MICHELL, JOHN

born 1724, Nottinghamshire, Eng. died April 21, 1793, Thornhill, Yorkshire British geologist and astronomer who is considered the father of seismology, the science of earthquakes. In 1760, the year in which he was elected a fellow of the Royal Society of London, Michell finished writing Conjectures Concerning the Cause, and Observations upon the Phnomena of Earthquakes, in which he presented the conclusions from his study of the disastrous Lisbon earthquake of 1755. He showed that the focus of that earthquake was underneath the Atlantic Ocean, and he proposed that the cause of earthquakes was high-pressure steam, created when water comes into contact with subterranean fires. His contributions to astronomy included the first realistic estimate of the distance between the Earth and a star and the suggestion, later verified by the British astronomer John Herschel, that binary stars are physically close to and in orbit around one other. Michell became Woodwardian professor of geology at the University of Cambridge in 1762 and rector of Thornhill in 1767. In 1750 he had published a major work on artificial magnets. He may have conceived the principle of the torsion balance independently of the French physicist C.-A. Coulomb. He hoped to use this instrument to determine the mean density of the Earth. Although he died before finishing his work, it was carried on by the British physicist Henry Cavendish in his determination of G, the gravitational constant (a measure of the strength of gravity).