They are, moreover, subject to many adventures and vicissitudes. Their aphelia--or the farthest points of their orbits from the sun--are usually, if not invariably, situated so near to the path either of Jupiter or of Saturn, as to permit these giant planets to act as secondary rulers of their destinies. By their influence they were, in all likelihood, originally fixed in their present tracks; and by their influence, exerted in an opposite sense, they may, in some cases, be eventually ejected from them. Careers so varied, as can easily be imagined, are apt to prove instructive, and astronomers have not been backward in extracting from them the lessons they are fitted to convey. Encke's comet, above all, has served as an index to much curious information, and it may be hoped that its function in that respect is by no means at an end. The great extent of the solar system traversed by its eccentric path makes it peculiarly useful for the determination of the planetary masses. At perihelion it penetrates within the orbit of Mercury; it considerably transcends at aphelion the farthest excursion of Pallas. Its vicinity to the former planet in August, 1835, offered the first convenient opportunity of placing that body in the astronomical balance. Its weight or mass had previously been assumed, not ascertained; and the comparatively slight deviation from its regular course impressed upon the comet by its attractive power showed that it had been assumed nearly twice too great.[244] That fundamental datum of planetary astronomy--the mass of Jupiter--was corrected by similar means; and it was reassuring to find the correction in satisfactory accord with that already introduced from observations of the asteroidal movements.