Pendulum the pendulum could be used to measure the force of gravity. During his expedition to Cayenne, French Guiana in 1671, Jean Richer found that a pendulum clock was minutes per day slower at Cayenne than at Paris. From this he deduced that the force of gravity was lower at Cayenne. In 1687, Isaac Newton in "Principia Mathematica" showed that this was because the Earth was not a true sphere but slightly oblate (flattened at the poles) from the effect of centrifugal force due to its rotation, causing gravity to increase with latitude. Portable pendulums began to be taken on voyages
Foucault pendulum vector diagrams the article discussing the Coriolis Effect for further details. The motion of ballistics with changing latitude is not helpful to understanding the change with latitude of the observed rotation time of the pendulum. (This discussion point is different from what is stated in the reference book.) There is only one point of connection to the Earth for the swinging pendulum and that point of connection doesn't move in relation to the Earth. Because the plane of the pendulum swing is free to swing in relation to the rotation of the structure of the connection point, the rotation of the Earth