"I refer to this fact for two reasons,--first, to verify Leverrier; and, second, to impress upon your minds the desirableness of locating our observatories in different parts of the earth. No European astronomer could have made this observation, because in their longitudes the sun would have set previous to the contact of the planet with its disk. I had the gratification of furnishing these observations to Leverrier himself, who reported upon them to the Academy of Sciences. The triumph of Leverrier was complete. It was after this that Arago, seeing the characteristics of his mind, said to him, 'Take up the movements of the planet Herschel,--watch them, analyze them, and tell us what it is that causes them.' Leverrier throws aside all other employments, and gives his mind to the investigation of this subject. He begins entirely back. He takes up the movements of the planets Jupiter and Saturn, and investigates them anew: he leaves nothing untouched. Finally, after having in the most absolute manner computed all the influence they exercise upon the planet Herschel, he says, 'I now know positively all existing causes that disturb the planet; but there is an outstanding power that disturbs it not yet accounted for, and now let me rise to a knowledge of that outstanding cause.' He did what no other man ever had attempted. He cleared up all difficulties;--he made all daylight before his gaze. And now, how shall I give to you an account of the train of reasoning by which he reached out into unknown space, and evoked from its bosom a mighty world? If you will give me the time, I will attempt to give you an idea of his mighty workings in the field of science.