the sun. Are we, then, to suppose that this comet is continually approaching the focus of our world, and some day will be precipitated upon its mass? These interesting questions and this hypothesis have been studied from different points of view by several astronomers, who have endeavoured to find the physical cause of this diminution. We shall return to this point further on; it is one that concerns the whole solar world.

SECTION IV.

BIELA'S OR GAMBART'S COMET.

History of its discovery; its identification with the comet of 1805-Calculation of its elliptic elements by Gambart-Apparitions previous to 1826-Peculiarities in the apparitions of 1832, 1846, and 1872.

SEVEN YEARS elapsed between the discovery of Encke's comet and that of Biela or Gambart, which likewise may be called a comet of short period, since it performs its revolution in less than seven years.

The comet was first observed by an Austrian major of the name of Biela, at Johannisberg, February 27, 1826; it was seen ten days after at Marseilles by the French astronomer Gambart. The latter, after having calculated the elements of the parabolic orbit, immediately recognised their resemblance to those of a comet which had been observed in 1805 and in 1772. The following table affords a comparison between the elements of these three orbits:

I especially wish to direct attention to these comparisons as examples of the employment of the most simple method for determining the periodical orbit of a comet, a method merely

suggestive and provisional, and for which direct calculations are immediately substituted. These calculations for the comet of 1826 were performed by Gambart and Clausen,* who both obtained accordant results, and assigned to the duration of the comet's revolution a period of six years and three quarters. Damoiseau, then, taking account of the perturbations, was able to predict its next return, which he fixed for the 27th of November, 1832. The comet made its appearance on the 26th, only

* It is customary to give to a periodical comet the name, not of the observer by whom it was first seen or observed, but that of the astronomer by whom the elliptic character of its orbit was first recognised. This comet that astronomers of both hemispheres persist in calling the comet of Biela ought, therefore, to bear the name of Gambart. It is not the only instance of injustice in the history of astronomy. Non periodical comets generally receive the name of their first observers thus we speak of Donati's comet, Coggia's comet, &c. But in our opinion the best system of denomination is that of designating comets by the year in which they have effected their perihelion passage, and affixing to them a numeral, according to the order of their discovery. Thus, we say comet I., 1858; comet II., 1858, &c. This method leaves no opening for small rivalries of the kind above alluded to.

[It seems natural, and is, in fact, unavoidable, that a comet should be known by the name of the astronomer with whom it is chiefly associated, whether as calculator or observer, without there being any fixed rule in the matter. Astronomers attach a particular name to a comet, not with the view of honouring the individual, but of having a convenient name for the comet; and although the system of quoting the year and the number is admirable for the majority of comets, still in the case of those that have become celebrated and are frequently referred to, some more distinctive and easily remembered appellation is needed.

But in the present case it seems a matter of justice that the comet should be named after Biela, who not only first discovered it, but who calculated its parabolic elements, remarked their similarity to those of the comets of 1772 and 1805, and thence concluded that the orbit was elliptic, and that the period was six years and nine months. This Biela communicated to the Astronomische Nachrichten, in a letter dated March 23, 1826. Gambart also calculated the parabolic elements of the comet, and remarked their resemblance to those of the comets of 1772 and 1805. His letter was dated March 22, and both appear in the same number of the Astronomische Nachrichten. Thus Biela and Gambart independently recognised the elliptic motion of the comet, while Biela was in in addition the first discoverer.

If we adopt the rule that a comet should be named after the astronomer who first recognised its periodicity, it is clear that Faye's comet-the subject of the next section should be named after Goldschmidt.-ED.]

one day earlier than the date assigned. Thus was perfected more and more the theory of cometary orbits based upon the principle of universal gravitation.

Including the previous apparitions of 1772 and 1805, the comet of six years and three-quarters has been observed on seven of its returns-in 1826, in 1832, in 1846, in 1852, and in 1872. It should have been observed in 1839, 1859, and 1866. 'In 1839,' says M. Delaunay, 'it could not be observed on account of the unfavourable position of its orbit at the time of its perihelion passage.' This passage, in fact, took place during the first days of July, and both before and after the comet was situated in close proximity to the sun, and consequently lost sight of in his rays. Nearly the same thing happened in 1859, the perihelion passage taking place in the first days of June. Lastly, in 1866, although the comet could not have been far distant from the earth about the time of its perihelion (the 26th of January), and notwithstanding the diligent search made for it with powerful instruments, it was not discovered. It was last seen at Madras by Mr. Pogson, on the 2nd and 3rd of December, 1872.

Gambart's comet has furnished some curious events in the history of physical astronomy. In the beginning of 1846 the comet divided into two distinct comets, which appear at the present day in the catalogues, with their respective orbits. Moreover, in 1832, like the comet of 1773, it had the privilege of exciting fears which at that epoch were certainly without foundation. The comet was to come into collision with the earth. There was more reason to believe in the possibility of such an encounter at the end of November 1872; and if it is not one of the twin comets that then just grazed the earth, it is at least one of their fragments. I here restrict myself to the simple mention of these events, which further on will receive the development they merit.

SECTION V.

FAYE'S COMET.

First comet whose periodicity, without comparison with previous dates, has been determined by calculation and verified by observation-M. Le Verrier demonstrates that it has nothing in common with the comet of Lexell-Slight eccentricity of Faye's comet and great perihelion distance-Dates of its return-Perturbations in the movements of Faye's comet inexplicable by gravitation alone: a problem to be solved.

A COMMUNICATION by Arago, published in 1844, in the Comptes Rendus of the Academy of Sciences, gives an account of the first researches relative to the fourth periodical comet, which we here subjoin :

This body was discovered at the Observatory of Paris by M. Faye, on November 22, 1843. This young astronomer But as the hastened to calculate its parabolic elements. number of observations increased M. Faye perceived that a parabola was quite inadequate to represent the series of positions occupied by the comet, and announced that he should determine its elliptic orbit, as soon as the state of the sky should have permitted him to pursue his observations of the new comet in regions so far removed from those in which it had first appeared that no doubt could possibly exist as to the certainty of his results. M. Faye therefore applied himself to the multiplying of observations, which had become extremely difficult to obtain, on account of the indistinctness of the comet. Matters were in this stage when a letter from