1847.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



II 



should at once have exerted all the influence which I might possess, either 

 directly, or indirectly through my friend Professor Challis, to procure the 

 publication of Mr. Adams's theory.* 



From some cause with which I am unacquainted, probably an accidental 

 one, I received no immediate answer to this inquiry. I regret this deeply, 

 for many reasons. 



M'hile I was expecting more complete information on Mr. Adams's 

 theory, the results of a new and most important investigation reached me 

 from anotlier quarter. In the Comptc Rendu o{ {he French Academy for 

 the lOih Nov., 1845, which arrived in this rimntry in December, there is 

 a paper by M. Le Verrier on the perturbations of Uranus produced by 

 Jupiter and Saturn, and on the errors in llie elliptic elements of Urumis, 

 consequent on the use of erroneous perturbations in the treatment of the 

 observations. It is impossible for me here to enter into details as to the 

 conclusions of this valuable memoir; I shall only say that, while the cor- 

 rectness of the former theories, as far as they went, was generally estab- 

 lished, many small terms were added ; that the accuracy of the calcula- 

 tions was established by duplicate investigations, following different cour- 

 ses, and executed with extraordinary labour; that the corrections to the 

 elements, produced by treating the former observations with these corrected 

 perturbations, were obiaiucd ; and that the correction to the ephenieris for 

 the present time, produced by the introduclion of the new perturbations 

 and the new elemenis, was investigated and found to be incapable of ex- 

 plaining the observed irregularity of Uranus. Perhaps it may be truly 

 said that the theory of Uranus was now, for the first time, placed on a 

 satisfuciory foundation. This important labour, as M. Le Verrier states, 

 was undertaken at the urgent request of M. Arago. 



In the Comptc Rendu for June 1, 1840, M. Le Verrier gave his second 

 memoir on the theory of Uranus. The lirst part contains the results of a 

 new reduction of nearly all the existing observations of Uranus, and their 

 treatment with reference to the theory of perturbations, as amended in the 

 former memoir. After concluding from this reduction that the observations 

 are absolutely irreconcilable with the theory, M. Le V«-rrier considers in 

 the second part all the possible explanations of the discordance, and con- 

 cludes that none is admissible, except that of a disturbing planet exterior 

 to Uranus. He then proceeds to investigate the elements of the orbit of 

 such a planet, assuming that its mean distance is double that of Uranus, 

 and that its orbit is in the plane of the ecliptic. The value of the mean 

 distance, it is to be remarked, is not fixed entirely by Bode'slaw, although 

 suggested by it ; several considerations are stated which compel us to 

 take a mean distance, not very greatly ditfering from that suggested by the 

 law, but which nevertheless, without the suggestion of that law, would 

 leave the mean distance in a most troublesome uncertainty. The peculiar- 

 ity of the form which the investigation takes is then explained. Finally, 

 M. Le Verrier gives as the most probable result of his investigations, that 

 the true longitude of the disturbing planet for the begioning of 1847 must 

 be about 3ii;i°, and that an error of 10° in this place is not probable. No 

 elements of the orbit or mass of the planet are given. 



This memoir reached me about the 23rd or 24lh of June. I cannot 

 sufficiently express the feeling of delight and satisfaction which I received 

 from it. The place which it assigned to the disturbing planet was the 

 same, to one degree, as that given by Mr. Adams's calculations, which I 

 had perused seven months earlier. To this time I had considered that 

 there was still room for doubt of the accuracy of Mr. Adams's investiga- 

 tions ; for I thmk ihat the results of algebraic and numerical computations, 

 so long and so complicated as those of an inverse problem of perturba- 

 tions, are liable to many risks of error in the details of the process : I 

 know that there are important numerical errors in the Mfcanique CHefte 

 of Laplace; in the Thenrie de la Lune of Plana; above all, in Bouvard's 

 first tables of Jupiter and Saturn ; and to express it in a word, I have 

 always considered the correctness of a distant mathematical result to be a 

 subject rather of moral than of mathematical evidence. But now I felt 

 DO doubt of the accuracy of both calculations, as applied to the perturba- 

 tion in longitude. I was, however, still desirous, as before, of learning 

 whether the perturbation in radius vector was fully explained, 1 therefore 

 addressed to M. Le Verrier the following letter; — 



No. 13.— G. B. Airy to M. Le Verkier. 



" Royal Observatory, Greenwich, 1846, June 20. 



"I have read, with very great interest, the account of your investigations 

 on the probable place of a pl.tnet disturbing the motions of Uranus, which 

 is contained in the Compte Rendu de I' Academic of June 1; and I now 

 beg leave to trouble you with the following question. It appears, from all 

 the later observations of Uranus made at Greenwich (which are most 

 completely reduced in the Greenwich Obserratinns of each year, so as to 

 exhibit the efl'ectof an error either in ihe tabular heliocentric longitude, or 

 the tabular radius vector), that the tabular radius vector is considerably loo 

 small. And I wish to inquire of you whether this would be a conse- 

 quence of the disturbance produced by an exterior planet, now in the 

 position which you have indicated? 



