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Archimedes' principle
圖片參考：http://upload.wikimedia.org/wikipedia/commons/e/e0/Submerged-and-Displacing.png

Archimedes' principle, or the law of upthrust, is :"a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid."In other words, when a body is partially or completely immersed in aliquid, then it experiences an upward buoyant force which is equal tothe weight of the fluid displaced by the immersed part of the body.
It is named after Archimedes of Syracuse, who first discovered this law. Vitruvius (De architectura IX.9–12) recounts the famous story of Archimedes making this discovery while in the bath (for which see eureka) but the actual record of Archimedes' discoveries appears in his two-volume work, On Floating Bodies.
This is true only as long as one can neglect the surface tension (capillarity) acting on the body.
The weight of the displaced fluid is directly proportional to thevolume of the displaced fluid (specifically if the surrounding fluid isof uniform density). Thus, among objects with equal masses, the onewith greater volume has greater buoyancy.
The density of the immersed object relative to the density of the fluid is easily calculated without measuring any volumes:

圖片參考：http://upload.wikimedia.org/math/8/8/9/88905ba13e85a42d1b704d14febb1c15.png


2008-03-23 16:57:18 補充：
Please go to the web page ( http://en.wikipedia.org/wiki/Buoyancy#Archimedes.27_principle) to see "Density" - detail explaining for the ship float.