round as possible. Early Roman ballistae The early Roman ballistae were made of wood, and held together with iron plates around the frames and iron nails in the stand. The main stand had a slider on the top, into which were loaded the bolts or stone 'shot'. Attached to this, at the back, was a pair of winches and a claw, used to ratchet the bowstring back to the armed firing position. A slider passed through the field frames of the weapon, in which were located the torsion springs (rope made of animal sinew), which were twisted around the bow arms, which in turn were attached to the bowstring. Drawing the bowstring back with the winches twisted the already taut springs, storing the energy to fire the projectiles. The ballista was a highly accurate weapon (there are many accounts right from its early history of single soldiers being picked off by the operators), but some design aspects meant it could compromise its accuracy for range. The lightweight bolts could not gain the high momentum of the stones over the same distance as those thrown by the later onagers, trebuchets, or mangonels; these could be as heavy as 90–135 kg (200-300 pounds). The Romans continued the development of the ballista, and it became a highly prized and valued weapon in the army of the Roman Empire. It was used, just before the start of the empire, by Julius Caesar during his conquest of Gaul and on both of his expeditions to Britain. Both attempted invasions of Britain and the siege of Alesia are recorded in his own commentarii (journal), The Gallic Wars (De Bello Gallico). First invasion of Britain The first invasion of Britain took place in 55 BC, after a rapid and successful initial conquest of Gaul, in part as an exploratory expedition, and more practically to try to put an end to the re-enforcements sent by the native Britons to fight the Romans in Gaul. A total of eighty transports, carrying two legions, attempted to land on the British shore, only to be driven back by the many British warriors assembled along the shoreline. The ships had to unload their troops on the beach, as it was the only one suitable for many kilometers, yet the massed ranks of British charioteers and javeliners were making it impossible. Seeing this, Caesar ordered the warships – which were swifter and easier