the distance of the
unknown planet. There is only one term unknown. The periodic time of
Herschel we will call 1, and its distance 1, and by resolving the
equation, we find the periodic time of the new planet to be a fraction
less than three times that of Herschel, or about 220 years. Now, if it
be required to perform 360 degrees in 220 years, it will perform about
a degree and a half in one year. Only one thing more remains to be
accomplished. If it is possible to get the position of the unknown body
at _any time_, we can trace it up to where it should be in 1847.

"First, then, let us suppose the sun, Herschel, and the new planet in
certain fixed positions, which we will represent as follows,--

[Illustration:
A B C

Sun. Herschel. Unknown, or
Leverrier Planet.
]

"It will be observed that a line drawn out from the sun to the right
will pass through Herschel, and if continued will intersect the new
planet. It is very apparent that, when these three orbs occupy the
position assigned them above, the influence of the unknown planet upon
Herschel will be exercised in the highest degree, and consequently that
Herschel will be drawn farther from the sun at that juncture than at
any other; and if we know where _Herschel_ is, when this effect is
produced, by prolonging the line through Herschel outward, it must pass
through the new planet. The delicate observations upon Herschel gave
this result, and showed when it was that it was swayed farthest from
the sun. By taking the place occupied by the planet at that time, and
increasing it onward one degree and a half per annum, we can point out
the place it must occupy at any given period. In September last we find
Leverrier communicating these results to his friends in Berlin. They
are provided with charts, on which every observed star is mapped down;
and if any new object presents itself in the heavens, it is immediately
subjected to a rigid scrutiny. On the very night on which Leverrier's
letter had been received, we find the telescope directed to the
designated point in the heavens. A stranger appears, but has only the
aspect of a fixed star. Long did the eye watch that night, but no
motion was found. When twenty-four hours rolled round, and it was once
more possible to fix the instrument upon this strange body, it had
moved in the precise degree and direction computed. The new planet was
found. The