An open glass tube is immersed in mercury in such a way that a length of $8$ $\mathrm{cm}$ extends above the mercury level. The open end of the tube is then closed and sealed and the tube is raised vertically up by additional $46$ $\mathrm{cm}$ . The length of the air column above the mercury, in the tube now, is (Atmospheric pressure $= 76 \mathrm{cm}$ of Hg)