Men sought for Neptune as the heroes sought the golden fleece.  The place of Uranus had been mapped for nearly one hundred years by these accidental observations.  On applying the law of universal gravitation, a slight discrepancy was found between its computed place and its observed place.  This discrepancy was exceedingly slight.  In 1830 it was only 20”; in 1840,190”; in 1884, 2’.  Two stars that were 2’ apart would appear as one to the keenest unaided eye, but such an error must not exist in astronomy.  Years of work were given to its correction.  Mr. John C. Adams, of Cambridge, England, finding that the attraction of a planet exterior to Uranus would account for its irregularities, computed the place of such a hypothetical body with singular exactness in October, 1841; but neither he nor the royal astronomer Airy looked for it.  Another opportunity for immortality was heedlessly neglected.  Meanwhile, M. Leverrier, of Paris, was working at the same problem.  In the summer of 1846 Leverrier announced the place of the exterior planet.  The conclusion was in striking coincidence with that of Mr. [Page 176] Clark.  Mr. Challis commenced to search for the planet near the indicated place, and actually saw and mapped the star August 4th, 1846, but did not recognize its planetary character.  Dr. Galle, of Berlin, on the 23d of September, 1846, found an object with a planetary disk not plotted on the map of stars.  It was the sought-for world.  It would seem easy to find a world seventy-six times as large as the earth, and easy to recognize it when seen.  The fact that it could be discovered only by such care conveys an overwhelming idea of the distance where it moves.

[Illustration:  Fig. 66.—­Perturbation of Uranus.]

The effect of these perturbations by an exterior planet is understood from Fig. 66.  Uranus and Neptune were in conjunction, as shown, in 1822.  But in 1820 it had been found that Uranus was too far from the sun, and too much accelerated.  Since 1800, Neptune, in his orbit from F to E, had been hastening Uranus in his orbit D from C to B, and also drawing it farther from the sun.  After 1822, Neptune, in passing from E to D, had been retarding Uranus in his orbit from B to A.

We have seen it is easy to miss immortality.  There is still another instance.  Lalande saw Neptune on May 8th and 10th, 1795, noted that it had moved a little, and that the observations did not agree; but, supposing the first was wrong, carelessly missed the glory of once more doubling the bounds of the empire of the sun.

[Page 177] It is time to pause and review our knowledge of this system.  The first view reveals a moon and earth endowed with a force of inertia going on in space in straight lines; but an invisible elastic cord of attraction holds them together, just counterbalancing this tendency to fly apart, and hence they circle round their centre of gravity.  The revolving earth turns every part of its surface to the moon in