Describe the method for measuring the equilibrium vapour pressure of water at a constant temperature.

(N/A) Process: Consider a transparent box equipped with a $U$-tube manometer containing mercury. $A$ drying agent,such as anhydrous calcium chloride $(CaCl_2)$ or phosphorus pentoxide $(P_4O_{10})$,is placed in the box for a few hours. This ensures the air inside the box is free from moisture (vapour).
After removing the drying agent by tilting the box,a watch glass (or petri dish) containing water is quickly placed inside the box.
Observation: The mercury level in the right limb of the manometer will be observed to increase slowly and eventually attain a constant value.
Conclusion: The observation indicates that:
$(i)$ Initially,the pressure inside the box increases due to the evaporation of water.
$(ii)$ After some time,the pressure inside the box becomes constant,indicating that the rate of evaporation equals the rate of condensation (equilibrium state).
$(iii)$ The volume of water in the watch glass decreases slightly until equilibrium is reached.