194 History of the Discovery of the Planet Neptune. 
more fortunate. Mr. Walker proceeded in the following manner. 
He first computed the orbit which best represented all the ob- 
servations which had been made at the Washington Observatory, 
as well as those which had been received from Europe. He 
then computed the planet’s tare — for a long series of 
preceding years, and sought among the records of astronomers, 
for observations of stars in the sciehbcnbaail of the computed 
path. Bradley, Mayer and Lacaille, have left us an immense col- 
lection of abservations, yet they seldom recorded stars so small 
as the body in question. Among the observations of Piazzi, so 
far as they have been received in this country, no one was found 
which could be identified with the planet. The Madras obser- 
vations were generally confined to the stars of Piazzi’s catalogue. 
The Paramatta catalogue seldom extends north of the thirty-third 
parallel of south declination ; and Bessel, in preparing his zones 
of 75,000 stars, did not sweep far enough south to comprehend 
the planet. The only remaining chance of finding an observa- 
tion of the planet, was among the observations of Lalande. The 
Histoire Celeste Frangaise embraces 50,0U0 stars, and Mr. Wal- 
ker soon found that Lalande had swept over the supposed path 
of the planet on the 8th and 10th of May, 1795. He accordingly 
computed more carefully the place of the planet for this period, 
making small variations in the elements of the orbit, so as to 
include the entire region within which the planet could possibly 
ave been confined. He then selected from the Histoire Celeste, 
all the stars within a quarter of a degree of the computed path. 
These stars were nine in number; of which siz had however 
been subsequently observed by Bessel, and of course were to be 
set down as fired stars. But three stars remained which required 
special examination ; and of these, one was too small to be mis- 
taken for the plan net, and a second was thought to be too far 
from the computed place. The remaining star was distant only 
two minutes from the computed place of the planet; it was 0 
the same magnitude, and was not to be found in Bessel’s ob- 
servations, although this part of the heavens must have been 
included in the field of his telescope. This discovery was made 
on the 2d of February, 1847; and on the first clear subsequent 
evening, Feb. 4th, the great equatorial of the Washington Ob- 
servatory was pointed to the heavens, and this star was missing. 
Where Lalande in 1795 saw a star of the ninth magnitude, there 
remained only a blank. The conclusion seemed almost certain, 
t Mr. Walker had here obtained the object of his search. 
_ accordingly computed the path npon this supposition, and fot 
that a single elliptic orbit would represent, with almost mathe- 
matical precision, the observation of 1795, and all the —- 
tions of 1846. 
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