" I imagine that it would not be sp, because the principal term of the 

 inequality would probably be analogous to the moon's variation, or would 

 depend on sin 2(r — v') ; and in that case the perturbation in radius vector 

 would have the sign — for the present relative position of the planet and 



* Here the Astronomer Royal explained to the meeting, by means of a diagram, the 

 nature of the eriors of the tabular radius vector. t 



Uranus. But this analogy is worth little, until it is supported by proper 

 symbolical computations. 



" By Ihe earliest opportunity I shall have the honour of transmitting to 

 you a copy of the Planctarii Reductions, in which you will find all Ihe 

 observations made at Greenwich to 1830 carefully reduced and compared 

 with the tables." 



Before I could receive M. Le Verrier's answer, a transaction occurred 

 which had some influence on the conduct of English astronomers. 



On the 29lh of June, a meeting of ihe Board of Visitors of llie Royal 

 Observatory of Greenwich was held, for the consideration of special 

 business. At this meeting. Sir J. Herschel and Professor Challis (among 

 other members of the Board) were present; I was also present, by invita- 

 tion of the Board. The discussion led, incidenlally, to the general ques- 

 tion of the advantage of distributing subjects of observation among differ- 

 ent observatories. I spoke strongly in favour of such distribution ; and 

 I produced, as an instance, the extreme probability of now discovering a 

 Dew planet in a very short time, provided the powers of one observatory 

 could be directed to the search for it. I gave, as the reason upon which 

 this jjrobability was based, the very close coincidence between the results 

 of J\lr. Adams's and M. Le Verrier's investigations of the place of the 

 supposed planet disturbing Uranus. I am authorised by Sir J. Herschel's 

 jirinted statement in Ihe Athemeum of October 3, to ascribe to the strong 

 expressions which I then used the remarkable sentence in Sir J. Herschel's 

 address, on September 10, to the British Association assembled at South- 

 ampton. " We see it [Ihe probable new planet] as Columbus saw America 

 from the shores of Spain. Its movements have been felt, trembling along 

 the far-reaching line of our analysis, with a certainty hardly inferior to 

 that of ocular demonstration."* And I am authorised by Professor Chal- 

 lis, in oral conversation, to state that the same expressions of mine induced 

 him to contemplate the search for the suspected planet. 



[M. Le Verrier's reply follows, in which he says that M. Bouvard cal- 

 culated incorrectly the orbit of Uranus, in ignorance of the exterior planet, 

 and that the error of the radius vector of Uranus arises from errors of its 

 eccentricity and longitude of perihelion. 



The following letter is from Professor Airy to Professor Challis, request- 

 ing the latter to undertake the search at Cambridge, with the Northumber- 

 land equatorial telescope, as the only instrument in England large enough 

 for the purpose.] 



In explanation of this letter, it may be necessary to stale that, in com- 

 mon I believe with other astronomers at that time, I thought it likely that 

 tlie planet would be visible only in large telescopes. I knew that the 

 Observatory of Cambridge was at this time oppressed with work, and I 

 thought that the undertaking- — a survey of such an extent as this seemed 

 likely to prove — would be entirely beyond the powers of its personal 

 establishment. Had Professor Challis assented to my proposal of assist- 

 ance, I was prepared immediately to place at his disposal Ihe services of 

 an eflicient assistant ; and for approval of such a step, and for liquidation 

 of the expense which must thus be thrown on the Koyal Observatory, I 

 should have referred to a Government which I have never known to be 

 illiberal v^hen demands for the benefit of science 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 Ihe Government refusing such 

 indemnity, 1 was prepared to take all consequtnces on myself. 



On the ISIh of July, I transmitted to Professor Challis "Suggestions for 

 the Examination of a Portion of the Heavens in search of the external 

 Planet vihich is presumed to exist and to produce disturbance in the mo- 

 tion of Uranus," and I accompanied them with ihe following letter: — 



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



" Royal Observatory, Greenwich, 1846, July 13. 



" I liave drawn up the enclosed 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." 



BIy '• Suggestions" contemplated the examination of a part of the hea- 

 vens 30° long, in Ihe direction of the ecliptic, and 10° broad. They en- 

 tered into considerable details as to the method which I proposed ; details 

 w Inch were necessary, in order to form an estimate of the number of hours' 

 work likely to be employed iu the sweep. 



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



No. 17.— Professor Challis to G. B. Airy. 

 [extracts.] 



" Cambridge Observatory, July 18, 1846. 

 "I have only just returned from my excursion. • * • I have de- 

 termined on sweeping for this hypothetical planet. • • • With re- 

 spect to your proposal of supplying an assistant I need not say anything, 

 as 1 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." 



* This sentence is copied from the written draft of the speech. Sir J. Herschel 

 appeared to suppoae that tlie serucuce had not been repotted in the public journals as 

 Bpoiien. I did, however, sea it so reported in an Englisti newspaper, to which I hid 

 access on tha l^outinent. 



3* 



