John Michell

For other people named John Michell, see John Michell (disambiguation).
The Reverend John Michell
Born (1724-12-25)25 December 1724
Eakring, Nottinghamshire
Died 29 April 1793(1793-04-29) (aged 68)
Thornhill, Yorkshire
Nationality English
Fields Physics, geology
Alma mater Queens' College, Cambridge
Known for Black holes, seismology, manufacture of magnets, mass of the Earth

John Michell (25 December 1724 – 29 April 1793) was an English clergyman and natural philosopher who provided pioneering insights in a wide range of scientific fields, including astronomy, geology, optics, and gravitation. Considered "one of the greatest unsung scientists of all time",[1] he was the first person known to propose the existence of black holes in publication, the first to suggest that earthquakes travel in waves, the first to explain how to manufacture artificial magnets, and the first to apply statistics to the study of the cosmos, recognizing that double stars were a product of mutual gravitation. He also invented an apparatus to measure the mass of the Earth. He has been called both the father of seismology and the father of magnetometry.

According to one source, "a few specifics of Michell's work really do sound like they are ripped from the pages of a twentieth century astronomy textbook." The American Physical Society (APS) has described Michell as being "so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later." The APS states that while "he was one of the most brilliant and original scientists of his time, Michell remains virtually unknown today, in part because he did little to develop and promote his own path-breaking ideas."[2]

Early life, education and professional positions

John Michell was born in 1724 in Eakring, in Nottinghamshire, the son of Gilbert Michell, a priest, and Obedience Gerrard. Gilbert was the son of William Michell and Mary Taylor of Kenwyn, Cornwall; Obedience was the daughter of Ralph and Hannah Gerrard of London.[3] He was educated at Queens' College, Cambridge, and later became a Fellow of Queens'. He obtained his M.A. degree in 1752 and B.D. degree in 1761.[4] He was Tutor of the College from 1751 to 1763 ; Praelector in Arithmetic in 1751; Censor in Theology in 1752; Praelector in Geometry in 1753 ; Praelector in Greek in 1755 and 1759; Senior Bursar in 1756; Praelector in Hebrew in 1759 and 1762 ; Censor in Philosophy and Examiner in 1760. "He was nominated Rector of St Botolph's, Cambridge, on 28th March 1760, and held this living until June 1763."[5] From 1762 to 1764, he held the Woodwardian Chair of Geology till he was obliged to relinquish it on his marriage.[5]

There is no surviving portrait of Michell; he is said to have been "a little short man, of black complexion, and fat."[6][7]

In 1910, Sir Edmund Whittaker observed that during the century after Newton’s death, "the only natural philosopher of distinction who lived and taught at Cambridge was Michell", although his "researches seem to have attracted little or no attention among his collegiate contemporaries and successors, who silently acquiesced when his discoveries were attributed to others, and allowed his name to perish entirely from Cambridge tradition." Michell proceeded to take up clerical positions in Compton and then Havant, both in Hampshire. During this period he unsuccessfully sought positions at Cambridge and as astronomer royal.[3][8]

In 1767, he was appointed rector of St. Michael's Church of Thornhill, near Leeds, Yorkshire, England, a post he held for the rest of his life. He did most of his important scientific work in Thornhill, where he died on 21 April 1793, at age 68. He is buried there.[2][7][9] After local pressure, a blue plaque went up on the church wall to commemorate him.

Scientific work

In 1750 Michell published at Cambridge a work of some eighty pages entitled "A Treatise of Artificial Magnets", in which he presented an easy and expeditious method of producing magnets that are superior to the best natural magnets. Besides the description of the method of magnetization which still bears his name, this work contains a variety of accurate observations about magnetism, and features 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.

Geology and seismology

Until the late 20th century Michell was considered important primarily because of his work on geology. His most important geological essay, written after the 1755 Lisbon earthquake, was entitled "Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes" (Philosophical Transactions, li. 1760). In this paper he introduced the idea that earthquakes spread out as waves through the Earth and that they involve the offsets in geological strata now known as faults. He was able to estimate both the epicentre and the focus of the Lisbon Earthquake, and may also have been the first to suggest that a tsunami is caused by a submarine earthquake.[10]

Michell’s essay not only provided insights on earthquakes but also, more broadly, represented an advance in the understanding of the geology of the Earth’s crust. He recognized that the Earth is composed "of regular and uniform strata", some of which have been interrupted by upheavals. "The most important part of Michell's Earthquake paper", in the view of one commentator, "is the account which it contains of what is now known as 'the crust of the Earth.'"[5] Exhibiting a remarkable knowledge of the geological strata in various parts of England and abroad,[10] he drew on his own observations to advance the understanding of sedimentary stratigraphy and was the first to define the Mesozoic stratigraphy in the U.K.[10][11]

