De bello Gallico, book 5, about clay shot being heated before slinging, so that it might set fire to thatch. "Whistling" bullets Some bullets have been found with holes drilled in them. It was thought the holes were to contain poison. John Reid of the Trimontium Trust, finding holed Roman bullets excavated at the Burnswark hillfort, has proposed that the holes would cause the bullets to "whistle" in flight and the sound would intimidate opponents. The holed bullets were generally small and thus not particularly dangerous. Several could fit into a pouch and a single slinger could produce a terrorizing barrage. Experiments with modern copies demonstrate they produce a whooshing sound in flight. The sling in medieval period Europe The Bayeux Tapestry of the 1070s portrays the use of slings in a hunting context. Frederick I, Holy Roman Emperor employed slingers during the Siege of Tortona in 1155 to suppress the garrison while his own men built siege engines. Indeed, slings seem to have been a fairly common weapon in Italy during the 11th and 12th centuries. Slings were also used by the Byzantines. On the Iberian peninsula, the Spanish and Portuguese infantry favoured it against light and agile Moorish troops. The staff sling continued to be used in sieges and the sling was used as a part of large siege engines. The Americas The sling was known throughout the Americas. In ancient Andean civilizations such as the Inca Empire, slings were made from llama wool. These slings typically have a cradle that is long and thin and features a relatively long slit. Andean slings were constructed from contrasting colours of wool; complex braids and fine workmanship can result in beautiful patterns. Ceremonial slings were also made; these were large, non-functional and generally lacked a slit. To this day, ceremonial slings are used in parts of the Andes as accessories in dances and in mock battles. They are also used by llama herders; the animals will move away from the sound of a stone landing. The stones are not slung to hit the animals, but to persuade them to move in the desired direction. The sling was also used in the Americas for hunting and warfare. One notable use was in Incan resistance against the conquistadors. These slings were apparently very powerful; in 1491: New Revelations of the Americas Before Columbus, historian Charles C. Mann quoted a conquistador as saying