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.’
- ::::::(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.
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)
- 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)
- On the Means of Discovering the Distance, Magnitude,
&c., of the Fixed Stars, ibid. (1784).
External links
References
- 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)
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