Time period of the simple pendulum is denoted by T

The formula for the period, T, is

where is the length of the pendulum measured from the pivot point to the bob's center of gravity.

 

Here the hollow bob is filled with water. If at the bottom of the bob there is hole through which water is leaking. This means that center of mass of the bob is shifting and thus effective lenght l, which is the length if the pendulum from point of suspension to center of mass is also changing.

 

So here the time period is no longer constant but it is changing.