In physics, the concept that as the speed of a moving fluid (liquid or gas) increases, the pressure within that fluid decreases. Originally formulated in 1738 by the Swiss mathematician and physicist Daniel Bernoulli, it states that the total energy in a steadily flowing fluid system is a constant along the flow path. An increase in the fluid's speed must therefore be matched by a decrease in its pressure. Bernoulli's principle applies in nozzles, where flow accelerates and pressure drops as the tube diameter is reduced. It is also the principle behind orifice or Venturi flow meters. These meters measure the pressure difference between a low-speed fluid in an approach pipe and the high-speed fluid at the smaller orifice diameter to determine flow velocities and thus to meter the flow rate. Bernoulli's principle is sometimes mistakenly used to explain the net force in a system that includes a moving fluid, such as lift on an airplane wing, thrust of a ship's propeller, or drifting of aB