is No. 6, and it is only necessary to explain that
Professor Challis was the Professor of Astronomy at Cambridge, and in
charge of the Cambridge Observatory, in which offices he had succeeded
Airy himself on his leaving Cambridge for Greenwich some eight years
earlier.

No. 6.--PROFESSOR CHALLIS _to_ G. B. AIRY.
[_Extract._]

"'CAMBRIDGE OBSERVATORY, _Feb. 13, 1844_.

[Sidenote: Challis mentions Adams to Airy, and suggests Adams' visit
to Greenwich.]

"'A young friend of mine, Mr. Adams of St. John's College, is working
at the theory of _Uranus_, and is desirous of obtaining errors of the
tabular geocentric longitudes of this planet, when near opposition,
in the years 1818-1826, with the factors for reducing them to errors
of heliocentric longitude. Are your reductions of the planetary
observations so far advanced that you could furnish these data? and
is the request one which you have any objection to comply with? If
Mr. Adams may be favoured in this respect, he is further desirous of
knowing, whether in the calculation of the tabular errors any
alterations have been made in Bouvard's _Tables of Uranus_ besides
that of _Jupiter's_ mass.'

"My answer to him was as follows:--

No. 7.--G. B. AIRY _to_ PROFESSOR CHALLIS.
[_Extract._]

"'ROYAL OBSERVATORY, GREENWICH, _1844, Feb. 15_.

"'I send all the results of the observations of _Uranus_ made with
both instruments (that is, the heliocentric errors of _Uranus_ in
longitude and latitude from 1754 to 1830, for all those days on which
there were observations, both of right ascension and of polar
distance). No alteration is made in Bouvard's _Tables of Uranus_
except in increasing the two equations which depend on _Jupiter_ by
1/50 part. As constants have been added (in the printed tables) to
make the equations positive, and as 1/50 part of the numbers in the
tables has been added, 1/50 part of the constants has been subtracted
from the final results.'

"Professor Challis in acknowledging the receipt of these, used the
following expressions:--

No. 8.--PROFESSOR CHALLIS _to_ G. B. AIRY.
[_Extract._]

"'CAMBRIDGE OBSERVATORY, _Feb. 16, 1844_.

"'I am exceedingly obliged by your sending so complete a series of
tabular errors of _Uranus_.... The list you have sent will give Mr