The arrangement shown in the figure measures the velocity $V$ of a gas of density $1 kg/m^{3}$ flowing through a pipe. The acceleration due to gravity is $9.81 m/s^{2}$. If the manometric fluid is water (density $1000 \: kg/m^{3}$) and the velocity $V$ is $20 m/s$, the differential head $h$ (in mm) between the two arms of the manometer is __________.
