The key works are Joseph Needham, Science and Civilisation in China, vol. 5, Chemistry and Chemical Technology, pt. 7, Military Technology: The Gunpowder Epic (Cambridge: Cambidge University Press, 1986) and Joseph Needham and Robin D.S. Yates, Science and Civilisation in China, vol. 5, pt. 6, Military Technology: Missiles and Sieges (Cambridge: Cambridge University Press, 1994). Professor Yates is currently working on another military volume for the Needham project, one that will deal with chariots and cavalry among many other topics.