The early gunpowder formula contained too little saltpeter (about 50%) to be explosive, but the mixture was highly flammable, and contemporary weapons reflected this in their deployment as mainly shock and incendiary weapons. One of the first, if not the first of these weapons was the fire arrow. The first possible reference to the use of fire arrows was by the Southern Wu in 904 during the siege of Yuzhang. An officer under Yang Xingmi by the name of Zheng Fan (鄭璠) ordered his troops to "shoot off a machine to let fire and burn the Longsha Gate", after which he and his troops dashed over the fire into the city and captured it, and he was promoted to Prime Minister Inspectorate for his efforts and the burns his body endured. A later account of this event corroborated with the report and explained that "by let fire (飛火) is meant things like firebombs and fire arrows." Arrows carrying gunpowder were possibly the most applicable form of gunpowder weaponry at the time. Early gunpowder may have only produced an effective flame when exposed to oxygen, thus the rush of air around the arrow in flight would have provided a suitably ample supply of reactants for the reaction.