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
each twenty-four hours. By an axial revolution in the same time
that the moon goes round the earth, the moon holds the same point
of its surface constantly toward the earth. If we were to add one,
two, four, eight moons at appropriate distances, the result would
be the same. There is, however, another attractive influence--that
of the sun. The sun attracts both earth and moon, but their nearer
affection for each other keeps them from going apart. They both,
revolving on thei