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HISTORY OF PHYSICAL ASTRONOMY.

maticians of great eminence. The researches of Rosenberger were morecomplete than those of any of his contemporaries. He determined theelements of 1082 and 1759 by an able discussion of all the observationsthat were in each case available to him. He also computed the perturba-tions occasioned by Mercury, Venus and Mars , as well as those due to thelarger planets of the system.

It was announced by astronomers that the comet, if visible at all to thenaked eye, would be so about the middle of October, when it attained itsnearest distance to the Barth. The event fully justified this prediction. Onthe evening of the 5 th of August the comet was seen for the first time, atthe Observatory of Rome. It gradually increased in brilliancy until itfinally was visible to the naked eye in the beginning of October.By the 12th of that month it appeared like a star of the secondmagnitude, and was accompanied by a tail seven or eight degreeslong. It soon afterwards plunged into the rays of the sun, andceased to be visible; but it was again discovered by astronomers earlyin the following year, after passing its perihelion. On account of itssouthern declination, its reappearance was not favourable for observationin Europe, but it was seen to great advantage at the Cape of Good Hope by Mr. Maclear, the Astronomer Royal for that station, and also by SirJohn Herschel , who was then engaged in prosecuting his sidereal observa-tions in the southern hemisphere . These astronomers continued toobserve it until the 5th of May, 1836, when it finally ceased to be visible *.

The various calculations which have been undertaken, for the purpose ofdetermining the elements of this famous comet by means of the observa-tions made on the occasion of its last appearance, generally concur inassigning noon of the 17th of November as the instant of the perihelionpassage. This result presents a very satisfactory agreement with thatwhich we have seen that Pontecoulant arrived at by the aid of theory.Having subsequently revised his calculations, this geometer rendered theaccordance still more complete. By employing improved values of themasses of the perturbing planets, he found that the passage of the perihe-lion would take place at noon on the 16th of November t.

It must be ackowledged that Rosenberger’s result did not coincide soclosely with that derived from observation, notwithstanding tlie elaboratecharacter of his researches. In consequence of the errors which in-evitably affect all observations in a greater or less degree, and the uncer-tainty which exists respecting the real values of the masses of the planets,it is not to be expected that an error of a few days may not occur in thecalculated time of the comet’s return. The fidelity with which it respondedto the deductions of the geometer on the occasion of its last appearanceforms one of the many magnificent triumphs which adorn the history of theTheory of Gravitation {.

* Some interesting particulars relative to the physical changes which this comet under-went, during the period of its visibility, are contained in Sir John Herschel ’s Account ofAstronomical Observations at the Cape of Good Hope .

t See the Connaissance des Temps for 1838. In this final revisal of his researchesPontecoulant employed the masses of Saturn, Jupiter, and the Earth , as assigned severallyby Airy, Bessel, and Encke.

J The following statement of the elements of the comet, derived from the observationsof 1759 and 1835, and the elements for 1835 as assigned by the theory of gravitation,will at once indicate the magnitude of the perturbations which the motion of the cometexperiences, and the accuracy with which they have been computed;—