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 there are two related points projected onto the surface of the Earth. These points can be used to determine the corresponding surface velocity components that are in opposition and not acting in the same plane of the swing. The magnitude of the difference between these two points (for a given latitude of the center-point) is a relative measure of the time to observe one full rotation. The ratio of the velocity vector difference to the corresponding points at the North Pole with the same equidistance from the center of the swing and the same projection to the surface can then