An open U - tube of uniform cross-section contains mercury when 54.4cm of water is poured in to one limb of the tube, then the difference in levels of liquid of the two sides is $\left({\mathrm{d}}_{\text{water }}=1\mathrm{gm}/{\mathrm{cm}}^3,\text{ and }{\mathrm{d}}_{\mathrm{Hg}}=13.6\mathrm{gm}/{\mathrm{cm}}^3\right)$