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The Chinese military essay of the Wujing Zongyao is written and built by scholars Zeng Gongliang (曾公亮), Ding Du (丁度), and Yang Weide (楊惟德) during the Song Dynasty. It is the first book in history to include formulas for gunpowder and its use for various bombs (thrown by sling or trebuchet catapult). It also describes the double-piston pump flamethrower and a thermoremanence compass, a few decades before Shen Kuo wrote of the first known magnetic mariners compass. Although making clear the importance of many weapons, it reserves high respect for the crossbow and ability of crossbowmen to eliminate charging units of nomadic cavalrymen.