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forces that, in addition to the electrostatic part appearing in Coulomb's law, are dependent on the velocities of the charges involved. Theories based on this type of assumption became quite successful, and their gravitational equivalents became the subject of scrutiny. One such law, based on the electrodynamic law of Wilhelm Weber, expressed the magnitude of gravitational attraction between bodies a distance, r , apart as

In the electrodynamic context, h had been measured and determined to be almost exactly times the speed of light.

If this gravitation law is applied to the perihelion problem, it produces an advance of just over 6" per century for Mercury. However, if one treats h as a parameter that takes a different value in the theory of gravity from that which it has in the electrodynamic theory, the whole of the anomalous advance of the perihelion of Mercury can be reproduced by taking h = 174 000 ms-1. The use

Michell (the 'father of seismology') began his scientific career as a geologist and, following the Lisbon earthquake of 1755, published a paper on the causes of such phenomena. It was in the context of geology that he designed his experiment to determine the density of the Earth. Later, he became interested in astronomy and here also he made significant contributions. He used a statistical analysis to show that the number of double stars observed was more than could be expected if they were just chance alignments of stars. He concluded that most such combinations were, in fact, pairs orbiting each other. Michell also hypothesized the existence of massive stars from which light could not escape but the presence of which could be inferred from the effect of their gravity on other stars - black holes as we would now call them.

A detailed history of nineteenth-century electrodynamical theories can be found in Whittaker (1951).

The first to claim that the law of electrical attraction had the same form as that for gravity appears to have been Joseph Priestley in 1767, based on the absence of electrical force inside a charged spherical shell, though the inverse square nature of the attraction had been suspected earlier by Daniel Bernoulli (Whittaker (1951), p. 53). The first person to measure the force directly was Charles Coulomb.

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