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I just saw a documentary about the Archimedes steam cannon on the History Channel. Archimedes supposedly designed a steam-powered cannon (heat up steam behind the projectile and release it all to fire the weapon) and may have used it to defend Syracuse against the Romans. Tests indicated that it could sink ships from several hundred feet.
What would have happened had this weapon been mass-produced? Apparently Archimedes was killed during the siege. If the Romans had captured him and forced him to say how to make more weapons...