The area of cross-section of one limb of an U-tube is twice that of the other. Both the limbs contain mercury at the same level. Water is poured in the wider tube so that the mercury level in it goes down by 1cm. the height of the water column is (density of water = 10 3 kgm −3 , density of mercury = 13.6×10 3 kgm −3 ).