Liquid pressure. Thrust on a horizontal surface.
It is named after Archimedes of Syracuse, who first discovered this law. According to Archimedes' principle, he was looking for a method in order to assertain that the gold given by the king to the crown maker was all used in the crown. He could easily measure the mass of the object but how much volume did it take up?
While under pressure from the king to remedy this question quickly!! Archimedes took time to have a bath at the bath house, upon stepping in to the bath he saw water lap over the edge
"EUREKA !!!!! ", he cried !! "I have found it".
"Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."
The weight of the displaced fluid is directly proportional to the volume of the displaced fluid (if the surrounding fluid is of uniform density). Thus, among completely submerged objects with equal masses, objects with greater volume have greater buoyancy.
Suppose a rock's weight is measured as 10 newtons when suspended by a string in a vacuum. Suppose that when the rock is lowered by the string into water, it displaces water of weight 3 newtons. The force it then exerts on the string from which it hangs would be 10 newtons minus the 3 newtons of buoyant force: 10 − 3 = 7 newtons. Buoyancy reduces the apparent weight of objects that have sunk completely to the sea floor. It is generally easier to lift an object up through the water than it is to pull it out of the water.
The density of the immersed object relative to the density of the fluid can easily be calculated without measuring any volumes:
s = w/b
Hydrostatic pressure from walter http://www.walter-fendt.de/ph14e/hydrostpr.htm
here we go with more from walter on upthrust
A video on buoyancy