Arab and Islamic traders spread knowledge of the traction trebuchet beyond the Orient. By the sixth century a.d., the armies of Byzantium and the Middle East were using these machines in their military campaigns. Archbishop John of Thessalonike described a battery of fifty traction trebuchets called petrobolos (’city-takers’) in his eyewitness account of the siege of that Macedonian city in 597. He claimed these machines flung so many stones that ‘neither earth nor human constructions could bear the impacts.’

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The traction trebuchet offered an impressive rate of fire: A 1991 experiment conducted on a model made to ancient standards at the University of Toronto showed that a well-coordinated trebuchet crew could fire four rounds per minute. Yet the traction trebuchet was not without its shortcomings. As this experiment revealed, the logistics of coordinating a team of more than twelve pullers was very difficult, and the unavoidable mechanics limited the throwing arm to only a small fraction of its rotational potential.

Perhaps these limitations inspired engineers of the Near East and Mediterranean to upgrade the traction trebuchet design. They attached a weight to the short end of the throwing arm, resulting in an engine known as the hybrid trebuchet. The counterweight, possibly an iron plate forged directly to the short end of the pivoted beam, extended the range of the machines. Attaching a sling to the longer end of the beam and adding wheels that allowed the trebuchet to gain the full advantage of motion made it possible for war engines to sling a rock against a castle wall with accuracy.

Although hybrid trebuchets may have been known as early as the eighth century, documented evidence indicates this design was gaining widespread acceptance among Arab and Byzantine armies during the eleventh and twelfth centuries. The Byzantine chronicler Anna Komnene alluded to this emerging technology when she mentioned several ‘unconventional engines’ that were employed at the siege of Nicaea in Asia Minor in 1097, which she claimed ‘amazed everyone’ with their ability to hurl gigantic stones. In the military vernacular of the eleventh-century Islamic world, the hybrid trebuchet was al-Ghadban, or ‘the furious one.’

In a military manual written for Saladin in 1187, Arabic engineer Murdi ibn Ali ibn Murdi al-Tarsusi depicted a hybrid trebuchet that he said had the same hurling power as a traction machine pulled by fifty men due to ‘the constant force [of gravity], whereas men differ in their pulling force.’ (Showing his mechanical proficiency, Tarsusi designed his trebuchet so that as it was fired it cocked a supplementary crossbow, probably to protect the engineers from attack.)

Improved firing power was certainly the primary advantage of the hybrid trebuchet. Such a machine used at the siege of Damietta in Egypt in 1218 fired stones weighing four hundred pounds at the city walls.

Nevertheless, if a trebuchet powered by a small counterweight was good, then one with a large counterweight would be even better. As European engineers adopted the trebuchet and improved it in the eleventh, twelfth, and thirteenth centuries (after encountering these machines during the Crusades), this premise was taken to its logical conclusion by developing the counterpoise trebuchet.

Unlike traction and hybrid trebuchets, there was no need for human intervention in operating of the counterpoise trebuchet. These machines, powered by either stationary weights, or by hanging buckets filled with sand, rocks, or rubble from the short end of the beam, used gravity to far surpass the capacity of any crew of pullers. With no pulling team beneath the trestle, the sling could be laid in a launching trough directly under the pivot, creating a greater throwing arc. The centripetal acceleration and power of the counterpoise trebuchet could be enhanced by mounting the machine on wheels so it could move during the throw. Larger engines could sling rocks weighing a ton or more three hundred yards, hitting a castle wall with devastating force.

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