332 ON THE THEOEY OF ENCKE'S COMET, 



previous statements, we find that the nature of the orbit described 

 by a body, is dependent upon the proportion which the projectile 

 force bears to the central force. With the initial velocity of 

 projection we have, I think, no concern for it is far beyond our 

 comprehension but we may fairly enquire whether any subse- 

 quent cause, can be assigned for the diversity of orbits which 

 manifest themselves in the case of comets. Arguing from the 

 elliptical, or nearly circular form of the orbits of all the permanent 

 bodies of our system, we should be disposed to expect that other 

 bodies following the same general laws, would run their races in 

 similar courses : undoubtedly this is merely an argument from 

 analogy, as there is no " a priori " argument against a body 

 having been originally projected under such circumstances that 

 it should describe an hyperbola or a parabola, instead of an 

 ellipse : but it leads me to a question which I shall have occasion 

 to dwell upon namely, " what are the causes which may deflect 

 a moving body from its original path ? " 



Starting from our own immediate system, we know that the 

 combined action of the sun and planets produces perturbations in 

 the orbits of the latter, though their mean paths remain un- 

 disturbed. We see a most interesting illustration of this in the 

 discovery of the Planet Neptune, which was due to the Mathema- 

 tical Investigations of Adams and Leverrier, who had previously 

 demonstrated that certain irregularities in the orbit of Uranus 

 could only be accounted for by perturbations caused by another 

 exterior planet : this planet was looked for and discovered in the 

 position indicated. 



Now here we are considering bodies whose masses have to 

 each other some considerable proportion, and although it is a 

 consequence of the Law of Universal Gravitation that each of 

 two bodies should attract each other in direct proportion to 

 their masses ; yet whilst a larger produces perceptible perturba- 

 tions in the orbit of a smaller body ; these perturbations are not 

 significant and there is no permanent derangement in the case 

 of the solar system. But far different is it in the case of a comet. 

 Here we have a body of such extreme trenuity that its mass is 

 not comparable with even the smallest of the members of our 

 own system. How easy then is it to conceive that, whatever 



