[Illustration: Figure 10.--HENRY KATER (1777-1835), English army officer and physicist. His scientific career began during his military service in India, where he assisted in the "great trigonometrical survey." Returned to England because of bad health, and retired in 1814, he pioneered (1818) in the development of the convertible pendulum as an alternative to the approximation of the "simple" pendulum for the measurement of the "seconds pendulum." Kater's convertible pendulum and the invariable pendulum introduced by him in 1819 were the basis of English pendulum work. (_Photo courtesy National Portrait Gallery, London._)] After his observations with the convertible pendulum, Capt. Kater designed an invariable compound pendulum with a single knife edge but otherwise similar in external form to the convertible pendulum[27] (fig. 13). Thirteen of these Kater invariable pendulums have been reported as constructed and swung at stations throughout the world.[28] Kater himself swung an invariable pendulum at a station in London and at various other stations in the British Isles. Capt. Edward Sabine, between 1820 and 1825, made voyages and swung Kater invariable pendulums at stations from the West Indies to Greenland and Spitzbergen.[29] In 1820, Kater swung a Kater invariable pendulum at London and then sent it to Goldingham, who swung it in 1821 at Madras, India.[30] Also in 1820, Kater supplied an invariable pendulum to Hall, who swung it at London and then made observations near the equator and in the Southern Hemisphere, and at London again in 1823.[31] The same pendulum, after its knives were reground, was delivered to Adm. Lütke of Russia, who observed gravity with it on a trip around the world between 1826 and 1829.[32] [Illustration: Figure 11.--THE ATTEMPT TO APPROXIMATE the simple (theoretical) pendulum in gravity experiments ended in 1817-18 when Henry Kater invented the compound convertible pendulum, from which the equivalent simple pendulum could be obtained according to the method of Huygens (see text, p. 314). Developed in connection with a project to fix the standard of English measure, Kater's pendulum was called "compound" because it was a solid bar rather than the fine wire or string with which earlier experimenters had tried to approximate a "weightless" rod. It was called convertible because it is alternately swung from the two knife edges (_a_ and _b_) at opposite ends. The weights (_f_ and _g_) are adjusted so that the period of the pendulum is the same from either knife edge. The distance between the two knife edges is then equal to the length of the equivalent simple pendulum.]