1400 and 1700 AD reported salt and natural gas production from dense networks of brine wells. Archaeological evidence of Song dynasty salt drilling tools