<Li> 1644: Even before the pendulum clock, French priest Marin Mersenne first determined the length of the seconds pendulum was 39.1 inches (990 mm), by comparing the swing of a pendulum to the time it took a weight to fall a measured distance . </Li> <Li> 1669: Jean Picard determined the length of the seconds pendulum at Paris, using a 1 - inch (25 mm) copper ball suspended by an aloe fiber, obtaining 39.09 inches (993 mm). </Li> <Li> 1672: The first observation that gravity varied at different points on Earth was made in 1672 by Jean Richer, who took a pendulum clock to Cayenne, French Guiana and found that it lost ​ 2 ⁄ minutes per day; its seconds pendulum had to be shortened by ​ 1 ⁄ lignes (2.6 mm) shorter than at Paris, to keep correct time . In 1687 Isaac Newton in Principia Mathematica showed this was because the Earth had a slightly oblate shape (flattened at the poles) caused by the centrifugal force of its rotation . At higher latitudes the surface was closer to the center of the Earth, so gravity increased with latitude . From this time on, pendulums began to be taken to distant lands to measure gravity, and tables were compiled of the length of the seconds pendulum at different locations on Earth . In 1743 Alexis Claude Clairaut created the first hydrostatic model of the Earth, Clairaut's theorem, which allowed the ellipticity of the Earth to be calculated from gravity measurements . Progressively more accurate models of the shape of the Earth followed . </Li> <Li> 1687: Newton experimented with pendulums (described in Principia) and found that equal length pendulums with bobs made of different materials had the same period, proving that the gravitational force on different substances was exactly proportional to their mass (inertia). </Li>

Could we ever construct a true simple pendulum