I heard about that ship burning device (mirrors and some sort of telescope maybe) Archimedes designed and the steam cannon. I saw a show on the History Channel that proved the ship burning device impractical because it took them about 100 mirrors and maybe half an hour to set a pieve of wood on fire. I think the cannon was proven to be unrealistic because of the amount of time it would take to heat the water (steam), and because it wasnt powerful enough to deliver a projectile to a long range target.