COMETS. 



inferred that the comet coming from some indefinitely distant 

 region of the universe, has entered the system in a certain direction r 

 h' A, which can be inferred from the visible arc g a g, and that it 

 must depart to another indefinitely distant region of the universe 

 following the direction h A', which is also ascertained from the 

 visible arc gag. 



If, on the other hand, it be ascertained that the visible arc, 

 such as g' a g', be part of a parabola, then, in like manner by the 

 properties of that curve, it will follow that it entered the system 

 coming from an indefinitely distant region of the universe in a 

 certain direction, p' p, which can be inferred from the visible 

 arcg' ag', and that after it ceases to be visible, it will issue from 

 the system in another determinate direction, p p', parallel to that 

 by which it entered. 



The comet, in neither of these cases, would have a periodic 

 character. It would be analogous to one of those occasional 

 meteors which are seen to shoot across the firmament never again 

 to reappear. The body arriving from some distant region, and 

 coming, as would appear, fortuitously within the solar attraction, 

 is drawn from its course into the hyperbolic or parabolic path, 

 which it is seen to pursue, and escapes from the solar attraction, 

 issuing from the system never to return. The phenomenon would 

 in each case be occasional, and, in a certain sense,' accidental, 

 and the body could not be said properly to belong to the system. 

 So far as relates to the comet itself, the phenomenon would consist 

 in a change of the direction of its course through the universe, 

 operated by the temporary action of solar gravitation upon it. 



13. But the case is very different, the tie between the comet 

 and the system much more intimate, and the interest and physical 

 importance of the body transcendently greater when the arc, such 

 as g" a g", proves to be part of an ellipse. In that case, the invisible 

 part of the orbit being inferred from the visible, the major axis 

 a a' would be known. The comet would possess the periodic 

 character, making successive revolutions like the planets, and 

 returning to perihelion a after the lapse of its proper periodic 

 time, which could be inferred by the harmonic law from the 

 magnitude of its major axis. 



Such a body would then not be, like those which follow hyper- 

 bolic or parabolic paths, an occasional visit to the system, con- 

 nected with it by no permanent relation, and subject to solar 

 gravitation only accidentally and temporarily. It would, on the 

 contrary, be as permanent, if not as strictly regular, a member of 

 the system as any of the planets, though invested, as will pre- 

 sently appear, with an extremely different physical character. 



It will therefore be easily conceived with what profound interest 

 154 



