Issue 6, 20th Febuary 1996: John Michell, Holey Brilliant
By Adam Hart-Davis.
[Computing, Law, Science and Technology, SciFi, Sport, The Unexplained, UP]
John Michell was born in 1724, and was described as ``a little short man, of a black complexion, and fat.'' He went to Cambridge, and in due course taught arithmetic, geometry, theology, Greek, Hebrew, and philosophy. And he was Professor of geology; so he must have been a busy chap, and a fantastic all-rounder. In 1767, when he was 43, he left Cambridge and the academic life, and went to the remote outback of Thornhill, now almost a suburb of Dewsbury, though perched on top of a steep hill. The living was good---the Rector was paid much more than most clerics---and he stayed there for the rest of his life, which was 26 years, and devoted all his spare time to science. I know at least one Cambridge Professor who would happily become a gentleman scientist if he was paid as much money as Michell was. He was the first person to suggest that earthquakes travel in waves. He worked out the inverse-square law of magnetism. He tried to work out how much pressure was exerted by sunlight. But he was really an astronomer at heart. He worked out that the Pleiades are a real collection of stars---and don't just look as if they are close together---and he was the first person to describe a black hole.
Ask anyone who invented black holes, and they usually say Einstein, maybe, or Stephen Hawking. But no; John Michell wrote a paper for the Royal Society in which he imagined a sphere 500 times as big as the sun, and then said that a body falling towards it from an infinite distance would move faster than light, ``and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, all light emitted from such a body would be made to return towards it, by its own proper gravity.'' Thus John Michell described a black hole, in a letter from Thornhill Rectory, in 1784. Michell had a wide circle of scientific friends. Joseph Priestley used to ride over from Leeds; also probably John Smeaton. Henry Cavendish was a frequent visitor, and Michell liked to play the violin with William Herschel, a German musician who had come over from Hannover, and went on, perhaps inspired by Michell, to become a famous astronomer.
The Old Rectory was a fine house, with a lovely lawn and great trees, and one can imagine a circle of scientific friends dining together and discussing the latest philosophical ideas; a sort of northern version of the Lunar Society of Birmingham. Michell invented and built an elegant piece of apparatus to measure the mass of the Earth. Unfortunately he died before he was able to use it, and he gave it to his lifelong friend Henry Cavendish. So it was Cavendish who first did the experiment, and it is always---rather unfairly---called Cavendish's Experiment. He put two small (1 kg) lead balls at the ends of a six-foot rod, which he suspended horizontally by a single fibre at its centre. Then he brought up two massive lead balls, one beside each of the small ones, so that the massive balls would tend to attract the small ones and twist the rod clockwise. After leaving the whole thing for some hours to settle down he then brought the massive lead balls to the other side of the small ones, in order to twist the rod anticlockwise. By measuring the angle between the two different positions of the rod, Cavendish was able to work out the attractive force exerted by each of the large lead balls on the 1 kg masses. But he already knew the attractive force exerted by the earth on a 1 kg mass---its weight. So by a tedious bit of maths, comparing the relative sizes of the attractive forces and the relative separations of the centres of the balls, he was able to work out the mass of the Earth.
The first man to measure the mass of the Earth was
Astronomer Royal Nevile Maskelyne in 1774, using a method called the Attraction
of Mountains. He worked out a mass of about five million million million tonnes.
Using Michell's apparatus, Cavendish calculated a figure close to six million
million million tonnes, which is close to what Newton had estimated a hundred
years earlier and close to the accepted figure of today.