A rubber ball filled with water, having a small hole is used as the bob of a simple pendulum. The period of such a pendulum

where L = separation between point of suspension and centre of gravity of ball water system.When the ball is completely filled, the c.g. lies at the centre of ball. As water is gradually lies at the centre of ball. As water is gradually draining out, the position of c.g. is gradually coming down. As a result L increases and T increases. When the ball is half filled, the c.g. of the system is at the lower most position. So L becomes maximum and T becomes maximum As the water is further coming out of the ball, position of c.g. is gradually moving up and L gradually decreases, so T also decreases when the ball is completely empty, the position of c.g. of the system lies at the centre of the ball. As a result time period of oscillation is reduced to the value as at the beginning.