used as units of distance. 1 inch is 25.4 millimetres. 1 ft is 12 inches or 304.8 mm.) After this initial description of Yan Su's device, the text continues to describe the work of Wu Deren, who crafted a wheeled device that would combine the odometer and south-pointing chariot: Chariots with differential gears Background and explanation The English engineer George Lanchester proposed that some south-pointing chariots employed differential gears. A differential is an assembly of gears, nowadays used in almost all automobiles except some electric and hybrid-electric ones, which has three shafts linking it to the external world. They are conveniently labelled A, B, and C. The gears cause the rotation speed of Shaft A to be proportional to the sum of the rotation speeds of Shafts B and C. There are no other limitations on the rotation speeds of the shafts. In an automobile, Shaft A is connected to the engine (through the transmission), and Shafts B and C are connected to two road wheels, one on each side of the vehicle. When the vehicle turns, the wheel going around the outside of the turning curve has to roll further and rotate faster than the wheel on the inside. The differential permits this to happen while both wheels are being driven by the engine. If the sum of the speeds of the wheels is constant, the speed of the engine does not change. In a south-pointing chariot, according to the hypothesis, Shaft B was connected to one road wheel and Shaft C was connected through a direction-reversing gear to the other road wheel. This made Shaft A rotate at a speed that was proportional to the difference between the rotation speeds of the two wheels. The pointing doll was connected (possibly through intermediate gears) to Shaft A. When the chariot moved in a straight line, the two wheels turned at equal speeds, and the doll did not rotate. When the chariot turned, the wheels rotated at different speeds (for the same reason as in an automobile), so the differential caused the doll to rotate, compensating for the turning of the chariot. The hypothesis that there were south-pointing chariots with differential gears originated in the 20th century. People who were familiar with modern (e.g. automotive) uses of differentials interpreted some of the ancient Chinese descriptions in ways that agreed with their own ideas. Essentially, they re-invented the south-pointing chariot, as