528 Prof. G. B. Airy on the History of the Discovery of 



were made by persons whose character and position offered a 

 guarantee that the assistance was fairly asked for science, and 

 that the money would be managed with fair frugality. In the 

 very improbable event of the Government refusing such in- 

 demnity, I was prepared to take all consequences on myself. 

 On the 13th of July, I transmitted to Professor Challis 

 '• Suggestions for the Examination of a Portion of the Heavens 

 in search of the external Planet which is presumed to exist 

 and to produce disturbance in the motion of Uranus," and I 

 accompanied them with the following letter : — 



No. 16. G. B. Airy to Professor Challis. 



"Royal Observatory, Greenwich, 1846, July 13. 

 " I have drawn up the inclosed paper, in order to give you 

 a notion of the extent of work incidental to a sweep for the 

 possible planet. 



" I only add at present that, in my opinion, the importance 

 of this inquiry exceeds that of any current work, which is of 

 such a nature as not to be totally lost by delay." 



My " Suggestions " contemplated the examination of a part 

 of the heavens 30° long, in the direction of the ecliptic, and 

 10° broad. They entered into considerable details as to the 

 method which I proposed ; details which were necessary, in 

 order to form an estimate of the number of hours' work likely 

 to be employed in the sweep. 



I received^ in a few days, the following answer: — 



No. 1 7. Professor Challis to G. B. Airy. 



[extracts.] 

 "Cambridge Observatory, July 18th, 1846. 

 "I have only just returned from my excursion. * * * I 

 have determined on sweeping for this hypothetical planet. 

 * * * With respect to your proposal of supplying an 

 assistant I need not say anything, as I understand it to be 

 made on the supposition that I decline undertaking the search 

 myself. * * * I purpose to carry the sweep to the extent 

 you recommend." 



The remainder of the letter was principally occupied with 

 the details of a plan of observing different from mine, and of 

 which the advantage was fully proved in the practical obser- 

 vation. 



On August 7, Professor Challis, writing to my confidential 

 assistant (Mr. Main) in my supposed absence, said, — 



No. 18. Professor Challis to the Rev. R. Main. 

 [extract.] 

 " Cambridge Observatory, August 7, 1846. 

 " I have undertaken to search for the supposed new planet 



