the Planet exterior to Uranus. 5 1 9 



I have departed from a strictly chronological order for the 

 sake of keeping in connexion the papers which relate to the 

 same trains of investigation. Several months before the date 

 of the last letter quoted, I had received the first intimation of 

 these calculations which have led to a distinct indication of 

 the place where the disturbing planet ought to be sought. 

 The date of the following letter is Feb. 13, 1844:— 



No. 6. Professor Challis to G. B. Airy, 



[extract.] 



" Cambridge Observatory, Feb. 13, 1844. 

 (i A young friend of mine, Mr. Adams, of St. John's College, 

 is working at the theory of Uranus, and is desirous of obtain- 

 ing errors of the tabular geocentric longitudes of this planet 

 when near opposition in the years 1818-26, 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 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 increasing the two equations which depend 

 on Jupiter by j^ part. As constants have been added (in the 

 printed tables) to make the equations positive, and as jy part 

 of the numbers in the tables has been added, j^ 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 



