John Michell
Encyclopedia
John Michell 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 experiment
er.
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:
. 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:
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.
. He may have been the first to suggest that a Tsunami
is caused by a subterranean earthquake.
In addition to his work on earthquakes, Michell drew on geological observations collected on many European travels to advance the understanding of sedimentary stratigraphy and was the first to define the Mesozoic
stratigraphy in the UK.
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 the gravitational weakening of starlight due to the surface gravity of the source ("gravitational redshift
"). 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
.
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
natural philosopher and geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
whose work spanned a wide range of subjects from astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
to geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
, and gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
. He was both a theorist and an experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
er.
Michell was educated at Queens' College, Cambridge
Queens' College, Cambridge
Queens' College is a constituent college of the University of Cambridge, England.The college was founded in 1448 by Margaret of Anjou , and refounded in 1465 by Elizabeth Woodville...
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
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
, in the same year as Henry Cavendish
Henry Cavendish
Henry Cavendish FRS was a British scientist noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and...
. In 1762 he was appointed Woodwardian Professor of Geology
Woodwardian Professor of Geology
The Woodwardian Professor of Geology is a professorship held in the Department of Earth Sciences at the University of Cambridge. It was founded by John Woodward in 1728...
, and in 1767 he became rector of Thornhill, West Yorkshire
Thornhill, West Yorkshire
Thornhill, is a village in Kirklees, West Yorkshire, England. Thornhill was absorbed into Dewsbury County Borough in 1910. It is located on a hill on the south side of the River Calder, and has extensive views of Dewsbury, Ossett and Wakefield...
, near Dewsbury
Dewsbury
Dewsbury is a minster town in the Metropolitan Borough of Kirklees, in West Yorkshire, England. It is to the west of Wakefield, east of Huddersfield and south of Leeds...
, 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 CambridgeCambridgeThe city of Cambridge is a university town and the administrative centre of the county of Cambridgeshire, England. It lies in East Anglia about north of London. Cambridge is at the heart of the high-technology centre known as Silicon Fen – a play on Silicon Valley and the fens surrounding the...
, 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 MagnetMagnetismMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
and ElectricityElectricityElectricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
.’
- (Cole MSS XXXIII, 156, British Library).
Gravity, magnetism & light
Michell conceived, sometime before 1783, the experiment now known as the Cavendish experimentCavendish experiment
The Cavendish experiment, performed in 1797–98 by British scientist Henry Cavendish was the first experiment to measure the force of gravity between masses in the laboratory, and the first to yield accurate values for the gravitational constant. Because of the unit conventions then in use,...
. 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
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
. 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
Henry Cavendish
Henry Cavendish FRS was a British scientist noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and...
, 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
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
.
At one point, Michell attempted to measure the radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
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. In this paper he suggested that earthquakes were experienced as seismic waves of elastic compression travelled through the Earth. He was able to estimate both the epicentre and focus of the 1755 Lisbon Earthquake1755 Lisbon earthquake
The 1755 Lisbon earthquake, also known as the Great Lisbon Earthquake, was a megathrust earthquake that took place on Saturday 1 November 1755, at around 9:40 in the morning. The earthquake was followed by fires and a tsunami, which almost totally destroyed Lisbon in the Kingdom of Portugal, and...
. He may have been the first to suggest that a Tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
is caused by a subterranean earthquake.
In addition to his work on earthquakes, Michell drew on geological observations collected on many European travels to advance the understanding of sedimentary stratigraphy and was the first to define the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
stratigraphy in the UK.
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 velocityEscape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
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
Schwarzschild radius
The Schwarzschild radius is the distance from the center of an object such that, if all the mass of the object were compressed within that sphere, the escape speed from the surface would equal the speed of light...
in general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
. Michell also suggested using a prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
to measure the gravitational weakening of starlight due to the surface gravity of the source ("gravitational redshift
Gravitational redshift
In astrophysics, gravitational redshift or Einstein shift describes light or other forms of electromagnetic radiation of certain wavelengths that originate from a source that is in a region of a stronger gravitational field that appear to be of longer wavelength, or redshifted, when seen or...
"). 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
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy and statistics. He summarized and extended the work of his predecessors in his five volume Mécanique Céleste...
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
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
under general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
.
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).