shu' or 'ground rat' mentioned in 1264 or the 'chong' mortar used by the armies of the Song Dynasty and invading Mongols during the 1270s), during the Yuan Dynasty (1271–1368) the term 'fire arrow' once implied to mean incendiary arrows during the Tang Dynasty was then used to describe the true rocket, producing a headache, as Needham says, for historians; the Huolongjing written by military officer Jiao Yu (fl. 14th to early 15th century) and the Song dynasty Chinese philosopher and politician Liu Bowen (1311–1375) during the early Ming Dynasty (1368–1644) described several types of rockets, one of them being a multistage rocket known as the 'huo long chu shui' or 'fire dragon issuing from the water' which, despite its name, was not launched from beneath the water from a primitive submarine but rather at near water-level maintaining a flat trajectory; defined as a two-stage rocket, it employed booster rockets that, when about to burn out of use, ignited a swarm of smaller rocket arrows fired from the front end of the missile shaped as a dragon's mouth. Multiple-tube seed drill: The wooden seed drill existed in China by the 3rd century BC, while the multiple-tube iron seed drill was first invented in China by the 2nd century BC, during the Han Dynasty (202 BC – 220 AD). The seed drill allowed for greater speed and regulation of distributing seeds in lined rows of crops instead of casting them out onto the farm field. N Natural gas as a fuel: On tomb brick reliefs of Sichuan province dating to the Han Dynasty (202 BC – 220 AD), scenes of borehole drilling for mining projects are shown. They show towering derricks lifting liquid brine through bamboo pipes to the surface so that the brine could be distilled in evaporation pans over the heat of furnaces and produce salt. The furnaces were potentially heated by natural gas brought by airtight jointed bamboo pipes for miles to towns and villages, yet gas brought up from perhaps below the surface could cause an explosion if it was not properly mixed with oxygen first, so the Han dynasty Chinese possibly built underground carburetor chambers and siphoned some of the gas off with exhaust pipes. Naval mine: The Huolongjing military manuscript written by Ming dynasty military writer, strategist, and philosopher Jiao Yu (fl. 14th to early 15th century) and Chinese military strategist, philosopher, statesman and poet