44 Mr. G. J. Stoney [Feb. 14, 



the year a.d. 902. He found by compar'ng the dates of the old ob- 

 servations with the modern ones, that the phenomenon is one which 

 recurs three times in a century, or more exactly, that the middle of the 

 swarm crosses the earth's path at intervals of 33j years. He further 

 showed that meteors which thus visit the earth three times in a cen- 

 tury must be moving in one or other of five orbits which he described; 

 and that therefore if means could be found for deciding between these 

 five orbits, the problem would be solved. The five possible orbits are 

 — the great oval orbit which we now know the meteors actually do 

 traverse every 33 years and a quarter ; a nearly circular orbit, very 

 little larger than the earth's orbit, which they would move round in 

 a few days more than a year ; another similar orbit in which their 

 periodic time would be a few days short of a year ; and two other small 

 oval orbits lying within the earth's orbit. But we owe even more to 

 Professor Newton. He also pointed out how it was possible to ascer- 

 tain which of these orbits is the true one, although the test he indi- 

 cated was one so diflncult of application that there was at the time 

 little hope that any astronomer would attempt it. Fortunately our 

 own Professor Adams, of Cambridge, was found able to grapple with 

 the difficulties of the problem, and willing to encounter its immense 

 labour, and to him we owe the completion of this great discovery. 



PROFESSOR ADAMS. 



A comparison of the dates of the successive showers which have 

 been recorded shows that the point where the path of the meteors 

 crosses the earth's orbit is not fixed, but that every time the meteors 

 come round they strike the earth's orbit at a point which is twenty- 

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

 in which the earth is travelling. In other words, the meteors do not 

 describe 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 seven- 

 teen 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 astronomers, 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 

 the 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 travelling 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 

 travelled in any of the four smaller orbits, the planets that would be 

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



