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and, regarding F₁ as a perturbing function, to find the variation of the constants of elliptic motion. Tisserand showed that the perturbations of all the elements are zero or periodic, and quite insensible, except that of the longitude of perihelion, which has a secular part. If h be assumed equal to the velocity of light, the effect would be to rotate the major axis of the orbit of Mercury in the direct sense 14" in a century.

Now, as it happened, a discordance between theory and observation was known to exist in regard to the motion of Mercury's perihelion; for Le Verrier had found that the attraction of the planets might be expected to turn the perihelion 527" in the direct sense in a century, whereas the motion actually observed was greater than this by 38". It is evident, however, that only 3/8 of the excess is explained by Tisserand's adoption of Weber's law; and it seemed therefore that this suggestion would prove as unprofitable as Le Verrier's own hypothesis of an intra-mercurial planet. But it was found later^[1] that 3/4 of the excess could be explained by substituting Riemann's electrodynamic law for Weber's, and that a combination of the laws of Riemann and Weber would give exactly the amount desired.^[2]

After the publication of his memoir on the law of force between electrons, Weber turned his attention to the question of diamagnetism, and developed Faraday's idea regarding the explanation of diamagnetic phenomena by the effects of electric currents induced in the diamagnetic bodies.^[3] Weber remarked that if, with Ampère, we assume the existence of molecular circuits in which there is no ohmic resistance, so that currents can flow without dissipation of energy, it is quite natural to suppose that currents would be induced in these molecular

1. By Maurice Lévy, Comptes Rendus, cx (1890), p. 545.
2. The consequences of adopting the electrodynamic law of Clausius (for which see later) were discussed by Oppenheim, Zur Frage nach der Fortpflanzungsgeschwindigkeit der Gravitation, Wien, 1895.
3. Leipzig Berichte, i (1847), p. 346; Ann. d. Phys. lxxiii (1848), p. 241; translated Taylor's Scientific Memoirs, v, p. 477; Abbandl. der K. Sächs. Ges. i (1852), p. 483; Ann. d. Phys. lxxxvii (1852), p. 145; trans. Tyndall and Francis' Scientific Memoirs, p. 163.