John Michell
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- This article is about the 18th Century English scientist. For the contemporary English writer, see John Michell (writer).
John Michell (1724 – April 29, 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. He earned his 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).
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 Queens’ 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).
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[edit] Work
[edit] Gravity, magnetism & light
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.
Michell 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 is now known as the Cavendish experiment.
[edit] Geology
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.
[edit] Effect of gravity on light
More recently, Michell's main "claim to fame" is considered to be his letter to Cavendish, published in 1784, on the effect of gravity on light. This paper was only generally "rediscovered" in the 1970's 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 would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. Michell also suggested using a prism to measure the gravitational weakening of starlight due to the surface gravity of the source ("gravitational shift"). 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 "resurrected" nearly two centuries later, these ideas had been reinvented by others.
The mathematician Pierre-Simon Laplace 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.
[edit] Some of Michell's contributions
- Observations On the Comet of January 1760 at Cambridge, Philosophical Transactions (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).
[edit] External links
- Black Holes - Part 1 - History (description of Michell's theory of black holes)
- Mystery at the Rectory: some light on John Michell by Richard Crossley
- John Michell biography (SEDS)
[edit] References
- 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)
This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
