10 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Jan. 



have received, and which they will continue to receive. Yet we should 

 do wrong if we considered that these two persons alone are to be regarded 

 as the authors of the discovery of this planet. I am confident that it will 

 be found that the discovery is a consequence of what may properly be 

 called a movement of the age; that it has been urged by the feeling of the 

 scicntilic world in general, and has been nearly perfected by the collateral, 

 but independent labours, of various persons possessing the talents or 

 powers best suited to the difl'erent parts of the researches. 



With this conviction, it has appeared to me very desirable that the au- 

 thentic history of tins discovery should be published as soon as possible ; 

 not only because it will prove a valuable contribution to the history of 

 science, but also beciuse it may tend to do justice to some persons who 

 otherwise would not receive in future times the credit which they deserve. 

 And as a portion of the history, I venture to oli'er to this Society a state- 

 ment of the circumstances which have come to my own knowledge. I 

 have thought that I could with propriety do this : not because I can pre- 

 tend to know all the history of the discovery, but because I know a con- 

 siderable part of it ; and because I can lay claim to the character of im- 

 partiality to this extent, that, though partaking of the general movement 

 of the age, I have not directly contributed either to the theoretical or to 

 the observing parts of the discovery. In a matter of Ibis delicacy I have 

 thought it best to acton my own judgment, without consulting any other 

 person ; I have, however, solicited the permission of my English corre- 

 spondents for the publication of letters. 



Without pretending to fix upon a time when the conviction of the irre- 

 concilability of the motions of Uranus with the law of gravitation first 

 fixed itself in the minds of some individuals, we may without hesitation 

 date the general belief in this irreconcilability from the publication of M. 

 Alexis liouvard's ruWfs 0/ Cn<n«s in 1821. It was fully shown in the 

 introduction to the tables, that, when every correction for perturba'ion 

 indicated by the best existing theories was applied, it was still impossible 

 to reconcile the observations of Flamsteed, Leniounier, Bradley, and 

 Mayer, with the orbit required by the observations made after 1781 : and 

 the elements of the orbit were adopted from the latter observations, leaving 

 the discordances with the former (amounting sometimes to three minutes 

 of arc) for future explanation. 



The orbit thus adopted represented pretty well the observations made 

 in the years immediately following the publication of tlie tables. But in 

 five or six years the discordance again growing up became so great, that 

 it could not escape notice. A small error was shown by the Rremsmiin- 

 ster Observations of 1825 and 182C: but, perhaps, I am not in error in 

 stating that the discordance was first prominently exhibited in the C'am- 

 brige Observations, the publication of which fram 1828 was conducted 

 under my superjntendence. 



[Here intervene letters from Mr. Hussey (183-1) and M. Bouvard (1837), 

 surmising that the perturbations might be produced by an unseen body.] 



I have departed from a strictly chronological order for the sake of keep- 

 ing in connexion the papers which relate to the same trains of investiga- 

 tion. Several months before the dale of the last letter quoted, I had re- 

 ceived the first intimation of those 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. Amy. 

 [extract.] 



" Cmnhridge Observatory, Feb. 13, 1844. 

 "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 ad- 

 vanced 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 calcula- 

 tion of the tabular errors any alterations have been made in Bouvard's 

 Tables of Uranus besides that of Ju/dter's mass." 



No. 7. — G. B. Aip.y to Professor Challis. 

 [extract.] 

 "Royal Observatory, Greenwich, 1844, Feb. lt>. 

 " 1 send all the results of the observations of Uranus made with both 

 instruments [that is, the heliocentric errors of Ui'anus in longitude and 

 latitude from l754to 1830, for all those days on which there were observa- 

 tions, 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 ^ part. As constants have been added (in 

 the printed tables) to make the equations positive, and as jij; part of tlie 

 numbers in the tables has been added, .^ part of the constants has been 

 sibtracted from the final results." 



No. 8. — Professor Challis to G. B. Airy. 



[extract.] 



" Cambridge ObseiTalory, Feb. 16, 1844. 

 "I am exceedingly obliged by your sending so complete a series of 

 tabular errors of Uranus. • • • 1 he list you have sent will give Mr. 

 Adams the means of carrying on in the most ellective manner the inquiry 

 in which he is engaged. 



No. 9. — Professor Challis to G. B. AiRV. 



