Kater's invariable pendulum, 1827.
© Science Museum / Science & Society Picture Library
The pendulum is packed into a wooden box, with its accesories, for safe transportation. In use, it swings from a knife-edged pivot at one end with a very heavy weight at the other. 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.