234 The Mathematical Electricians of the 



and, regarding F\ 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 A 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 attrac- 

 tion 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 f 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 Terrier's own 

 hypothesis of an intra-mercurial planet. But it was found 

 later* that f of the excess could be explained by substituting 

 Eiemann's electrodynamic law for Weber's, and that a com- 

 bination of the laws of Biemann and Weber would give exactly 

 the amount desired.f 



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.^ Weber remarked 

 that if, with Ampere, 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 



* By Maurice Levy, Comptes Eendus, ex (1890), p. 545. 



t The consequences of adopting the electrodynamic law of Clausius (for which 

 see later) were discussed by Oppenheim, Zur Frage nach der Fortpflanzungs- 

 geschwindigJceit der Gravitation, Wien, 1895. 



I Leipzig Berichte, i (1847), p. 346 ; Ann. d. Phys. Ixxiii (1848), p. 241 ; 

 translated Taylor's Scientific Memoirs, v, p. 477 ; Abhandl. der K. Sachs. Ges. i 

 (1852), p. 483; Ann. d. Phys. Ixxxvii (1852), p. 145; trans. Tyndall and 

 Francis' Scientific Memoirs, p. 163. 



