Velocity of air in the test section of a low speed open circuit subsonic wind tunnel is given by  $$V_{2} = \sqrt{\frac{2\left ( p_{1} – p_{2} \right )}{\rho \left [ 1 – \left ( \frac{A_{2}}{A_{1}} \right )^{2} \right ]}}$$ Pressure difference, $p_{1}-p_{2}$ is measured by means of a manometer. $$p_{1}-p_{2}= \gamma \Delta h$$ Here, $\gamma$ = density of mercury * acceleration due to gravity

$\Delta h$ = Difference in heights of the liquid in the manometer between its two side.

Therefore, $$V_{2}=\sqrt{\frac{2\gamma \Delta h}{\rho \left [ 1-\left ( \frac{A_{2}}{A_{1}} \right )^{2} \right ]}}$$ $$\gamma = \left ( 1.36 \times 10^{4} \right )\left ( 9.8 \right )=1.33\times 10^{5}N/m^{2}$$ $$h = 10.5\,cm = 0.105\,m$$ $$\Rightarrow  V_{2} = \sqrt{\frac{2\times \left ( 1.33\times 10^{5} \right )\times 0.105}{1.225\left [ 1-\left ( \frac{1}{10} \right )^{2} \right ]}}=151.8\,m/s$$