John Michell (25 December 1724 – 29 April 1793) was an English natural philosopher and geologist whose work spanned a wide range of subjects from astronomy to geology, optics, and gravitation. He was both a theorist and an experimenter.
Michell was educated at Queens' College, Cambridge and later became a Fellow of Queens'. He obtained his M.A. in 1752 and B.D. in 1761. In 1760 he was elected a Fellow of the Royal Society, in the same year as Henry Cavendish. In 1762 he was appointed Woodwardian Professor of Geology, and in 1767 he became rector of Thornhill, West Yorkshire, near Dewsbury, where he died.
Gravity, magnetism and light 
Michell conceived, sometime before 1783, the experiment now known as the Cavendish experiment. It was the first to measure the force of gravity between masses in the laboratory and produced the first accurate values for the mass of the Earth and the gravitational constant. He invented and built, independently of co-inventor Charles Augustin de Coulomb, a torsion balance for the experiment but did not live to put it to use. His apparatus passed to Henry Cavendish, who performed the experiment in 1798. In 1987, gravity researcher A.H. Cook wrote:
The most important advance in experiments on gravitation and other delicate measurements was the introduction of the torsion balance by Michell and its use by Cavendish. It has been the basis of all the most significant experiments on gravitation ever since.
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.
At one point, Michell attempted to measure the radiation pressure of light by focusing sunlight onto one side of a compass needle. The experiment was not a success: the needle melted.
In scientific biographies written during the early 20th century, Michell's historical importance is ascribed to his work on geology. His most important geological essay was entitled "Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes" (Philosophical Transactions, li. 1760), which showed a remarkable knowledge of the strata in various parts of England and abroad. In this paper he suggested that earthquakes were experienced as seismic waves of elastic compression travelled through the Earth. He was able to estimate both the epicentre and focus of the 1755 Lisbon Earthquake. He may have been the first to suggest that a Tsunami is caused by a subterranean earthquake. In addition to his work on earthquakes, Michell drew on geological observations collected on many European travels to advance the understanding of sedimentary stratigraphy and was the first to define the Mesozoic stratigraphy in the UK.
Effect of gravity on light 
More recently, Michell has become known for his letter to Cavendish, published in 1784, on the effect of gravity on light. This paper was rediscovered in the 1970s and is now recognised as anticipating several astronomical ideas that had been considered to be 20th century innovations. Michell is now credited with being the first to study the case of a heavenly object massive enough to prevent light from escaping (the concept of escape velocity was well known at the time). Such an object, which he called a dark star, would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. The classical minimum radius for escape assuming light behaved like particles of matter is numerically equal to the Schwarzschild Radius in general relativity. Michell also suggested using a prism to measure what is now known as gravitational redshift, the gravitational weakening of starlight due to the surface gravity of the source. Michell acknowledged that some of these ideas were not technically practical at the time, but wrote that he hoped they would be useful to future generations. By the time that Michell's paper was rediscovered nearly two centuries later, these ideas had been reinvented by others.
The mathematician Pierre-Simon Laplace soon after suggested the same idea of high-gravity objects trapping light in his book Exposition du Systeme du Monde in 1796. This sort of high-gravity object under Newtonian theory is commonly referred to as a dark star, and can be thought of as being the predecessor of the modern idea of a black hole under general relativity.
Selected publications 
- Observations On the Comet of January 1760 at Cambridge, Philosophical Transactions (1760)
- Conjectures Concerning the Cause and Observations upon the Phaenomena of Earthquakes, ibid. (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)
- Michell, John (1784), "On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose. By the Rev. John Michell, B. D. F. R. S. In a Letter to Henry Cavendish, Esq. F. R. S. and A. S.", Philosophical Transactions of the Royal Society of London (The Royal Society) 74: 35–57, ISSN 0080-4614
|Wikisource has original works written by or about:
- Alan Ellis, Black Holes - Part 1 - History,Journal of the Astronomical Society of Edinburgh, 39, (1999). Description of Michell's theory of black holes
- Mystery at the Rectory: some light on John Michell by Richard Crossley
- John Michell biography, SEDS
- Venn, J.; Venn, J. A., eds. (1922–1958). "John Michell". Alumni Cantabrigienses (10 vols) (online ed.). Cambridge University Press.
- He was thus described by a contemporary commentator: "John Michell, BD is a little short Man, of a black Complexion, and fat; but having no Acquaintance with him, can say little of him. I think he had the care of St. Botolph’s Church Cambridge, while he continued Fellow of Queen's College, where he was esteemed a very ingenious Man, and an excellent Philosopher. He has published some things in that way, on the Magnet and Electricity.’ Cole MSS XXXIII, 156, British Library.
- Cook, A.H. (1987), "Experiments in Gravitation", Three Hundred Years of Gravitation, Cambridge University Press, p. 52, ISBN 0-521-34312-7 Unknown parameter
- Geoscientist, Vol 24, No.4, May 2011
- John Michell "On the means of discovering the distance, magnitude etc. of the fixed stars ..." Philosophical Transactions of the Royal Society (1784) 35-57, & Tab III
- Russell McCormmach and Christa Jungnickel, Cavendish, American Philosophical Society, Philadelphia, 1996, ISBN 0-87169-220-1.
- Clyde R Hardin, "The scientific work of the Reverend John Michell", Annals of Science, 22 27-47 (1966)
- Russell McCormack, "John Michell and Henry Cavendish: Weighing the stars", British Journal for the History of Science 4 126-155 (1968)
- Gary Gibbons, "The man who invented black holes [his work emerges out of the dark after two centuries]", New Scientist, 28 June pp. 1101 (1979)
- Simon Schaffer, "John Michell and black holes", Journal for the History of Astronomy 10 42-43 (1979)
- Jean Eisenstaedt, "De l'influence de la gravitation sur la propagation de la lumière en théorie newtonienne. L'archéologie des trous noirs", Archive for History of Exact Sciences, 42 315-386 (1991)
- Jean Eisenstaedt, Avant Einstein Relativité, lumière, gravitation, Paris: Seuil (2005)