The period of oscillation of a free pendulum increases with the arc of oscillation. This effect is known technically as the Circular Error. Its theoretical amount for the simple pendulum is readily calculated, and the following tables give it. I have called it the Circular Equation in place of the Circular Error, employing the word equation in its astronomical meaning of a theoretical numerical adjustment of a result, and distinguishing it thus from discrepancies of unknown source and amount which may also present themselves. The relation between arc and rate in the maintained motion of the pendulum of a clock will be a subject for examination experimentally and otherwise in future numbers of these studies. It may contain several elements, but must include the Circular Equation among them.