The astronomers of the present day understand this well, and can describe with exactness the pathway of each planet. This knowledge does not come merely from watching one year how the planets travel, and remembering for another year, but is much more a matter of close and difficult calculation. Many things have to be considered, such as the planet’s distance from the sun, the sun’s power of attraction, the planet’s speed, the nearness and weight of other neighboring planets. All these questions were gone into, and astronomers sketched out the pathway in the heavens which they expected Uranus to follow. He would move in such and such an orbit, at such and such distances from the sun, and at such and such rates of speed. But Uranus would not keep to these rules. He quite discomfited the astronomers. Sometimes he went fast, when, according to their notions, he ought to have gone more slowly; and sometimes he went slowly, when they would have looked for him to go more fast, and the line of his orbit was quite outside the line of the orbit which they had laid down. He was altogether a perplexing acquaintance, and difficult to understand. However, astronomers felt sure of their rules and modes of calculation, often before tested, and not found to fail. They made a guess at an explanation. What if there were yet another planet beyond Uranus, disturbing his motions--now drawing him on, now dragging him back, now so far balancing the sun’s attraction by pulling in the opposite direction as to increase the distance of Uranus from the sun? It might be so. But who could prove it? Hundreds of years might pass before any astronomer, in his star-gazing, should happen to light upon such a dim and distant world. Nay, the supposed planet might be, like Uranus, actually seen, and only be mistaken for a “variable star,” shining but to disappear. There the matter seemed likely to rest. There the matter probably would have rested for a good while, had not two men set themselves to conquer the difficulty. One was a young Englishman, a student of Cambridge, John Couch Adams; the other, a young Frenchman, Urbain Jean Joseph Leverrier,--both being astronomers. Each worked independently of the other, neither knowing of the other’s toil. The task which they had undertaken was no light one--that of reaching out into the unknown depths of space to find an unknown planet.