The Science of the Metronome
The metronome works rather like an upside-down pendulum. A spring is wound up to keep it moving (just like a watch) and the speed of the "tick" depends on how high the weight is from the bottom of the arm.
As the weight is moved up, the centre of gravity moves up the arm. This means that the spring has to work a bit harder to reverse the swing and so it takes longer for each "tick".