In 1760, as a result of this work, he was elected a member of the Royal Society.[10]

A 1788 letter to Cavendish indicated that Michell continued to be interested in geology several decades after his paper on earthquakes.[5]

Magnetism

Michell studied magnetism and discovered the "inverse-square law", the fact that the magnetic force exerted by each pole of a magnet decreases in proportion to the square of the distance between them.[10] His 1750 paper Treatise of Artificial Magnets, which was written for seamen and instrument makers and intended as a practical manual on how to make magnets, included a list of the "Properties of Magnetical Bodies" that represented a major contribution to the understanding of magnetism.[8]

Gravity

Main article: Cavendish experiment
Michell's torsion balance, used in the Cavendish experiment

Michell devised a torsion balance for measuring the mass of the Earth, but died before he could use it. His instrument passed into the hands of his lifelong friend Henry Cavendish, who first performed in 1798 the experiment now known as the Cavendish Experiment. Placing two 1-kg lead balls at the ends of a six-foot rod, he suspended the rod horizontally by a fibre attached to its centre. Then he placed a massive lead ball beside each of the small ones, causing a gravitational attraction that led the rod to turn clockwise. By measuring the rod’s movement, Cavendish was able to calculate the force exerted by each of the large balls on the 1-kg balls. From these calculations, he was able to provide an accurate estimate of the gravitational constant and of the mass and average density of the Earth. Cavendish gave Michell full credit for his accomplishment.[9][10]

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.[12]

Double stars

Michell was the first person to apply the new mathematics of statistics to the study of the stars, and demonstrated in a 1767 paper that many more stars occur in pairs or groups than a perfectly random distribution could account for. He focused his investigation on the Pleiades cluster, and calculated that the likelihood of finding such a close grouping of stars was about one in half a million. He concluded that the stars in these double or multiple star systems might be drawn to one another by gravitational pull, thus providing the first evidence for the existence of binary stars and star clusters.[2][9][10] His work on double stars influenced Herschel’s research on the same topic.[9]

Black holes

It was Michell who, in a paper for the Philosophical Transactions of the Royal Society of London, read on 27 November 1783, first proposed the idea that there were such things as black holes, which he called "dark stars". This insight has been called his "most far-sighted accomplishment". Having accepted Newton’s corpuscular theory of light, which posited that light consists of minuscule particles, he reasoned that such particles, when emanated by a star, would be slowed down by its gravitational pull, and thought that it might therefore be possible to determine the star's mass based on the reduction in speed. This insight led in turn to the recognition that a star's gravitational pull might be so strong that the escape velocity would exceed the speed of light. Michell calculated that this would be the case with a star more than 500 times the size of the Sun. Since light would not be able to escape such a star, it would be invisible. In his own words:

If there should really exist in nature any bodies, whose density is not less than that of the sun, and whose diameters are more than 500 times the diameter of the sun, since their light could not arrive at us; or if there should exist any other bodies of a somewhat smaller size, which are not naturally luminous; of the existence of bodies under either of these circumstances, we could have no information from sight; yet, if any other luminous bodies should happen to revolve about them we might still perhaps from the motions of these revolving bodies infer the existence of the central ones with some degree of probability, as this might afford a clue to some of the apparent irregularities of the revolving bodies, which would not be easily explicable on any other hypothesis; but as the consequences of such a supposition are very obvious, and the consideration of them somewhat beside my present purpose, I shall not prosecute them any further.
— John Michell, 1784[13]

Michell suggested that there might be many "dark stars" in the universe, and today astronomers believe that black holes do indeed exist at the centers of most galaxies.[2] Similarly, Michell proposed that astronomers could detect "dark stars" by looking for star systems which behaved gravitationally like two stars, but where only one star could be seen. Michell argued that this would show the presence of a "dark star". It was an extraordinarily accurate prediction. All of the dozen candidate stellar black holes in our galaxy (the Milky Way) are in X-ray compact binary systems.[14]

A few years after Michell came up with the concept of black holes, the French mathematician Pierre-Simon Laplace suggested essentially the same idea in his 1796 book, Exposition du Système du Monde.[2]

It has been written that Michell was so far ahead of his time in regard to black holes that the idea "made little impression" on his contemporaries.[2][10] "He died in quiet obscurity", states the American Physical Society, "and his notion of a 'dark star' was forgotten until his writings re-surfaced in the 1970s."[2]

Telescopes

Michell constructed telescopes for his own use. One of them, a reflecting telescope with a 10-foot focal length and a 30-inch aperture, was bought by William Herschel after Michell's death. Although it was unusable because the prime mirror had been damaged, Herschel used it as a model to build a similar instrument.[9]

Other professional activities

Michell also wrote a paper on surveying that his biographer has described as "elegant" in theory. Michell was elected a member of the Royal Society. He was first invited to meetings of the Royal Society in 1751 as a guest of Sir George Savile, who would become his patron. He later attended meetings "one to four times a year", while at Cambridge. His paper on the cause of earthquakes was read before the Society beginning on 28 February 1760, leading to a recommendation by Savile and another member that Michell be invited to join the Society. He was elected a member on 12 June 1760.

