History of the Discovery of the Planet Neptune. 191 
If we could determine for each day the precise effect produced 
by the unknown body, we could deduce from it the direction in 
which Uranus is drawn; that is, we should know the direction 
of the disturbing body. But the problem is far from being thus 
simple. The amount of the disturbance cannot be deduced di- 
rectly from the observations, unless we know the exact orbit 
which Uranus would describe provided it were free from this dis- 
turbing action; and this orbit in turn cannot be computed, un- 
less we know the amount of the disturbances. Le Verrier there- 
fore computes for every nine degrees of the entire circumference, 
the effect which would be produced by supposing a planet situa- 
ted in different parts of the ecliptic. He finds that when he lo- 
cates the supposed disturbing planet in one part of the ecliptic, 
the discrepancies between the observed and computed effects are 
enormous. By varying the place of the planet, the discrepancies 
become smaller, until at a certain point they nearly disappear. 
Hence he concludes that there is but one point of the ecliptic 
where the planet can be placed, so as to satisfy the observations 
of Uranus. Having thus determined its approximate place, he 
proceeds to compute more rigorously its effects; and on the first 
of June, 1846, he announces as the result of his investigations, 
that the longitude of the disturbing planet for the beginning of 
1847, must be about 325°. 
The result thus obtained by Le Verrier, differed but one degree 
from that communicated by Mr. Adams to Professor Airy, more 
than seven months previous. Upon receiving this intelligence, 
Professor Airy expressed himself satisfied with regard to the gen- 
eral accuracy of both computations, and immediately wrote to 
errier, enquiring as he had done before of Mr. Adams, wheth- 
er his theory explained the error of the tables in respect to the 
distance of Uranus from the sun. Le Verrier showed that it did 
this perfectly. Professor Airy was now so well convinced of the 
existence of a planet yet undiscovered, that he was anxious to 
have a systematic search for it forthwith undertaken. Ob- 
Servatory of Cambridge is provided with one of the finest tele- 
Scopes of Europe, presented by the late Duke of Northumberland. 
Professor Airy urged upon the director, Professor Challis, to un- 
dertake the desired search, and recommended the examination of 
a belt of the heavens, ten degrees in breadth, aud extending thirty 
degrees in the direction of the ecliptic. 'This belt was to be 
Swept over at least three times. If any star in the second sweep 
a different position from that observed in the first, it might 
be presumed that it was the planet. If two sweeps failed of de- 
tecting the planet, it might be caught in the third. 
Professor Challis commenced his search, July 29th, and con- 
tinued it each favorable evening, recording the exact position of 
every star down to the eleventh magnitude. Meanwhile Le Ver- 
