__Rate of Flow of Liquid:__

For streamlining the flow of a perfectly incompressible liquid, the amount of liquid flowing through any cross-section of a tube in a given time interval remains constant.

**The rate of flow of a liquid through a tube means the volume of liquid flowing through any cross-section of the tube per second.**

Suppose a liquid flows through a tube of the cross-sectional area a with a uniform velocity v. The volume of liquid flowing through any cross-section of the tube per second is equal to the volume of a cylinder of length v and cross-sectional area α.

∴ The volume of liquid flowing per second

= the rate of flow of the liquid

= velocity of flow x area of cross-section of the tube

= vα

Therefore, the mass of liquid flowing per second

= velocity of flow x area of cross-section of the tube x density of the liquid

= vαρ [ρ = density of the liquid]

__Equation of Continuity:__

For a streamline flow of a fluid (liquid or gas) through a tube, the mass of the fluid flowing per second through any cross-section of the tube remains constant. This is known as the **equation of continuity**.