

tern toward some quarter of the universe wholly different from that from which 

 they came. 



We have stated that none of the planets depart much above or below the 

 plane of the earth's orbit ; it is quite otherwise with comets, which follow no 

 certain law in this respect. Some of them sweep the system nearly in the 

 plane in which the planets move ; others rush through it in curves, oblique in 

 various degrees to this plane ; while some move in planes perpendicular to it. 

 The planets also move round the sun all in one direction. Comets, on ths other 

 hand, rebel against this law, and move, some in one direction and some in 

 another. 



So far then as observation informs us, we are left to decide between two 

 suppositions : 1. That the comet has entered the system for the first time ; and 

 that having swept behind the sun, it has emerged in a different direction, never 

 to return : 2. That it moves in a large orbit, of which the sun is not the cen- 

 tre, but. on the contrary, is placed very near the path of the body itself; that 

 the comet is visible only in that part of its orbit which is in the immediate 

 neighborhood of the sun, but is invisible throughout that large part, which per- 

 haps extends, through depths of space, far beyond our most remote planet. If 

 the latter supposition be adopted, it would follow that the same comet, after 

 emerging from our system, would, after the lapse of a certain time, return to it, 

 arid pursue the same path, or nearly the same path, round the sun as before. 

 If then we find, after the lapse of a certain time, a comet following the same 

 path w r hile visible, as a former comet was observed to follow, we infer that 

 they also followed the same path during that much longer period in which they 

 were beyond the sphere of our observation, and consequently we infer, with a 

 high degree of probability, that the comets which thus follow precisely the 

 same track, must be the same comet. We say with probability, because there 

 is a possibility, although it be a bare possibility, that two different comets 

 should move precisely in the same orbit. 



Now, let us suppose that, during the appearance of a comet, its path from 

 day to day, or perhaps from hour to hour, is so carefully observed, that we 

 could delineate it with a corresponding degree of accuracy in any plan or 

 model of the system. This path would, as we have seen, form a very small 

 fragment of its entire orbit ; but if the nature of that orbit were known, the 

 principles of geometry would also enable us to complete the curve. Thus, if 

 a small arc of a large circle be traced upon paper, every one conversant with 

 geometry will be able to complete the circle, even though he be not told with 

 what centre the small arc was described, or with what length of radius. It 

 is the same with other curves. Newton has proved that every mass of matter 

 which is moved through the system, under the attracting influence of the sun. 

 must, by its motion, trace one or other of those curves called conic sections: 

 and that the curve must be so placed, that the centre of the sun shall be in that 

 point which is called its focus. Now, conic sections are of three kinds ; the 

 ellipse, which is a curve of oval form, such that a point moving on it would re- 

 trace the same course every revolution. But the other two species (called the 

 parabola and hyperbola), consist of two branches diverging from their point of 

 connexion in two different directions, and proceeding in those directions with- 

 out ever again reuniting. If a body (as it might do by the established law of 

 ! gravitation) entered our system by one branch of such a curve, it would, after 

 | sweeping behind the sun, emerge by the other branch never to return. Thus it 

 i appears, that either of the two suppositions which we have just made, would be 

 | compatible with the law of gravitation ; and it is possible that some comets misrht 

 i mov'e in ellipses, returning continually over the same path at stated intervals, 

 [ while others, moving in parabolas, or hyperbolas, entering our system for the first 



J 



