All of which is fine, but this wasn't lost in the dark ages, this was supposedly used once, never used again, and never even recorded or noticed by the invading

All of which is fine, but this wasn't lost in the dark ages, this was supposedly used once, never used again, and never even recorded or noticed by the invading Romans, who might have been very interested in the novel weapon which was setting their ships on fire. Wikipedia has an article on steam cannons: http://en.wikipedia.org/wiki/Steam_cannon http://en.wikipedia.org/wiki/Steam_cannon which notes that there were various attempts to make a viable steam cannon during the heyday of the steam engine, but nobody managed to make a viable one until World War 2. Also, Mythbusters apparently did an episode about Archimedes and steam cannons. I can't find out much about it, but it doesn't look like it was an overwhelming success. In conclusion, I don't see a helluva lot of advantage for the steam cannon over the catapult.