Henry Kater (1777-1835) making observations of a pendulum, with two assistants. Engraving by J. Basire, 1822, after J. Goldingham, 1821. Henry Kater's most substantial work was the invention of Kater's pendulum, enabling the strength of gravity to be determined, first at London and subsequently at various stations throughout the country. He is also the inventor of the floating collimator and is considered the inventor of the prismatic compass, although it was patented by someone else. Kater was actively involved in the Royal Society of London. The illustration shows the operation of measuring the swing of the pendulum in comparison to a clock pendulum. The measurements were carried out early in 1821 in the state of Madras, India.