260 Comets. 



the sphere of the sun's predominant attraction to the action 

 of one or other of the planets near which they may have 

 passed, in such a manner as to diminish their velocity, and 

 render it compatible with elliptic motion.* A similar cause 

 acting the other way, might with equal probability, give rise 

 to a hyperbolic motion. But whereas in the former case, 

 the comet would remain in the system, and might make an 

 indefinite number of revolutions, in the latter it would return 

 no more. This may possibly be the cause of the exceedingly 

 rare occurrence of a hyperbolic comet as compared with el- 

 liptic ones. 



All the planets without exception, and almost all the sa- 

 tellites, move in one direction round the sun. Retrograde 

 comets, however, are of very common occurrence, which cer- 

 tainly would go to assign them an exterior or at least an in- 

 dependent origin. Laplace, from a consideration of all the 

 cometary orbits known in the earlier part of the present 

 century, concluded, that the mean or average situation of the 

 planes of all the cometary orbits, with respect to the ecliptic, 

 was so nearly that of perpendicularity, as to afford no presump- 

 tion of any cause biassing their directions in this respect. 

 Yet we think it worth noticing that among the comets which 

 are as yet known to describe elliptic orbits, not one whose 

 inclination is under 17" is retrograde ; and that out of thii'ty- 

 six comets which have had elliptic elements assigned to 

 them, whether of great or small excentricities, and without 

 any limit of inclination, only five are retrograde, and of these 

 only two, viz., Halley's and the Great Comet of 1843, can be 

 regarded as satisfactorily made out. Finally, of the 125 

 comets whose elements are given in the collection of Schuma- 

 cher and Olbers, up to 1823, thenumberof retrograde comets 

 under 10'^ of inclination is only two out of nine, and under 

 20°, seven out of twenty-three. A plane of motion therefore, 

 nearly coincident with the ecliptic, and a periodical return, 

 are circumstances eminently favourable to direct revolution 

 in the cometary as they are decisive among the planetary 

 orbits. 



* The velocity in an ellipse is always less than in a parabola, at equal dis- 

 tances from the sun ; in a hyperbola always greater. 



