IV 



actual orbit of the meteors. Professor Newton pointed out a line of 

 investigation by which it was possible that this discrimination might 

 be made. Bodies revolving round the sun in these several orbits 

 would be differently acted upon by the surrounding planets. The 

 perturbations in these five orbits would accordingly be different, and 

 would probably lead to a different rate of shift of the node of the 

 orbit along the plane of the ecliptic. If then, the perturbations in all 

 the five orbits can be so fully investigated that the rate of the shift 

 of the node in each can be computed, it will then be seen which of 

 the five computed amounts accords with that which Professor Newton 

 deduced from the observations, viz., an advance of 29' in 33| years, or 

 50*2" annually. 



This was an invaluable suggestion, and the key to the complete solu- 

 tion of the problem, although there was at the time little hope that any 

 mathematician could be found competent to grapple with the difficulties 

 of the problem, which involved the investigation of a kind of per- 

 turbation which had never been attempted, viz., the perturbation by 

 a planet of a body revolving in the reverse direction in an orbit round 

 the sun, which is nearly coincident with the orbit of the planet. 

 Fortunately our own Professor J. Couch Adams was able to cope 

 with all the difficulties of the problem, and after five months' labour 

 found himself in a position to make known which of the five orbits 

 is the real orbit of the meteors. This marvellous achievement, how- 

 ever, would not have been possible without the discoveries that had 

 been made by Professor Newton : 1st, of the amount of the average 

 shift of the node ; 2nd, of the fact that the choice lay between five 

 orbits which he defined ; and 3rd, that a discrimination between 

 these was theoretically possible by the method afterwards success- 

 fully employed by Adams. Thus one of the most marvellous dis- 

 coveries of the century in dynamical astronomy is due to the 

 associated efforts of these two great men. Professor Newton in- 

 dicated the problem and pointed out how it was to be attacked, and 

 Professor Adams successfully grappled with its immense difficulty. 



One other example of Professor Newton's contribution to our 

 knowledge of astronomy must here suffice. It may be treated briefly. 

 Most comets come into that portion of space which is occupied by 

 the solar system from great distances outside. Such comets move 

 either in parabolas, or in ellipses or hyperbolas which approximate to 

 parabolas. By an examination of the orbits of 247 comets, Professor 

 Newton establishes the fact that the planes of the orbits of these 

 non-periodic comets lie in all positions indifferently, and that such 

 comets exhibit no preponderance of direct over retrograde motion ; 

 whereas all the known periodic comets, which are about fifteen 

 in number, move in planes which are but moderately inclined to the 

 planes in which the principal planets move, and show such a marked 





