COMETS. 



the heavens on various occasions of the appearances of the most 

 conspicuous of them had nevertheless been from time to time for 

 some centuries observed and recorded with such a degree of pre- 

 cision as the existing state of astronomical science permitted ; and 

 even when their places were not astronomically ascertained, the 

 date of their appearanc^ was generally preserved in the historic 

 records, and in many cases the constellations through which they 

 passed were indicated, so that the means of obtaining at least a 

 rude approximation to their position in the firmament were thus 

 supplied. 



10. Such observations, vague, scattered, and inexact as they 

 were, supplied, however, data by which, in several cases, it was 

 possible to compute the real motion of these bodies through space, 

 their positions in relation to the sun, the earth, and the planets, 

 and the paths they followed in moving through the system, with 

 sufficiently approximate accuracy to conclude with certainty that 

 they were one or other of the conic sections, the place of the sun 

 being the focus. 



This was sufficient to bring these bodies under the general 

 operation of the attraction of gravitation. 



It still remained, however, to determine more exactly the 

 specific character of these orbits. Are they ellipses more or less 

 eccentric ? or parabolas ? or hyperbolas ? Any of the three classes 

 of orbits would, as has been shown, be equally compatible with 

 the law of gravitation. 



11. It might be supposed that the same course of observation 

 as that by which the orbit of a planet is traced would be applicable 

 equally to comets. Many circumstances, however, attend this 

 latter class of bodies, which render such observations impossible, 

 and compel the astronomer to resort to other means to determine 

 their orbits. 



A spectator stationed upon the earth keeps within his view each 

 of the other planets of the system throughout nearly the whole of 

 its course. Indeed, there is no part of the orbit of any planet in 

 which, at some time or other, it may not be seen from the earth. 

 Every point of the path of each planet can therefore be observed ; 

 and, although without waiting for such observation, its course 

 might be determined, yet it is material here to attend to the fact, 

 that the whole orbit may be submitted to direct observation. The 

 different planets, also, present peculiar features by which each 

 may be distinguished. Thus, as has been explained, they are 

 observed to be spherical bodies of various magnitudes. Their 

 surfaces are marked by peculiar modes of light and shade, which, 

 although variable and shifting, still, in each case, possess some 

 prevailing and permanent characters by which the identity of the 

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