" Cambridge Observatory Sept. 22, 1845 

 " My friend Mr. Adams (who will probably deliver this note to you) 

 has completed his calculations respecting the perturbation of the orbit of 

 Uranus by a supposed ulterior planet, and has arrived at results which he 

 would be glad to communicate to you personally, if you could spare him 

 a few moments of your valuable time. His calculations are founded on 

 the observations \ou were so good as to furnish him with some time ago; 

 and from his character as a mathematician, and his practice in calculation, 

 I should consider the deductions from his premises to be made in a trust- 

 worthy manner. If he should not have the good fortune to see you at 

 Greenwich, he hopes to be allowed to write to you on this subject." 



No. 10. — G. B. Airy to Professor Challis. 



" Riiyal Observatory, Greenwich, 184.'), Sept. 2.9. 



" I was, I suppose, on my way from France, when Mr. Adams called 

 here : at all events, I had not reached home, and therefore, to my regret, I 

 have not seen him. Would you mention to Mr. Adams that I am very 

 much interested with the subject of his investigations, and that 1 should 

 be delighted to hear of them by letter from him 1" 



On one of the last days of October, 1845, Mr. Adams called at the 

 Royal Observatory, Greenwich, in my absence, and left the following im- 

 portant paper : — 



No. 11. — J. C. Adams, Esq. to G. B. Airy. 



" According to my calculations, the observed irregularities in the motion 

 of Uranus may be accounted for by supposing the existeuce of an exterior 

 planet, the mass and orbit of which are as follows: — 



Mean Distance (assumed nearly in accordance 



with Bode's law) 38-4 



Mean Sidereal Motion in 305-25 days . . 1°30''9 

 Mean Longitude, 1st October, 1845 . . . 323 34 



Longitude of Perihelion. .... 315 55 



Eccentricity 01610. 



Mass (that of the Sun being unity), . . . 0-0001656. 



For the modern observations I have used the method of normal places, 

 taking the mean of the tabular errors, as given by observations near three 

 consecutive oppositions, to correspond with the mean of the times ; and 

 the Greenwich observations have been used down to 1830 ; since which, 

 the Cambridge and Greenwich observations, and those given in the As- 

 tronomische Nachtiekten, have been made use of. The following are the 

 remaining errors of mean lougitude : — 



Observation — Theory. 



1780 -f0"-27 1801 — 0"-04 18-22 -fO" 30 



1783 —0 -23 1804 -fl -76 1825 -)-l ■92 



1780 —0 -90 1807 —0 -21 1828 +2 -25 



1789 -f 1 -82 1810 -fO -56 1831 —1 -06 



1792 —0-91 1813 —0 -94 1834 —1 -44 



1795 +0 -09 1816 —0 -31 1837 —1 -62 



1798 —0 -99 1819 —2 -00 1840 -fl -73 



The error for 1780 is concluded from that for 1781 given by observation, 



compared with those of four or five following years, and also with Lemon- 



nier's observations in 1769 and 1771, 



" For the ancient observations, the following are the remaining errors : — 



Observation — Theory. 

 1090 -f44'-4 1750 — l"-6 1703 — 5"-I 



1712 4-0-7 1753 -f 5 -7 17G9 +0-6 



1715 — 6 -S 1750 — 4-0 1771 +11 '8 



The errors are small, except for Flamsteed's observation of 1690. This 

 being an isolated observation, very distant from the rest, I thought it best 

 not to use it in forming the equations of condition. It is not improbable 

 however, that this error might be destroyed by a small change in the as- 

 sumed mean motion of the planet." 



I acknowledged the receipt of this paper in the following terms : 



No. 12.— G. B. Airy to J. C. Adams, Esq. 



" Royal Observatory, Greenwich, 1845, Nov. 5. 

 " I am very much obliged by the paper of results which you left here a 

 few days since, showing the perturbations on the place of Uranus pro- 

 duced by a planet with certain assumed elements. The latter numbers are 

 all extremely satisfactory : 1 am not enough acquainted with Flamsteed's 

 observations about 1690 to say whether they bear such an error but I 

 think it extremely probable. 



" But I should be very glad to know whether this assumed perturbation 

 will explain the error of the radius vector of Uranus. This error is now 

 very considerable, as you will be able to ascertain by comparing the nor- 

 mal equations, given in the Greenwich observations for each year, for the 

 times be/ore opposition with the times after opposition." 



I have before staled, that I consider the establishment of this error of 

 the radius vector of Uranus to be a very important determination, I there- 

 fore considered that the trial, whether the error of the radius vector would 

 be explained by the same theory which explained the error of longitude' 

 would be truly an exferimcntum cruets. And I wailed with much anxiety 

 for Mr. Adams's answer to my query. Had it been in the afliimative, I 



