Kater�s invariable pendulum, 1828-1831.

UNKNOWN - JUNE 19: One of 14 scenes by Lieutenant E N Kendall portraying the coasts and islands of the South Atlantic, made during the voyage of HMS 'Chanticleer' under Captain Henry Foster. The purpose of the voyage was to ascertain the formation and outline of coasts and the direction of the principal ocean currents in both hemispheres. To do this Foster took gravity observations and measurements, particularly by means of a Kater invariable pendulum, shown in use. The rate at which a pendulum swings varies with its length and the force of gravity. The rate of swing of a pendulum of fixed (�invariable�) length can be used to determine the force of gravity at different places around the globe. Gravity decreases with the distance from the centre of the Earth, so the pendulum can be used to determine the shape of the planet. (Photo by SSPL/Getty Images)