A pendulum is constructed as a light thin-walled sphere of radius \(R\) filled up with water and suspended at the point \(O\) from a light rigid rod (Fig. 4.11). The distance between the point \(O\) and the centre of the sphere is equal to \(l\). How many times will the small oscillations of such a pendulum change after the water freezes? The viscosity of water and the change of its volume on freezing are to be neglected.