Length of mercury columns in two arms is equal.
Height to which water is poured in one arm, h = 13.6 cm
Let h' be the rise in the mercury level in the other arm.
Density of mercury = 13.6 × 103 kg m-3
Density of water = 103 kg m-3
Pressure due to water on one arm = Pressure on mercury column in the other arm
or 13.6 × 103 × g = h' × 13.6 × 103 × g
h' = 1 cm
The rise in mercury level in the other arm will be 1 cm.