The Greeks were the first people to develop and use these siege engines, and the Romans adopted a lot of the Greek culture, including the siege engines. They made a lot of improvements, and around 400 B.C., they came up with the Gastrophetes. This was a big step forward in siege engineering, since the gastrophetes (or "bellybow") could launch an arrow further and with more power than an ordinary bow. It couldn't, however, be used for sieges against towns, since a stone wall wouldn't break down because of a little arrow.
The Romans then developed the first "real" siege engine, the Oxybeles. The oxybeles was in fact just a big gastrophetes mounted on a tripod for stability. It would launch big arrows, or javelins, towards its target. Most cities, however, began to build walls and towers to protect themselves. A bigger siege engine was needed. This became the Ballista.
The ballista could launch stones up to 100 kilos in weight more than 200 meters! They also used arrows or javelins, and that's where their name "shield piercer" comes from. Its major drawback was it's complicity, size and high cost. The siege engineers wanted something smaller, cheaper and simpler, so they invented the Scorpio, in fact a smaller version of the ballista. Despite their small size, they could launch an arrow more than 300 meters, with deadly accuracy.
The Cheiroballista was the first siege engine that employed steel as a spring frame. This was a great improvement, because the visibility through the spring frame was greatly improved. Together with it's high mobility, accuracy and power this was the most deadly siege engine ever made.
The Gastrophetes was first designed in 400 B.C., when the siege engineers wanted more power out of the composite bow. The bow was made out of a layer of horn, wood and animal sinew. With a normal bow the power stored in the bow couldn't be held by the archer for long. He also couldn't fire a bow that was too large, because he then wouldn't be able to pull back the bowstring. In the gastrophetes, the bow was larger and had more power stored in it because the archer would span the bow by using it's own weight, rather then it's hands. Therefore an U-shaped extension was made at the back end of the bow, where the archer would rest his belly on (that's why it's also called a "belly-bow"). The slider had a ratchet on both sides of the bow, which prevented the bow from firing when the archer took his weight of it. This was a far better and easier way to span the bow, and the bow could be made bigger and stronger, increasing the distance an arrow could be shot.
The Oxybeles was designed in 375 B.C., because the composite bow developed so fast that it grew too large and too powerful to be operated by a single human archer. The bow was placed on a tripod and a winch was fitted to draw it back. It was still made out of horn, wood and sinew, but it was bigger and more powerful then the gastrophetes bow. Because of that, the range was greater, and by placing the bow on a stable tripod the accuracy of the shot was also greatly improved. However, the limits of the materials were nearly reached, and the engineers still wanted more power. This led to the invention of the torsion spring bundle.
The Ballista. The Roman engineers wanted a bigger machine with a greater range, so in 50 B.C., they invented the ballista. The ballista was, unlike the Greek Catapulta, built almost entirely out of wood. The ballista used an older spring frame type that was more powerful as the arms could travel further, but it was also more expensive so it was used primarily for big stone throwing machines. The ballista also had the ratchet device on the outside, made out of steel. It was larger than the catapulta, in fact, some of the machines were the biggest ever made in the Roman period! They were also very expensive, because of the size and of the older, but more powerful spring frame.
The Scorpio was invented in 50 B.C., following the design of the Greek Catapulta. It also had a spring frame type made out of one piece, encased in steel. This was cheaper than the spring frame type of the Ballista, but it was also less powerful. That's why the arms are bent, so they could travel further to give a bit more speed and power to the projectile. The smaller types were far more dangerous than their size might indicate, they could launch a arrow over 350 meters!
The Cheiroballista was the last siege engine invented by the Romans, being developed in 100. a.D. The spring frame was made out of metal, and the skeins were encased in bronze cilinders to protect them from the weather. The arms were also made out of steel, which were a lot smaller than the big arms of the Ballista. It was also highly mobile because it's small size. It could be loaded onto a mule cart to be driven to the battlefield, and some of them even had their own wheels. Combined with the power and accuracy it was the most deadly and advanced catapult ever designed.
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