Neutral buoyancy diver maintains neutral buoyancy by continuous correction, usually by controlled breathing, as neutral buoyancy is an unstable condition for a compressible object in a liquid. The mathematician Archimedes discovered much of how buoyancy works more than 2000 years ago. In his research, Archimedes discovered that an object is buoyed up by a force equal to the weight of the water displaced by the object. In other words, an inflatable boat that displaces 100 pounds (45 kilograms) of water is buoyed up by that same weight of support. An object that floats in the water is known as being "positively" buoyant.
The Legend of Zelda: A Link to the Past and Four Swords by a GameCube sequel, "", which continued the story and expanded upon the gameplay concepts while including a single-player adventure. In 2005, the "Four Swords" prequel "" for the Game Boy Advance became the next handheld "Zelda" game, also co-developed with Capcom. A sequel was announced for the Nintendo DS entitled "The Legend of Zelda: Four Swords DS". However, this was cancelled and replaced by "". To celebrate the 25th anniversary of "The Legend of Zelda" series, Nintendo released "The Legend of Zelda: Four Swords Anniversary Edition" on DSiWare as a free limited-time only download for the Nintendo DSi and