Invariable pendulum no. 10 with agate plane no. 10, 1827, by Thomas Jones. In box with telescope, thermometers and accessories

The pendulum is shown packed into a wooden box, with its accessories, 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, if it swings under similar conditions each time. This usually requires vacuum apparatus. Gravity decreases with the distance from the centre of the Earth, so the pendulum can be used to determine the shape of the planet.