Hydrostatics, fluid statics, or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an immersed body. Hydrostatics is a subcategory of fluid statics, which is the study of all fluids, both compressible or incompressible, at rest. Archimedes is credited with the discovery of Archimedes' principle, which relates the buoyancy force on an object that is submerged in a fluid to the weight of fluid displaced by the object. Herons' fountain is a device invented by Heron of Alexandria that consists of a jet of fluid being fed by a reservoir of fluid. The fountain is constructed in such a way that the height of the jet exceeds the height of the fluid in the reservoir, apparently in violation of principles of hydrostatic pressure. Pascal's law is a fundamental principle of fluid mechanics that states that any pressure applied to the surface of a fluid is transmitted uniformly throughout the fluid in all directions, in such a way that initial variations in pressure are not changed. Due to the fundamental nature of fluids, a fluid cannot remain at rest under the presence of a sheer stress. Fluids can exert pressure normal to any contacting surface. If a point in the fluid is thought of as an infinitesimally small cube, then it follows from the principles of equilibrium that the pressure on every side of this unit of fluid must be equal. For a barotropic fluid in a conservative force field like a gravitational force field, the pressure exerted by a fluid at equilibrium becomes a function of force exerted by gravity. The hydrostatic pressure can be determined from a control volume analysis of an infinitesimally small cube of fluid. Any body of arbitrary shape which is immersed, partly or fully, in a fluid will experience the action of a net force in the opposite direction of the local pressure gradient, 4-4-2-7-7-8-3-28-16.