Peofessoe W. Thomson on the Motion of Mercury. 265 



for which the observations cannot be blamed, unless we suppose that 

 such astronomers as Lalande, Cassini, Bouguer, &c., could have com- 

 mitted errors amounting to several minutes of time, and even varying 

 progressively from one epoch to another ! 



" Now, what is remarkable is, that it suiBces to augment by 38" the 

 centenary movement of the perihelion of Mercury's orbit to represent 

 all the transit observations to within a second, and most of them even 

 to less than half a second. This result, so precise and simple, which 

 gives at once to all the comparisons an accuracy superior to that which 

 has been hitherto attained in astronomical theories, shows clearly that 

 the increase in the motion of the perihelion is necessary, and that 

 when made to fulfil this condition, the tables of Mercury and the Sun 

 possess all desu'able accuracy." 



M. Le Verrier proceeds in his letter to examine the diiFerent sup- 

 positions by which it may be attempted to explain the perturbation in 

 Mercury's motion, which he has thus discovered. An increase of i\jth on 

 the supposed mass of Venus would account for it ; but the periodical 

 disturbances in the Earth's motion, and the secular variation of the 

 inclination of the Earth's axis to the plane of the echptic produced by 

 Venus, do not allow any such change in our estimate of her mass. On 

 the other hand, a planet revolving round the Sun inside Mercury's 

 orbit, might produce precisely the variation in the perihelion to be 

 explained, without sensibly disturbing the motions of Venus, the Earth, 

 or any of the superior planets. M. Le Verrier shows, for instance, that 

 a planet equal in mass to Mercury, and revolving in a circular orbit in 

 the same plane, at a little less than half the distance from the Sun, would 

 fulfil these conditions ; and, therefore, that a less mass in a larger orbit, 

 or a greater mass in a smaller orbit, would do the same. But, considering 

 that so large a mass could scarcely have escaped observation, either in 

 its transits across the Sun's disc, or by its own brilliant illumination, 

 wliich would render it visible to us dm-ing total ecHpses of the Sun, 

 even if, on account of its nearness to the Sun, not ordinarily seen as 

 a morning and evening star alternately, M. Le Verrief thinks it most 

 probable that the disturbing mass consists in reality of a series of cor- 

 puscules circulating between the Sun and Mercmy. 



Here, then, by a profound appreciation of purely astronomical data, 

 the great French physical astronomer, leaving those remote regions 

 where, independently of our own countryman, Adams, he tracked 

 the unseen planet by its disturbing influence on the remotest body 

 of the then known list, has been led to conclude that the iimer- 

 most of the recognized planets is also disturbed by planetary matter, 

 not previously reckoned among the influencing masses of our system. 



