[UPSC CSE 2008] Mechanics of Continuous media (Q289, 15M)

What do you understand by streamline motion and critical velocity of a viscous liquid through a capillary tube. Capillaries of lengths $ l $, $ 2l $ and $ \frac{l}{2} $ are connected in series. Their radii are $ r $, $ \frac{r}{2} $ and $ \frac{r}{3} $ respectively. If the streamline flow is maintained and the pressure across the first capillary is $ P_1 $, deduce the pressures across the second and the third capillaries.