THE SWING OF A PENDULUM GRADUALY DECREASES WITH TIME HOWEVER EVEN THEN IT IS REGARDED AS A RELIABLE TIME KEEPING DEVICE WHY
Any phenomenon that repeats itself regularly can serve as a measure of time. In pendulum,one second was defined as the time taken by a simple pendulum of length one meter in going from one extreme position to other extreme position. So pendulum was considered as a time keeping device. Later on this definition was modified but still it is a reliable time keeping device.