A cylindrical tube of length 100 cm contains some mercury inside. If the length of remaining air column remains constant at all temperatures, the height of mercury (in cm) is (neglect the expansion of cross –section ; and take  ${\gamma }_r$  of  $Hg=18\times {10}^{-5}{}^0{C}^{-1}$ and  ${\gamma }_g$ of tube = $2.7\times {10}^{-5}{}^0{C}^{-1}$  )