John Michell
John Michell was an English natural philosopher and clergyman who provided pioneering insights into a wide range of scientific fields including astronomy, geology, optics, and gravitation. He was the first to have proposed the existence of stellar bodies comparable to black holes, and was the first to apply statistics to astronomy, providing the earliest evidence for the physical nature of double stars and star clusters.
He was first to have suggested that earthquakes travelled in waves and the first to have measured the velocity of an earthquake, that being the 1755 Lisbon earthquake. Michell further invented an apparatus to measure the mass of the Earth that was later used by Henry Cavendish to measure the gravitational constant. He also explained how to manufacture an artificial magnet and provided the first accurate and comprehensive statement of the law of magnetic force. As a result, he has been called the father of both seismology and of magnetometry.
Michell served as the Woodwardian Professor of Geology at the University of Cambridge starting in 1762. He was elected a Fellow of the Royal Society in 1760.
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. He was educated at Queens' College, Cambridge, and later became a Fellow of Queens'. The family was of Cornish origins. He obtained his M.A. degree in 1752, and his B.D. degree in 1761. 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 28 March 1760, and held this living until June 1763." From 1762 to 1764, he held the Woodwardian Chair of Geology until he was obliged to relinquish it on his marriage.There is no surviving portrait of Michell; he is said to have been "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."
In 1910, Sir Edmund Whittaker observed that during the century after Isaac 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.
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, aged 68. He is buried there. 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". 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. He was the first to calculate the velocity of an earthquake.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.'" Exhibiting a remarkable knowledge of the geological strata in various parts of England and abroad, 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.
In 1760, as a result of this work, he was elected a member of the Royal Society.
A 1788 letter to Henry Cavendish indicated that Michell continued to be interested in geology several decades after his paper on earthquakes.
Magnetism
Michell studied magnetism and discovered that the magnetic force exerted by each pole of a magnet decreases according to an inverse-square law, i.e. in proportion to the square of the distance between them. 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. His paper provided the first accurate and comprehensive statement of the law of magnetic force.Gravity
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.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.
Double stars and variable stellar luminosity
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. His work on double stars may have influenced his friend William Herschel's research on the same topic.Another consequence of Michell's statistical work was the realization that fixed luminosity for stars was a poor approximation. Hershel had developed a model of the Milky Way using this approximation and a large number of star directions. While the shape was correct, various appendages to the model were not. The variable luminosity meant the evidence for the edge of the galaxy was suspect and Hershel lost faith in his model.
Black holes
In a paper for the Philosophical Transactions of the Royal Society of London, read on 27 November 1783, Michell was the first to propose the existence of celestial bodies similar to black holes. 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 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:Michell suggested that there might be many such objects in the universe, and today astronomers believe that black holes do indeed exist at the centers of most galaxies. Similarly, Michell proposed that astronomers could detect them 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 star from which light was not escaping. It was an extraordinarily accurate prediction. All of the dozen candidate stellar black holes in our galaxy are in X-ray compact binary systems.
Michell's ideas about gravity and light interested William Herschel, who tried to test them with his powerful telescopes. A few years after Michell came up with the concept of invisible, light-trapping stars, the French mathematician Pierre-Simon Laplace suggested essentially the same idea in his 1796 book, Exposition du Système du Monde.
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. "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."