Ballistic Machines.

[Longmane]

Hopefully this piece from the same book will follow on well from the earlier one on sieges.


Ballistic Machines
Machines that threw projectiles were known by many names in their time, although today we refer to them all as catapults.  There are a few simple forces that can provide ballistic power without explosives or motors. Levers and gravity can be harnessed to provide flinging power, while the power of both tension and torsion derive from a material being bent so that it will spring or unwind back to its original state.     

Tension engines worked by bending wood; it would spring back to shape when the tension was released, thus flinging a projectile with the force of its movement.  Crossbows and longbows work on this principle, and some larger forms of crossbows could act as siege weapons, throwing larger projectiles.

These great crossbows were built on a frame and used a windlass at the back of the frame to wind the bolt on its string far back.  When the windlass was released, the wooden bows tension thrust its heavy bolt forward with speed and great force.  But wood's ability to bend and snap back is limited by its tendency to crack. Wooden bows could not throw anything larger than a bolt and could not take aim at walls, but only at people.

Torsion is the force exerted by a rope that has been twisted tightly and tries to untwist.  It is the principle of a child's toy boat or aeroplane that uses a rubber band wound up tight to drive paddles or propellers as it unwinds.

Torsion had been used to drive throwing machines since ancient times.  The Romans had a throwing machine called an "onager," a wild donkey.  It used a very thick band of rope, highly resistant to being twisted, as the torsion spring.  When a lever was inserted into the torsion spring and cranked back so that the rope was forced to twist, on release it sprang into the air.  The lever had a sling on the end with a heavy stone.  As it sprang into the air, it struck a bar that stopped its movement, and the stone flew out of the sling. The onager's simple torsion spring provided great velocity and force.

Medieval uses of the torsion spring are not as clear.  There is evidence that torsion machines of this kind were known in the time of Charlemagne.  Artists' illustrations show a machine similar to the Roman onager, but instead of a sling at the end of the lever, there is a spoon-shaped cup for the rock to be placed in.  It was probably called a mangonel.  Turkish medieval sources picture a device similar to the Roman one, called a manjaniq, used by Muslim armies.

In the 14th century, there were large crossbows that did not use bent wood, but rather had two separate arms with torsion springs.  Different types were known variously as ballistae and espringals (and in other languages, springarda or springolf ).  They were more often used by a fortress's defenders, since they shot bolts at individuals, rather than rocks at walls.

The espringal was built into a wooden frame, mounted on a tower.  On each side, the frame had a torsion spring made of very thick horsehair rope that was resistant to twisting.  Levers inserted into each spring were pulled back by ropes attached to the fi ring mechanism.  The espringal's firing system was like a crossbow, with a long groove for a bolt.  The operator cranked the bolt back, pulling on the levers and the torsion springs.  Released, the torsion springs untwisted and the levers shot the bolt forward, through the groove and out toward the target.  The bolts were long and heavy.  They could be expected to pierce wooden shields, steel armour, and sometimes more than one body.

The third type of throwing machine used levers and gravity.  Since ancient times, people had known that if a lever is put over a fulcrum, like a see-saw, and the lengths are not equal, it takes a much heavier weight on the short end to balance a lighter weight on the long end.  If the short end is suddenly weighted, the long end will fly into the air very fast.  Unlike tension and torsion, which depend on the strength of bent wood or twisted rope, lever-based machines can throw very heavy objects with relative ease.  As long as the lever's arm and the stand with the fulcrum hinge are strong enough, there is no load limit.

The perrier used only the lever to fling large stones.  The perrier depended on a sudden downward pull by men or horses.  Its frame lifted the lever's short arm above the men's heads, with a rope dangling down, and the long end rested on the ground with a sling.  They could load a heavy rock into the sling.  When the payload was in place, men with ropes pulled the short end down, as hard as they could, and the long arm with its rope swung upward suddenly, flinging the projectile into the air.  In order to achieve significant force, the pull had to be both sudden and hard.  Many ropes attached to a bar allowed many men or horses to pull.  Sudden pull could be achieved by having the throwing arm restrained by a latch as the men began pulling, so the latch could suddenly be released. The perrier may have been in use by the 11th century.

The trebuchet used a lever with a very heavy counterweight on its short end.  The long arm, with a sling on the end, was winched to the ground, forcing the boxy counterweight to lift into the air.  Men loaded a large stone into the sling as the long end was held down firmly.  When the long arm was released, the counterweight fell to the ground, suddenly lifting the long throwing arm and releasing its sling-propelled payload into the air.  Because the machine's power depended on gravity to pull the counterweight down, not on men or horses to tug it hard, the trebuchet was the strongest of the throwing machines.

Trebuchets could be built larger and stronger to throw ever-larger payloads.  Instead of a windlass, the winching could be accomplished by one or two wheels, the way the tallest cranes raised loads.  Several men stood inside the wheel and walked on its steps, using their weight and a pulley system to magnify the force. The counterweight, perhaps by now a large wooden bucket filled with many large stones, slowly lifted into the air.  The throwing arm was lashed down, the men got out of the wheels, and the counterweight could be released.

Rocks are the best-known catapult payload, and they were the most commonly used.  A machine could deliver a series of rocks to the same spot on a wall if the rocks were the same weight and the machine had not been moved.  This pounded the wall over and over, increasingly weakening it. Iron shot was even better than stone shot, but it was more expensive.

As a siege went on, trebuchets were loaded with new payloads that were intended to frighten or harm the people inside.  The trebuchet was now aimed to fling over the wall, not at it.  Most often, armies threw dead animals or even dead human body parts.  Severed heads were a common payload. Corpses spread disease, a deadlier attack than any rock.  An assault with corpses was also a psychological terror weapon, especially if the heads or other body parts belonged to people known to those inside.  Trebuchets could also fling manure.

A shrapnel effect came from "beehives"—clay pots packed with rocks.  They burst open on contact, and the rocks flew into the town to smash windows and injure people.  Armies also threw incendiary mixes, such as hot tar and quicklime.  Incendiary mixes were often called naphtha; there are a few existing recipes. Quicklime was the key ingredient, because water causes combustion on contact.  Other ingredients were flammable substances: pine pitch, tar, oil, animal fat, and dung.

Greek fire was the most famous incendiary compound of the time.  The name "Greek fire" caught on because it was invented in Constantinople, after they had lost territory to the invading Muslims.  Byzantine soldiers used catapults to fling pots of Greek fire at besieging Muslim armies.  It caught fire on contact, and even water did not put it out.  They could pump it at attacking ships and burn up whole fleets; in this way, they saved Constantinople in the seventh century when other cities were conquered by the invading Arabs. 

The secret composition of Greek fire was carefully guarded for a long time, but eventually first Muslims and then Christian Europeans learned how to make it.  It became a component of trebuchet attacks during sieges.  However, there is no surviving account of what was in Greek fire. Many scholars speculate that it must have contained quicklime or saltpeter, and others believe it had to use petroleum as a main ingredient.  The use of petroleum in some form seems very likely, since Greek fire was described as a liquid that burned even on top of water.