Aerostatics
A subfield of fluid statics, aerostatics is the study of gases that are not in motion with respect to the coordinate system in which they are considered. The corresponding study of gases in motion is called aerodynamics. Aerostatics studies density allocation, especially in air. One of the applications of this is the barometric formula. An aerostat is a lighter than air craft, such as an airship or balloon, which uses the principles of aerostatics to float. Basic laws. Treatment of the equations of gaseous behaviour at rest is generally taken, as in hydrostatics, to begin with a consideration of  the general equations of momentum for fluid flow, which can be expressed as: formula_1, where formula_2 is the mass density of the fluid, formula_3 is the instantaneous velocity, formula_4 is fluid pressure, formula_5 are the external body forces acting on the fluid, and formula_6 is the momentum transport coefficient. As the fluid's static nature mandates that formula_7, and that formula_8, the following set of partial differential equations representing the basic equations of aerostatics is found. formula_9 However, the presence of a non-constant density as is found in gaseous fluid systems (due to the compressibility of gases) requires the  inclusion of the ideal gas law: formula_10, where formula_11 denotes the universal gas constant, and formula_12 the temperature of the gas, in order to render the valid aerostatic partial differential equations: formula_13, which can be employed to compute the pressure distribution in gases whose thermodynamic states are given by the equation of state for ideal gases.