If an incompressible, non-viscous fluid flows through a tube of non-uniform cross section, the product of the area of cross section and the fluid speed at any point in the tube is constant for a streamline flow.

Let A_{1} and A_{2} denote the areas of cross section of the tube where the fluid is entering and leaving. If v_{1} and v_{2} are the speeds of the fluid at the ends A and B respectively, and ρ is the density of the fluid, then the liquid entering the tube at A covers a distance v_{1} in one second.

So volume of the liquid entering per second = A_{1} × v_{1}

Mass of the liquid entering per second at point A = A_{1} v_{1} ρ

Mass of the liquid leaving per second at point B = A_{2} v_{2} ρ

Since there is no accumulation of fluid inside the tube, the mass of the liquid crossing any section of the tube must be same. Therefore,

A_{1} v_{1} ρ = A_{2} v_{2} ρ

**A _{1} v_{1} = A_{2} v_{2}**

This expression is called **equation of continuity**.