Caption:

Model. Made around 1755, this working model was used to demonstrate the varying motion of comets around the Sun. Called a cometarium, it was used by Stephen Demainbray in his public lectures on natural philosophy. Using elliptical pulley wheels, the model demonstrates that comets to move quickly near to the Sun and slowly when more distant. This is shown by two dials, the uppermost showing equal arcs next to equal time, while the other shows the comet's path against the same period. A bead representing the comet moves in an elliptical groove and slides on arm that rotates round the large Sun globe. The model could also be used illustrate the elliptical motion of Mercury around the Sun. This type of instrument was invented by J T Desaguliers (1683-1744).