John Michell

John Michell was a well-known British geologist and astronomer and later regarded as the 'Father of Seismology' in his study of earthquakes. He is also credited with the idea of Binary Stars, the demonstration of an inverse square law in magnetism, and was the inventor of the torsion balance before instigating the experiment, later completed by Cavendish, to weigh the Earth.

A paper written by the Rev John Michell in 1783 was discovered in the 1970s. This is the first known discussion of the concept of a black hole, an object from which no light nor anything else can escape.

At the time the 'corpuscular' theory of light was the vogue. This regarded light as being made up of 'corpuscles' or particles similar in some respects to the modern idea of the photon. It was therefore considered a possibility that light could be affected by gravity in the same way as ordinary matter. Over one hundred years prior to this in 1676 Ole Rømer had discovered that the speed of light was finite through observed variations in the period of Jupiter's moon Io. Observations of stellar aberration by James Bradley in 1728 produced further confirmation and a more accurate value for the speed of light of 295,000 kilometers per second compared to today's figure of 300,000 km per second. The Newtonian concept of escape velocity as being the minimum velocity needed to escape from a planet's surface to infinity was well understood. For a spherical mass M of radius R it is simply: sqrt(2GM/R) where G is the Gravitational constant. The escape velocity thus increases as the object's mass increases and also increases if the mass remains the same but the radius gets smaller.

Michell pondered a body so massive that the escape velocity at its surface was equal to the speed of light. It is the first known reference to an object later called a black hole.

See also: black hole

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The following text comes from a 1911 Encyclopaedia. Please update as needed, and combine with the above.

Michell was educated at Queens College, Cambridge. His name appears fourth in the Tripos list for 1748-1749; and in 1755 he was moderator in that examination. He became M.A. in 1752, and B.D. in 1761. He was a fellow of his college, and was appointed Woodwardian Professor of Geology in 1762, and in 1767 rector of Thornhill in Yorkshire, where he died.

He was elected a fellow of the Royal Society in the same year as Henry Cavendish (1760). In 1750 he published at Cambridge a work of some eighty pages entitled A Treatise of Artificial Magnets, in which is shown an easy and expeditious method of making them superior to the best natural ones. Besides the description of the method of magnetization which still bears his name, this work contains a variety of accurate magnetic observations, and is distinguished by a lucid exposition of the nature of magnetic induction. He was the original inventor of the torsion balance, which afterwards became so famous in the hands of its second inventor Charles-Augustin de Coulomb. Michell described it in his proposal of a method for obtaining the mean density of the earth. He did not live to put his method into practice; but this was done by Henry Cavendish, who made, by means of Michell's apparatus, the celebrated determination that now goes by the name of Cavendish's experiment (Phil. Trans., 1798).

His most important geological essay was that entitled Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes (Phil. Trans., li. 1760), which showed a remarkable knowledge of the strata in various parts of England and abroad.

Michell's other contributions to science are:

Observations On the Comet of January 1760 at Cambridge, Phil. Trans. (1760)
A Recommendation of Hadley's Quadrant for Surveying, ibid. (1765)
Proposal of a Method for measuring Degrees of Longitude upon Parallels of the Equator, ibid. (1766)
An Inquiry into the Probable Parallax and Magnitude of the Fixed Stars, ibid. (1767)
On the Twinkling of the Fixed Stars, ibid. (1767)
On the Means of Discovering the Distance, Magnitude, &c., of the Fixed Stars, ibid. (1784).