450 THE POPULAR SCIENCE MONTHLY. 



nine minutes (i. e., nearly half a degree) farther on in the direction in 

 which the earth is traveling. In other words, the meteors do not de- 

 scribe exactly the same orbit over and over again : their path in one 

 revolution is not exactly the same as their path in the next revolution, 

 although very close to it. Thus, their path in a. d. 126 was that 

 which is represented by the strong oval line in the diagram, but, in the 

 seventeen centuries which had since elapsed, it has gradually shifted 

 round into the position represented by the dotted ellipse. This kind 

 of motion is well known to astronomei's, and its cause is well known. 

 It would not happen if the sun were the only body attracting the 

 meteors, but arises because the planets also draw them in other direc- 

 tions ; and although the attraction of the planets is very weak com- 

 pared with the immense power of the sun, still they are able to drag 

 th meteors a little out of their course round the sun, and in this way 

 occasion that shifting round of the orbit of which we are speaking. 

 Now, in the case of meteors which are really traveling in the large 

 orbit, this shifting of the orbit must be due to the attraction of the 

 planets Jupiter, Saturn, Uranus, and the Earth, while, if they had 

 traveled in any of the four smaller orbits, the planets that woidd be 

 near enough and large enough to act sensibly upon them would be the 

 Earth, Venus, and Jupiter. Accordingly, if any one could be found 

 able to calculate how much effect would be produced in each of the 

 five cases, the calculated amount of shifting of the orbit could be 

 compared with the observed amount, which is 29' in thirty-three and a 

 quarter years, and this would at once tell which of the five possible 

 orbits is the true one. 



These papers of Professor Newton's were published in 1864. Be- 

 fore the computations which he had indicated could be attempted, it 

 was necessary that the direction in which the meteors enter the earth's 

 atmosphere should be known much more accurately than it then was, 

 in order to enable astronomers to compute the exact forms and posi- 

 tions of the five possible orbits. This observation, then, was of the 

 greatest importance in 1866, and it was on this account that all the 

 astronomers on that occasion devoted nearly all their efforts to deter- 

 mining with the utmost precision the exact point of the constellation 

 Leo from which the meteors seemed to radiate. This important direc- 

 tion was ascertained during the great meteoric shower on the morn- 

 ing of the 14th of November, 1866, and immediately after Professor 

 Adams and his two assistants in the Cambridge Observatory set to 

 work at their arduous task. This great calculation required the solu- 

 tion of a problem in mechanics which had never before been at- 

 tempted, and involved an immense amount of tedious labor. Amid 

 all these difficulties Professor Adams triumphed ; and after months of 

 toil he was able to announce in the following March that, if the meteors 

 are moving in the large orbit, Jupiter would produce a shifting of the 

 orbit in each revolution amounting to 20', the attraction of Saturn 



