According to
legend,
Archimedes helped
defend the city of
Syracuse (no, not in
New York) by building a
mirror that would focus the
sun's rays onto a
ship and set it
aflame. Supposedly, the
mirror was not
parabolic. It approximated a
parabola with seven flat pieces (the center a
hexagon, each
flange a
rectangle). The
six flanges were each
connected to the center by a
hinge, and they could
move together to adjust
focus.
Unfortunately, this is probably a myth. Why? First, assuming that the mirror could focus enough sunlight to work, you'd have to keep it focused on the same spot on a moving ship long enough to get the fire going. Also, early histories only say that Archimedes found out how to 'hurl fire' at the enemy ships. Nothing about mirrors is mentioned; it was probably the product of a hyperactive imagination (maybe ol' Arch did something involving catapults instead).