Astronomy. 119 



c the identity is no longer admitted by any one.' I say that after this 

 discussion, no one will hesitate respecting the assertion of M. Babinet 

 (Persojine ne s'arretera au dire de M. Babinet). 



" Let us first fix precisely the state of the question. I have determin- 

 ed the position of Neptune by means of the perturbations which it pro- 

 duces on Uranus. Accordingly, when such perturbations take place, 

 I can find directly where Neptune is. But when there are no pertur- 

 bations, this is impossible. Let this not be forgotten. Again, the action 

 of one planet on another depends, at a given moment, only on its rela- 

 tive situation in the heavens and on its mass. The only facts, there- 

 fore, which I could conclude from the perturbations of Uranus, while 

 they existed, were the direction in which Neptune was to be found, its 

 distance from the sun, and its mass. Let us see how I have arrived at 

 the determination of these three quantities. 



" First. — Is it true that the direction in which I have placed Neptune 

 contains an enormous error, except for the epoch of M. Galleys discov- 

 ery, or for very few years before and after ? NO ; this is false. I place 

 before the Academy of Sciences a chart of the respective situations of 

 Neptune, in the orbit I have theoretically assigned to it and in the orbit 

 resulting from direct observation. The latter positions have been taken 

 from Mr. Walker, so as to avoid all suspicion of my having attempted 

 to obtain a smaller deviation. According to this figure, the following 

 are the minimum deviations from my theory : 



In 1857 + 4° -0 



1847 + 1 



1837—0-7 



1827— 2 -0 



1817— 3 1 



1807 —4-5 



1797 _ 6-6 

 It follows that during sixty five years, my theory, deduced from indi- 

 rect considerations, assigns to Neptune a series of positions never dif- 

 fering from those obtained by the direct orbit by more than one fifty- 

 fifth, at the most, of the circumference of a circle. And this is called 

 a small number of years, when it is known that Neptune has had a 

 sensible effect on Uranus for only twenty-five or thirty years at the ut- 

 most ! The fifty -fifth part of the circle ! This is what is called an enor- 

 mous error, when it is known that the data which served as basis to my 

 theory are only known to a tenth. But I do not insist on this subject, 

 <* s I hear M. Babinet declare, that when he spoke of enormous errors^ 

 he had not calculated them, and imagined them much more considera- 

 ble than they are in reality. But, it will be said, if we go beyond these 

 sixty-five years, we should find more considerable deviations. Yes, 

 without doubt. That results from the nature of the question ; it cannot 

 be avoided. I have said that I determine the position of Neptune by 

 means of the perturbations it produces on Uranus, When there are 

 Perturbations, lean say where Neptune shall be found ; but to ask of 

 me to do so, long after the perturbing action has disappeared, is simply 

 to ask an impossibility — a sort of miracle. Now, in examining my 

 plate — which in a few days I shall place before the public, and in which 

 1 have traced the course of Uranus— it appears clearly that this planet 



