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HISTORY OP PHYSICAL ASTRONOMY.

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j sible to assign the minimum time during which the comet has been re-| volving within the limits of the solar system. Le Verrier found that it; must have been introduced into the system at least as early as the year; 1747; whence it followed that it had made at least thirteen revolutionsj in an elliptic orbit previous to its discovery.

j The interest excited by the periodic comet of 1843 had only abated in a[ small degree when the solar system was enriched by another discoveryS of the same class. On the 29th of August, 1844, De Yico, the director; of the Observatory at Rome , discovered a comet, which was soon after-| wards found by M. Faye to revolve in an ellipse with a period of about five! years and a half. MM. Laugier and Mauvais, of the Royal Observatory of' Paris , instituted a comparison between the elements of this comet and thoseof one observed in 1585 by Tycho Brahe and Rothmann. Halley had cal-- culated the elements of the last-mentioned comet on the supposition thati it revolved in a parabola, but the French astronomers shewed that the1 motion might be satisfied better by an elliptic orbit with a period of fivei years and two months. From the close resemblance which the elements' of this comet bore to those of the comet of 1844, MM. Laugier and■ Mauvais concluded that the two bodies were identical, the actual dis-cordances of the elements being ascribed by them to the effects of planetaryperturbation. The grounds upon which this conclusion rested were indeedvery plausible, for the eccentricities of both orbits were almost the same,the places of the perihelia did not differ more than 30°, and the distancebetween the nodes was only 22°. It is manifest, however, that theidentity could only be established beyond all doubt by a comparison ofthe elements of both comets when rigorously determined for somecommon epoch. Le Verrier investigated the subject by tracing backthe comet of 1844 through the anterior revolutions up to 1585, andthen comparing the elements for that epoch with those of the recordedcomet. The results at which he arrived shew how unsafe it is in researchesof this nature to adopt any conclusion which is not verified by rigorouscalculation. As we have mentioned already, the distance between theperihelia of the comets observed in 1844 and 1585 was only 30°, and itwas reasonably enough supposed that this displacement might have beenproduced by the action of the planets. Le Verrier , however, found that,when the orbit of the comet of 1844 was traced back, the perihelion,instead of approaching towards that of the comet of 1585, on the contraryreceded further and further from it, insomuch that, at the epoch of 1585,the perihelia of the two comets were diametrically opposite to each other.The nodes also of the comet of 1844 continually receded from those of thecomet of 1585, so that, in order to coincide with the latter at the epoch ofcomparison, they would require to move through 338°, and not 22° duringthe period comprised between 1844 and 1585. The elements of bothcomets, when thus determined for the common epoch of 1585, offered dis-cordances of such magnitude as appeared to Le Verrier to be incompatiblewith identity, and he therefore came to the conclusion that the two cometswere totally distinct bodies. He also ascertained in the course of thesame researches that the comet of 1844 was not identical with Lexell ’scomet. He discovered, however, such a strong resemblance betweenthe elements of the new comet and those of a comet observed in1678 by La Hire, that he considered himself fully justified in concludingthat they were identical. Thus, although the comet of 1844 has doubtlessformed a part of the solar system for many ages, and has frequently ap-proached very near the earth, history records only one instance of its