The volume of water flowing per second through a tube of length l and radius r under a pressure difference P is given by

v = πpr^{4}/8ηl,

Where η = coefficient of viscosity.

Given that the pressure difference is equal to height of water i.e.,

p = egλ

and volume flowing per second is 40 ml. Hence.

When another tube of same length but radius (r/2) is connected in series with this tube, let P_{1} and P_{2} be the pressure difference across these tube respectively. They are connected to the same pressure head.

Dividing equation (i) in equation (ii), we get