On pouring 15 cm of water and spirit each into the respective arms of the U-tube, the mercury level will rise in the arm containing the spirit.$ho_{m}=$ density of mercury. Let us select two points A and B lying in the same horizontal plane asshown. Thus according to Pascal’s law, Pressure at $A=$ Pressure at $B$ or $P_{0}+h_{w} \rho_{w} g=P_{0}+h_{s} \rho_{s} g+h_{m} \rho_{m} g$ where $P_{0}=$ atmospheric pressure or $h_{w} \rho_{w}=h_{s} \rho_{s}+h_{m} \rho_{m}$ or $h_{m} \rho_{m}=h_{w} \rho_{w}-h_{s} \rho_{s}$ Here, $h_{w}=$ height of water column $=10+15=25 \text{ cm}$ $h_{s}=$ height of spirit column $=12.5+15=27.5 \text{ cm}$ $\rho_{w}=1\ \mathrm{g}\ \text{cm}^{-3} ; \rho_{s}=0.8\ \mathrm{g}\ \text{cm}^{-3}$$\rho_{m}=$ density of mercury $=13.6\ \mathrm{g}\ \text{cm}^{-3}$∴ Substituting in equation (i), we get$h_{m} \times 13.6=25 \times 1-27.5 \times 0.8$or $h_{m}= \frac{25-22.00}{13.6}$$=0.2206 \text{ cm}$$=0.221 \text{ cm}$ or $h_{m}=0.221 \text{ cm}$i.e. mercury will rise in the arm containing spirit, by 0.221 cm