184
HISTORY OF PHYSICAL ASTRONOMY.
had applied to Mr. Adams. In his letter to the French mathematician,he remarks, that the tabular radii vectores of Uranus were too small inreceut years ; and he asks whether this would result from the disturbanceof an exterior planet occupying the position indicated by the theory. “ Iimagine,” says Mr. Airy, “ that it would not be so, because the principalterm of the inequality would probably be analogous to the moon’s varia-tion, or would depend on sine 2 (v — v '); and in that case the perturba-bation in radius vector would have the sign — for the present relativeposition of the planet and Uranus . But this analogy is worth little untilit is supported by proper symbolical computations.”
The reply of Le Vender to the Astronomer Royal was at once promptand satisfactory. He stated that his theory accounted for the errors ofradius vector as well as those of longitude; and he explained the circum-stances which caused them to disappear. We have already mentionedthat the irregularities of Uranus , if due to a disturbing planet, must haveproceeded partly from the errors in the elements of the planet’s own orbit,and partly from the perturbations occasioned by the undiscovered planet.Now, Le Vender found that the errors of radius vector almost whollyvanished when he applied to the elements the corrections derived fromthe errors of longitude. It was clear, then, that the errors directly de-pendent on perturbation were quite insignificant, compared with thosearising from the distortion of the orbit, and this circumstance afforded asatisfactory explanation of the apparent inconsistency between theory andobservation, alluded to by the Astronomer Royal in his letter to LeVender. The latter, in fact, found that both the eccentricity and thelongitude of the perihelion ought to be increased; and it happened, in con-sequence of the actual position of the planet, that the application of thesecorrections had the effect of increasing the tabular value of the radius vectoralmost to the whole extent required by the observations*.
The results of Mr. Adams, which received so remarkable a confirmationfrom those of M. Le Vender, appeared now to the Astronomer Royal tobe established beyond all doubt by the satisfactory communication he re-ceived from the latter of these mathematicians. At a meeting of theBoard of Visitors of the Observatory of Greenwich , held on the 20thJune, 1846, Mr. Airy mentioned the near agreement of the results ob-tained by Le Verrier and Adams relative to the supposed existence of aplanet exterior to Uranus ; and he suggested in strong terms the desir-ableness of some Observatory devoting its resources to a systematicsearch for the undiscovered body. In pursuance of this view he wrote aletter to Mr. Challis, dated the 9th July, recommending an examinationof the heavens with the Northumberland refractor, and offering to supplyhim with an assistant if he should not have leisure to superintend theoperation personally f. On the 18th of the same month he wrote a secondletter to Mr. Challis on the same subject, inclosing in it a paper headed,“ Suggestions for the Examination of a portion of the Heavens in search
* Airy’s Hist. Statement, No. 14.
f Airy’s Hist. Statement, No. 15. It is perhaps right to mention, for the use of someof our readers, that at Greenwich the object of the observations is to determine with theutmost possible precision the apparent positions of the celestial bodies, rather than toacquire a knowledge of the physical structure of the heavens. Telescopes of great powerare not, therefore, required at the Royal Observatory. In the present instance it wasnecessary to observe all the stars, in the part of the heavens appointed for examination,down to the tenth magnitude, but at Greenwich there were no telescopes sufficiently power-ful for this purpose. The instrument alluded to by Mr. Airy is a magnificent refractor pre-sented to the University of Cambridge by the late Duke of Northumberland.