COMETS. 



be expected to he much more considerable than when one planet 

 acts upon another, as well because of the extreme comparative 

 lightness of the comet, as of the great eccentricity of its orbit, 

 which sometimes actually or nearly intersects the paths of several 

 planets, and especially those of the larger ones, yet still such 

 planetary attractions jire only disturbances, and cannot be sup- 

 posed to efface that character which the orbit receives from the 

 predominant force of the immense mass of the sun. . While there- 

 fore we may be prepared for the possibility, and even the pro- 

 bability, that the same periodic comet on the occasion of its 

 successive re-appearances, may follow a path g" a g" in passing 

 to and from its perihelion, differing to some extent from that 

 which it had followed on previous appearances, yet in the main 

 such differences cannot, except in rare and exceptional cases, be 

 very considerable, and for the same reason the intervals between 

 its successive periods, though they may differ, cannot be subject 

 to any very great variation. 



16. If then, on examining the various comets whose appearances 

 have been recorded, and whose places while visible have been 

 observed, and on computing from the apparent places the arc of 

 the orbit through which they moved, it be found that two or more 

 of them, while invisible, moved in the same path, the presumption 

 will be that these were the same body re-appearing after having 

 completed its motion in an elliptic orbit ; nor should this pre- 

 sumption of identity be hastily rejected because of the existence 

 of any discrepancies between the observed paths, or any inequality 

 of the intervals between its successive re-appearances, so long 

 as such discrepancies can fairly be ascribed to the possible 

 disturbances produced by planets which the comet might have 

 encountered in its path. 



17. Many comets, however, have been recorded , but not observed. 

 Historians have mentioned, and even described, their appearances, 

 and in some cases have indicated the chief constellations through 

 which such bodies passed, although no observations of their appa- 

 rent places have been transmitted by which any close approximation 

 to their actual paths could be made. Nevertheless, even in these 

 cases, some clue to their identification is supplied. The intervals 

 between their appearances alone is a highly probable test of 

 identity. Thus if comets were regularly recorded to have 

 appeared at intervals of fifty years (no circumstance affording 

 evidence of the diversity of these objects), they might be assumed, 

 with a high degree of probability, to be the successive returns of 

 an elliptic comet having that interval as its period. 



18. The appearances of about 400 comets had been recorded in 

 the annals of various countries before the end of the seventeenth 



156 



