John Michell (1724 - April 21, 1793)

John Michell was born in 1724 in Nottinghamshire, but the exact place and date of his birth are unknown. He studied at Queens' College of Cambridge University where he received an MA degree in 1752, and a BD in 1761. Also in 1761, he was elected Fellow of the Royal Society. From 1762 to 1764, he held the Woodwardian Chair of Geology, and in 1767, was appointed rector of St. Michael's Church of Thornhill, near Leeds, Yorkshire, England, a post he held for the rest of his life. Reverend John Michell died in Thornhill on April 21, 1793 at age 68, and is buried there.

In 1750, Michell studied artificial magnets and developed a method for magnetization. As a geologist, he constructed a torsion balance for measuring gravitational forces; this device became famous only by its second inventor, Charles-Augustin de Coulomb (1736-1806), and was eventually used by Henry Cavendish (1731-1810), who used the device of his life-long friend, John Michell, in his famous experiment to measure gravity between two test masses; from his measures, Cavendish was able to give a very acurate estimate of the Earth's mass (6*10^18 metric tons). In 1760, Michell constructed a theory of earthquakes as wave motions in the interior of the Earth, and suspected a connection between earthquakes and volcanism.

In 1767 he published an investigation on double stars and clusters, and calculated the probability of finding chance alignments of stars (asterisms). In particular, he investigated the Pleiades cluster, and calculated a probability of 1/496,000 to find such a group as a chance alignment anywhere in the sky. Also, he found that much too many pairs and close groups of stars were visible in the sky to assume that all these were chance alignments. and concluded that many of them should be physical pairs or groups, held together by

"the influence of some general law [..], to whatever cause this may be owing, wether to their mutual gravitation, or some other law or appointment of the Creator."

Later, in a letter to Cavendish of 1784, Michell published thoughts about the effect of gravity on light, including an early concept of Black Holes, masses dense enough to prevent light from escaping.

As an amateur astronomer, John Michel was also an active telescope builder. His main instrument was a self-made 10-foot [focal length] reflector of 30-inch aperture. This instrument was puchased by William Herschel after Michell's death, and served as a model for a similar instrument. At that time it was no more usable, as the prime mirror was damaged.

The work of Michell had some impact on William Herschel, and in particular, stimulated his work on double stars.


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