Michell followed his work in seismology with work in astronomy, and after publishing his findings in 1767 he served on an astronomical committee of the Royal Society.[3]

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.

Personal life

Michell was a man of "wide latitude in religious belief".[3] He was described by a contemporary as "a little short man, of black complexion, and fat", and was "esteemed a very ingenious Man, and an excellent Philosopher." During his years at Thornhill, he welcomed many distinguished visitors, including Benjamin Franklin, Joseph Priestley (the discoverer of oxygen), and Henry Cavendish (the discoverer of hydrogen).[2] Michell wrote to Franklin in 1767 describing his first visit to Thornhill.[15] Priestley, who lived in Leeds for a time, saw Michell "pretty often", by his own account, with Michell supplying Priestley "with metals for his chemical experiments and with information and assistance, including proofreading, for his book on the history of optics". Michell also helped the engineer John Smeaton "revise the text of his book on the Eddystone Lighthouse, now regarded as a 'classic of engineering literature.'"

Michell’s first wife was a Miss Williamson, "a young lady of considerable fortune", whom he married in 1764 and who died in 1765.[5] On 13 February 1773, in Newark, Nottinghamshire, he married Ann Brecknock, daughter of Matthew and Ann Brecknock of Nottinghamshire.[3] They had one child, Mary, who married Thomas Turton of Leeds.[16]

Michell’s younger brother Gilbert was a merchant in London who later lived with Michell in Thornhill, where the two brothers were active in local real estate, purchasing many properties in the West Riding of Yorkshire.[3]

Biography

Michell is the subject of the book Weighing the World: The Reverend John Michell of Thornhill (2012) by Russell McCormmach.[17]

Selected publications

References

  1. ↑ "On-line: SCIENCE AND TECHNOLOGY". Exnet.com. Retrieved 6 October 2014.
  2. 1 2 3 4 5 6 7 8 "This Month in Physics History: November 27, 1783: John Michell anticipates black holes". APS Physics.
  3. 1 2 3 4 5 6 McCormmach, Russell (2012). Weighing the World. Springer.
  4. ↑ "Michell, John (MCL742J)". A Cambridge Alumni Database. University of Cambridge.
  5. 1 2 3 4 5 Geikie, Sir Archibald (1918). Memoir of John Michell, M.A., B.D., F.R.S., fellow of Queens' college, Cambridge, 1749, Woodwardian professor of geology in the university 1762. Cambridge University Press.
  6. ↑ Soter, Steven; Tyson, Neil deGrasse, eds. (2000). "John Michell and Black Holes (excerpt)". Cosmic HoriZons: Astronomy At The Cutting Edge. New Press.
  7. 1 2 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.
  8. 1 2 "John Michell (1724-93): Fatherof Magnetometry?" (PDF). HEP.
  9. 1 2 3 4 5 "Michell". Messier.
  10. 1 2 3 4 5 6 7 8 "John Michell and Black Holes". AMNH.
  11. ↑ Geoscientist, Vol 24, No.4, May 2011
  12. ↑ Cook, A.H. (1987), "Experiments in Gravitation", in Hawking, S.W. and Israel, W., Three Hundred Years of Gravitation, Cambridge University Press, p. 52, ISBN 0-521-34312-7
  13. ↑ 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. 74: 35–57. doi:10.1098/rstl.1784.0008.
  14. ↑ Wilkins, Alasdair (23 December 2010). "The forgotten genius who discovered black holes over 200 years ago". io9. Retrieved 10 September 2014.
  15. ↑ "To Benjamin Franklin from John Michell, 6 September 1767". Founders.archives.gov. Retrieved 6 October 2014.
  16. ↑ Clerke, A. M. (2004). "Michell, John (1724/5–1793), astronomer". rev. Michael Hoskin. Oxford Dictionary of National Biography. Retrieved 14 January 2015. (subscription required (help)).
  17. ↑ "Weighing the World". springer.com. Retrieved 6 October 2014.

Bibliography

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