I've been told that a clock will run faster with fully wound mainspring because the spring pushes the gears faster. The clock will slow down as the spring winds down and exerts less force. I've also heard the opposite, that a clock will run faster as mainspring winds down because the lower tensions on the gears allows them to move faster. (Sort of like a person can walk faster when he isn't trying to walk faster.) Both seem wrong to me. The pendulum should keep it accurate throughout the range of the spring until the power impulse is too small to keep the pendulum swinging. The period of pendulum (the time to complete 1 oscillation) is only dependent on the length of the pendulum. Weight of the bob has no effect, other than inertial. A pendulum with a period of 1 second will stay at that period unless the length is changed, for example by moving the bob up or down (and altering the length by moving the center of gravity). How would the strength of the spring change anything? A stronger spring would impart more force to the pendulum, possibly increasing the amplitude but not changing the period. The weaker spring would impart less force, and when too weak to keep the amplitude large enough, the escapement stops, and then the pendulum stops as the amplitude gets smaller and smaller, but not slower and slower. Am I missing something?