**PRESSURE** — the physical quantity characterizing intensity of force operating per acre in the perpendicular (normal) direction to this area; plays an important role in the physical, chemical and biological phenomena, it is considered in medical practice during creation of various devices and during the development of new methods of treatment.

, as well as any force, results from interaction of bodies with each other. At hypodispersion of force of F operating on this surface of S, pressure of p made on this surface, W calculate on a formula: p = F/S. If forces are distributed on a body surface unevenly, then specify average D. or find D. in various points of a surface. At the same time conditionally understand such small site of a surface of dS as «point», on Krom distribution of the operating force of dF can be considered uniform. Thus, D. is equal in this point of a surface to dF/dS.

Take D. tested by unit of a plane surface under the influence of evenly distributed unit of force for unit of D. In the International System of Units (SI) dimension of H 1 m is taken for D.'s unit^{ 2 } (pascal, Pa), in the SGS system — dynes/cm^{ 2 } .

Often put stand-alone units of D. into practice: the atmosphere is physical, equal 1,1013*10^5 Pa, and also mm of mercury. and mm w.g. In the first case as D.'s unit serves D. made by a mercury column (at t ° 0 °), in - the second — a water column (at t ° 4 °) 1 mm high. In bioacoustics and meteorology apply as unit of D. of millibars (MB) equal 10^{ 2 } Pas.

Atmospheric and barometric D. is the D. made by sole weight atmospheres (see) on any body which is in it. Of gas — D. rendered by gas on vascular walls in Krom it is, owing to thermal agitation of its molecules. Of gas with sufficient approach in many cases can be calculated on Klapeyron's equation — Mendeleyeva: p = mRT/Mv, where m — the mass of gas, T — the absolute temperature of gas, M — a pier. the weight (weight), v — volume, and R — a universal gas constant (see. Gaza ). If the vessel is filled with mix of gases, then D., a cut each gas separately would make, occupying the volume of all mix, is called partial D. of gas. Under Dalton's law the general D. of gas mixture is equal to the sum of the partial D. making mix of gases. Influence of change on a live organism of the general D. of a gaseous fluid or partial D. of one of its components is used in medical practice (see. Hyperbaric oxygenation , Hyperoxia ). Hydrodynamic D. (RD) is called D. tested by a body in fluxion: RD = Rp — RS where Rp — total D., and RS — hydrostatic D. Gidrostaticheskoye D. is the D. made by sole weight liquids (see) on the body which is in it. If at hydrostatic D.'s definition also the atmospheric D. operating on the free surface of liquid, then measured by D. is considered call absolute hydrostatic. D.'s measurement is taken with the help barometers (see), manometers (see), and also various sensors D. (see. Sensors ).

See also Osmotic pressure .

**Bibliography:** Burdun G. D. Reference book on the International System of Units, M., 1971, bibliogr.; Kikoina. To. and To and - to about and I. K N. Molecular physics, L., 1976.

*V. P. Mishin.*