Lightning rod
Balls of solid glass occasionally were used in a method thought to prevent lightning strikes to ships. It is worth noting here not because it worked, which it did not, but because it reveals a lot about pre-scientific thought. Glass objects do not conduct electricity well. They are seldom struck by lightning. Therefore, goes the theory, there must be something about glass that repels lightning. Hence the best method for preventing a lightning strike to a wooden ship was to bury a small solid glass ball in the tip of the highest mast. The random behavior of lightning ensured that the method gained a good bit of credence even after the development of the marine lightning rod soon after Franklin's initial work.