﻿52 On the Probable Character ofCometary Orbits. 



at the period when it first became attached to the solar system, 

 the nebulosity about the sun was probably much greater than 

 it now is. 



Lastly, we have already noticed that, out of 174 parabolic 

 comets, the number which have a retrograde motion is consider- 

 ably greater than the number which have a direct motion, and 

 also that the number which have an inclination to the ecliptic 

 greater than 45° is considerably greater than the number which 

 have an inclination less than 45°. 



If these facts are not due to mere chance (and that they 

 are due to chance is improbable), they are themselves an evi- 

 dence that the parabolic comets are not, for the most part, 

 visitors to the solar system for the first time*. But if these 

 comets are periodic, and if they have become so by some cause 

 which has acted equally on comets approaching the sun with 

 direct and retrograde motions and in any directions, then we 

 should expect that the comets with a retrograde motion would 

 be more numerous than those which have a direct motion, and 

 also that those whose planes are inclined to the ecliptic at an 

 angle greater than 45° would be more numerous than those 

 whose planes have an inclination less than 45°. This we should 

 expect, because a greater number of those with a direct motion 

 than of those with a retrograde motion and a greater number of 

 those with an inclination less than 45° than of those with an 

 inclination exceeding 45° would be subsequently changed by the 

 attractions of the planets into comets of comparatively short 

 periods; and therefore, of those left unchanged, a majority 

 would have retrograde motions and large inclinations to the 

 ecliptic. It is a confirmation of this theory to find that, if we 

 take into account all known comets, 48*9 f per cent, and 51*1 

 per cent, respectively, or numbers very nearly equal, have direct 

 and retrograde motions. 



The sum of what I have endeavoured to prove is : — 



I. That the great majority of comets whose orbits are undi- 

 stinguishable from parabolas are moving in elliptic orbits, and 

 are permanent members of the solar system. 



II. That these comets have not, like the comets of short pe- 

 riod, become permanently attached to the solar system through 

 changes which have been produced in their orbits by the per- 

 turbing attractions of the planets ; and that the loss of velocity 

 which they have experienced in passing through the resisting 



* The probability that out of 174 comets not more than 73 will have 

 direct motion is about ¥ V« 



t For this and other statements see Chambers's 'Astronomy/ pp. 346- 

 349). 



