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Medieval siege engines
The projectiles hurled from many early weapons were propelled by the use of either torsion or counterpoise techniques. The former relys on using the power captured in a tense twisted core of ropes. The latter exploits the force created when weight is opposed on a lever. Direct tension is another method sometimes employed, as in a bow or crossbow, but the stresses on a single device like a bow arm can often prevent construction on a large scale.
The ballista
This is one of the earliest siege weapons using the torsion method of propulsion and would have first seen action in Britain when the Romans invaded. Looking like a large tripod-mounted crossbow, the two arms of the machine are attached to twisted cores of ropes. As the bowstring is cranked back the rope cores strain to hold increasing pressure. A release mechanism shoots the weapon when the aimer is ready, sending a bolt, or dart, racing to its target over a considerable range.
The catapult
The second torsion engine is the onager or mangonel (commonly referred to as a catapult). Again the Romans employed these machines. They used them to throw missiles high into the air. Though the size of projectile is relatively small compared to the bulk of the engine, special ammunition-like, ceramic-encased incendiaries could be shot accurately. Both the ballista and mangonel were so successful that they continued to be used through the medieval period long after the Romans had departed.
Counterpoise machines
The medieval period also saw the introduction of counterpoise machines in which a launching arm pivots on a bar, like a see-saw. Originating on the continent, the perrier and trebuchet (illustrated) could be used to devastating effect.
Perrier
The perrier is usually the smaller of the two. The initial power is provided by a team of men pulling down on ropes attached to one end. This offsets the weight of the missile in the sling at the other end and levers the arm with such force that the missile is slingshot from the machine. This machine is relatively fast to reset and so has a quick 'rate of fire'.
Trebuchet
The trebuchet is heavier, bigger and slower to reset. A ballast box provides the downforce on the lever, and by its sheer size and ability this machine proved to be one of the most effective of the engines devised.
Apart from the destructive capability of the siege engines, they must also have had a considerable psychological effect on the occupants of a fortress. Imagine watching them being built, knowing what was to come.
Impressive examples of all four of these siege engines are on permanent display at Caerphilly Castle, Caerphilly, South Wales. For further details contact Caerphilly Castle directly on 029 2088 3143.
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