Ambracia was besieged by the Romans, who dug mines under the walls. The defenders filled a clay jar with chicken feathers, which they then lit, using bellows to blow the acrid smoke down the tunnel; unable to approach the pot due to defensive spears, the Romans were forced to abandon their works. Quicklime, sulfur and smoke The 15th-century engineer Taccola recommended quicklime, although its use went back to ancient times, and might well have been a component of Greek fire. Quicklime reacts violently with water, and can cause blindness and burns. While quicklime was used in some naval battles, it does not appear to have been standard issue on board ships, due to the danger of the quicklime blowing back and burning the user. Other substances smoked rather than flamed. Sacks of burning sulfur were effective at clearing enemy mines due to the toxic smoke produced. Any smoke could be used in small confines; the Greek military writer Aeneas Tacticus recommended burning wood and straw to drive out enemy sappers by the smoke. Gunpowder and cannon The discovery of gunpowder was probably the product of centuries of alchemical experimentation. Saltpetre was known to the Chinese by the mid-1st century AD and there is strong evidence of the use of saltpetre and sulfur in various largely medicinal combinations. The impetus for the development of gunpowder weapons in China was increasing encroachment by tribes on its borders. The earliest known formula for gunpowder can be found in a Chinese work dating probably from the 9th century. The Chinese wasted little time in applying it to warfare, and they produced a variety of gunpowder weapons, including flamethrowers, rockets, bombs, and mines, before inventing firearms. European descriptions of gunpowder first appear in Opus Maius and Opus Tertium, written by the English philosopher Roger Bacon in the mid-13th century, although the mixture was not very effective. The composition of gunpowder varied throughout the period, and did not settle into the current ratios of saltpetre, sulfur and charcoal until the 17th century. The years 904–906 saw the use of incendiary projectiles called 'flying fires' (fei-huo). Needham (1986) argues that gunpowder was first used in warfare in China in 919 as a fuse for the ignition of another incendiary, Greek fire. Initially, gunpowder mixtures were utilised through traditional engines and throwing mechanisms; containers and grenades were thrown by mangonels and trebuchets, and explosive rockets and arrows were