N ov. 19, 1869.] 215 [Kirkwooa. 



ON COMETS AND METEORS. 



By Daniel Kirkwood, LL. D., Professok in Indiana University. 



Bead before ilie American PMlosopMcal Society, Nov. 19, 1869. 



The comets which passed their perihelia in August, 1862, and January, 

 1866, will ever be memorable in the annals of science, as having led to the 

 discovery of the intimate relationship between comets and meteors. 

 These various bodies found revolving about the sun in very eccentric 

 orbits may all be regarded as similar in their nature and origin, differing 

 mainly in the accidents of magnitude and density. The recent researches, 

 moreover, of Hoek, Leverrier and Schiaparelli, have led to the conclusion 

 that such objects exist in great numbers in the interstellar spaces ; that 

 in consequence of the sun's progressive motion they are sometimes 

 drawn towards the centre of our system ; and that if undistvirbed by any 

 of the large planets they again pass off in parabolas or hyperbolas. 

 When, however, as must sometimes be the case, they approach near 

 Jupiter, Saturn, Uranus or Neptune, their orbits may be transformed 

 into ellipses. Such, doubtless, has been the origin of the periodicity of 

 the August and November meteors, as well as of numerous comets. 



In the present paper it is proposed to consider the probable con- 

 sequences of the sun's motion through regions of space in which 

 cosmical matter is widely diffused ; to compare these theoretical de- 

 ductions with the observed phenomena of comets, aerolites and falling 

 stars ; and thus, if possible, explain a variety of facts in regard to those 

 bodies, which have hitherto received no satisfactory explanation. 



1. As comets now moving in elliptic orbits owe their periodicity to the 

 disturbing action of the major planets, and as this planetary influence is 

 sometimes sufficient, especially in the case of Jupiter and Saturn, to 

 change the direction of cometary motion, the great majority of periodic 

 comets should move in the same direction with the planets. Now, of the 

 comets known to be elliptical , 70 per cent, have direct motion. In this 

 respect, therefore, theory and observation are in striking harmony. 



2. AVhen the relative positions of a comet and the disturbing planet 

 are such as to give the transformed orbit of the former a small perihelion 

 distance, the comet must return to the point at which it received its 

 greatest perturbation ; in other words, to the orbit of the planet. The 

 aphelia of the comets of short period ought therefore to be found, for the 

 most part, in the vicinity of the ovMts of the major planets^. The actual 

 distances of these aphelia are as follows : 



A. P. S VOL. XI — 2b 



