The storey of the discovery of the isochronism of a pendulum by Galileo Galilei's observation of a swinging lamp in Pisa Cathedral is well known. Galileo believed that the pendulum took the same time to swing both wide arcs and small arcs, i.e. That it was isochronous, whereas, in fact, the shorter arcs take slightly less time than the wide swings. Vicenzio, Galileo's son, in 1641, drew a design of a form of duplex clock escapement, given to him by his father. The ratchet-shaped locking teeth, formed on the periphery of the escape wheel, are locked by the detent which is shown in the lifted position. The pendulum staff carries two arms; the lower arm is shown receiving impulse, while the upper is lifting the detent. No original examples of this escapement of this escapement are known to exist.