John Michell (December 25, 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 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.
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.’
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- (Cole MSS XXXIII, 156, British Library).
Work
Gravity, magnetism & 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 didn’t 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. [1]
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.
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.
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 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 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.
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).
External links
References
- ^
Cook, A.H. (1987), "Experiments in Gravitation", in Hawking, S.W. and Israel, W., Three Hundred
Years of Gravitation, Cambridge University Press, pp. p.52, ISBN 0521343127
- 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)
This article incorporates text from the Encyclopædia
Britannica Eleventh Edition, a publication now in the public
domain.
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