Who was Archimedes and what did he discover?

Archimedes was a Sicilian mathematician who lived circa 287-212 BC. He discovered the principles of hydrostatics, namely the science of how solid bodies will act when placed in liquids.

He is reputed to have made this discovery when in the bath and having had a blinding flash of genius, shouted Eureka (I have found it) and ran naked through the street.