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 longitudinal (north-south) direction the surface velocity vectors on either side of the swing are balanced in the same direction and the center-point of the pendulum swing is moving along with the direction of rotation (see Figure 2A). For the equatorial pendulum with a swing in the latitudinal direction (along the equator) the surface velocity vectors on either side of the swing are not balanced in the same direction but are acting within the same "x"-"y" plane as the pendulum swing (see Figure 2B). These vectors are all of the same magnitude, that of the equatorial velocity. On one side