HISTORY OF PHYSICAL ASTRONOMY.
172
positions did not differ widely from the truth*. Without some assumedvalue of the mean distance, the operation of eliminating the unknownquantities would have been totally impracticable; but, when this objectwas once accomplished, a more accurate value might be afterwards arrivedat, either by varying the hypothesis and comparing the results with thosederived in the .first instance, or by introducing into the second solutiona correction to the assumed mean distance in the shape of a new un-known quantity. The results which Mr. Adams obtained by means ofhis first essay were sufficiently satisfactory to encourage him to pursuehis researches. The observations which he employed were most defectivethroughout the period included between the years 1818 and 1820, and itwas precisely in that period also that the errors of his theory were greatest.This circumstance induced him to apply to the Astronomer Royal, throughhis friend Professor Challis, for the Greenwich Observations of the planetthen in process of reduction. Professor Challis’ letter to the AstronomerRoyal on this occasion is dated the 13th February, 184-lj. He mentionsin it that Mr. Adams, a young friend of his, was engaged in researcheson the theory of Uranus . He then specifies the nature of his request,expressing also a desire to know whether, in comparing the observations ofthe planet with the theory, any alteration had been made in the tables ofBouvard, except that depending on the correction to the value of Jupiter ’smass. The Astronomer Royal a day or two afterwards returned a suitablereply to Professor Challis’ letter, and kindly transmitted, for the use ofMr. Adams, all the Greenwich Observations of the planet from 1750 to1830 completely reduced |.
Furnished with these valuable data, Mr. Adams was now in a con-dition to pursue his researches with the prospect of ultimate success.He accordingly undertook a more comprehensive investigation of the pro-blem by considering the orbit of the disturbing planet to be elliptic. Byrepeating the approximation several times, introducing on each occasionmore and more terms of the perturbation, he succeeded in gradually reduc-ing the errors of Uranus , until its computed places finally exhibited a verygeneral and satisfactory accordance with those derived from observation.In the autumn of 1846, he considered his researches were so far maturedas to warrant the presumption that his results might be employed with astrong probability of success in searching for the new planet. Accordinglyin the month of September, before leaving Cambridge, he placed in thehands of Professor Challis a paper containing numerical values of the
* The following curious law regulating the mean dislanees of the planets from the sunwas first announced by Bode, the German astronomer. It is said, however, that he wasindebted for the original notion of it to Titian , a professor of Wurtemberg.
Mercury ...... = 4 =4
Venus.= 4 -|- 3.2° = 7
Mars.= 4 + 3.2 2 = 16
Ceres (as the mean of the smaller planets) = 4 + 3.2 3 = 28
Jupiter.= 4 + 3.2 4 = 52
Saturn.= 4 + 3.2 s = 100
Uranus.. = 4 + 8.2" = 196
If we compare these numbers with the actual distances (supposing the earth’s distanceto be in both cases represented by 10), we shall find a very remarkable agreement. It iseasy to perceive that as the planets recede from the sun, their successive distances becomemore nearly double each other.
f Airy’s Historical Statement of circumstances connected with the discovery of the planetbeyond Uranus , No. 6.
| Airy’s Historical Statement, No. 7.