356 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [46 



By the comparison of these theories with observation, M. Le Verrier 

 was led to two interesting results. He found that in order to bring the 

 theories of Mercury and Mars into accordance with observation, it was 

 necessary and sufficient to increase the secular motion of the perihelion of 

 Mercury, and also the secular motion of the perihelion of Mars. 



Hence M. Le Verrier inferred that there existed, on the one hand, in 

 the neighbourhood of Mercury, and on the other, in the neighbourhood of 

 Mars, sensible quantities of matter, the action of which had not been taken 

 into account. 



This conclusion has been verified with respect to Mars. The matter 

 which had not been considered turns out to belong to the Earth itself, the 

 mass of which had been taken too small, having been derived from too small 

 a value of the solar parallax. A similar increase of the mass of the Earth 

 is indicated by the theory of Venus, and a corresponding increase of the 

 solar parallax is likewise derived from the lunar equation in the motion of 

 the Sun. 



With respect to Mercury, a similar verification has not yet taken place, 

 but the theory of the planet has been established Avith so much care, and 

 the transits of the planet across the Sun furnish such accurate observations, 

 as to leave no doubt of the reality of the phenomenon in question; and the 

 only way of accounting for it appears to be to suppose, with M. Le Verrier, 

 the existence of several minute planets, or of a certain quantity of diffused 

 matter circulating about the Sun within the orbit of Mercury. 



The results which M. Le Verrier had thus obtained from his researches 

 on the motions of the interior planets added to the interest with which he 

 now entered upon similar researches on the system of the four great planets 

 which are the most distant from the Sun. Such researches might furnish 

 information respecting matter, hitherto unknown, existing in the neighbourhood 

 of these planets. Possibly they might afford indications of the existence of 

 a planet beyond Neptune, and at any rate they would provide materials 

 which would facilitate future discoveries. 



As I shall have occasion to explain later on, the theories of the mutual 

 disturbances of the larger planets are far longer and more complicated than 

 those of the smaller, so that all that M. Le Verrier had yet done might be 

 almost regarded as merely a prelude to what still remained to be done. 

 Increased difficulties, however, far from deterring, seemed rather to stimulate 

 him to greater exertions. 



