When a fluid ( liquid or gas ) flows in a tube of flow in streamline motion, the rate of flow of mass of the fluid through any cross-sectional area of the tube remains the same, this means that the quantity of fluid entering one end of a flow per second is the same as leaving the tube at the other end of a flow tube per second. It is known as the equation of continuity.

Let us consider two sections of a flow tube at the points $A$ and $B$ having cross-sectional areas $\alpha_1$ and $\alpha_2$ respectively, let $v_1$ and $v_2$ be the velocities at A and B respectively, $\rho_1$ and $\rho_2$ be the densities of the fluid at $A$ and $B$ respectively.

The mass of fluid crossing the cross-section $\alpha_1$ per second is $v_1\alpha_1\rho_1$ and the mass of fluid crossing the cross-section $\alpha_2$ per second is $v_2\alpha_2\rho_2$

Since there are no sources or sinks with the volume between the sections A and B, by using the law of conservation of matter we can write

$v_1\alpha_1\rho_1=v_2\alpha_2\rho_2$

Since the fluid is incompressibe, therefore $\rho_1=\rho_2$