武经总要
In the 3rd century, the Chinese engineer Ma Jun invented the south-pointing chariot. This was a wheeled vehicle that employed differential gearing in order to lock a figurine of an immortal in place on the end of a long wooden staff, the figure having its arm stretched out and always pointing to the southern cardinal direction. Although the authors of the "Wujing Zongyao" were mistaken in believing that the design of the south-pointing chariot was not handed down (as it was reinvented during the Song period and combined with an odometer), they described a new device which allowed one to navigate. This was the 'south pointing fish' (a thermoremanence compass), essentially a heated iron (or preferably steel) object cut in the shape of a fish and suspended in a bowl of water. The "Wujing Zongyao" part 1 volume 15 text stated: