HUBERT ANSON NEWTON. 27*7 



directions of the normal to the plane of the orbit equally probable. 

 On the first hypothesis, therefore, we should expect a uniform 

 distribution in inclination ; on the second, a frequency proportioned 

 to the sine of the inclination. It was shown by a diagram in which 

 the actual and the two theoretical distributions are represented 

 graphically, that the actual distribution agrees pretty well with 

 the theory of external origin and not at all with that of internal 

 origin. It was also shown that the curve of actual distribution 

 cannot be made to agree with Kant's hypothesis by any simple 

 and reasonable allowances for perturbations. On the other hand, 

 if we assume the external origin of comets, and ask how the curve 

 of sines must be modified in order to take account of perturbations, 

 it is shown that the principal effect will be to increase somewhat 

 the number of inclinations between 90 and 135 at the expense of 

 those between 45 and 90. It is apparent at once from the diagram 

 that such a change would make a very good agreement between the 

 actual and theoretical curves, the only important difference remaining 

 being due to comets of short periods, which mostly have small 

 inclinations with direct motion. These should not weigh very much, 

 the author observes, in the general question of the distribution of 

 inclinations, because they return so frequently and are so easily 

 detected that their number in a list of observed comets is out of all 

 proportion to their number among existing comets. But this group 

 of comets of short periods can easily be explained on the theory of an 

 external origin. For such comets must have lost a large part of 

 their velocity by the influence of a planet. This is only likely to 

 happen when a comet overtakes the planet and passes in front of it. 

 This implies that its original motion was direct and in an orbit of 

 small inclination to that of the planets, and although it may lose a 

 large part of its velocity, its motion will generally remain direct and 

 in a plane of small inclination. This very interesting case of the 

 comets of short periods and small inclinations, which was treated 

 rather briefly in this paper, was discussed more fully by Professor 

 Newton at the meeting of the British Association in the following 

 year.* 



Many years later, Professor Newton returned to the same general 

 subject in a very interesting memoir " On the Capture of Comets by 

 Planets ; especially their Capture by Jupiter," which was read before 

 the National Academy in 1891, and appeared in the Memoirs of the 

 Academy two years later.! It also appeared in the American 

 Journal of Science in the year in which it was read. J This contains 



* Eep't Brit. Assoc. Adv. Sci. for 1879, p. 272. 



t Mem. Nat. A cad., vol. vi, 1st memoir. 



J Amer. Jour. Sci., ser. 3, vol. xlii, pp. 183 and 482. 



