(16) That of Pons, with a period of 71.56 years.
(17) That of Others, with a period of 72.65 years. (18) That of Halley, with a period of 76.08 years. The orbits of sixty comets appear to be ellipses. These are probably periodical. The paths of about two hundred cannot be dis tinguished from parabolas. These are possibly periodical, as the ellipse, when very eccentric, cannot near perihelion be distinguished front a parabola. Only about a dozen have orbits pos sibly hyperbolic; and, as we have said. in only two cases is the hyperbolic character of the path established upon fairly reliable evidence.
In the month of June, 1770, Messier discovered a comet which remained visible a long time, and enabled Lexell to ascertain the orbit to be an ellipse whose major axis was only three times the diameter of the earth's orbit, and corre sponded to a periodic revolution of five and one half years. This result suggested grave difficul ties. It had been found impossible to identify this comet with any previously observed, and yet it was difficult to conceive that a bright comet with so short a period of return should have previously escaped observation. What was still more remarkable, it was never seen again, though anxiously looked for in the places where Lexell's orbit would have brought it. It became popularly called `Lexell's lost comet,' and gave occasion to many sarcasms by the wits of the clay at the expense of astronomers, who had boasted of having found the key to the cometary movements. In the present day the explanation is complete. The comet was never seen before /770, because its orbit previously had been totally different, its nearest point to the sun having been as distant as the path of .Jupiter. Its appearance that year arose out of the fact that in 1767 it was in such close contact with Jupiter, moving in the same direction and nearly in the same plane, that the attraction of that planet entirely changed its orbit. But why has the comet not since been seen? Its passage to the point of perihelion in 1776 took place by day; and in 1779, before another return, it again encountered the vast body of Jupiter, and suffered a fresh orbital derangement—the attraction of the planet deflecting it into more distant regions. and so changing the form of the orbit that if it had again been visible, it would probably not have been recognized as identical with Lexell's comet.
The celebrated comet of 1680, which furnished Newton with the occasion for proving that comets revolve around the sun in conic sections. and that, consequently, they are retained in their orbits by the sante force as that which regulates the movements of the planets, appears to have been about, the most remarkable for brilliancy of any of which we have authentic accounts. ibis comet is supposed to be identical with the one that appeared about the time of Caesar's death (u.c. 44), with that which was seen in
the reign of Justinian in the year 531. and with another in the year 1106, in the reign of Henry II.; the period of revolution, according to the orbit calculated for it. by Whiston. being about 575 years. There is, however, some doubt among astronomers as to the real form of its orbit, the one assigned to it by Eneke giving it a period of 8.813 years. The tail of a comet is nearly always away from the sun, frequently assuming a curved form. It increases in length with its proximity to the sun, but does not acquire its greatest length till after passing the perihelion. These are usual characteristics of comets, which were exemplified by this one in a remarkable de gree. The phenomena might be accounted for. if we were to regard the train as vaporization produced by the intense heat to which the body of the comet is exposed in its approach to the sun. In the nineteenth century the comets most remarkable for brilliancy were the comets of 1811. 1843, 1858 (Donati's). 1861, 1880. 1881, and the great comet (1) of 1882. Spectroscopic investigation, so far as yet pursued, points to the conclusion that comets are self-luminous; and do not shine merely by reflecting solar light. It has been discovered. in determining the tracks of those streams of dark bodies that cause meteoric showers, that some of the tracks coincide with the orbits of well-known comets. From this it is inferred that star-showers and comets may be only different manifestations of the same thing. (See METEORS.) What the matter of the millets consists of is, of course, only a subject for speculation. The composition of the nebulosity and the tail is, at all events, something of al most inconceivable tenuity, as shown by three considerations. (1) Stars seen through them suffer no diminution of brigbtness, though the light must have to traverse sometimes millions of miles of the eometary atmosphere. (2) Though the thickness of the tail of a comet may he millions of miles, and its length of course much greater, the comets have never been ob served to cause any sensible disturbance of the planetary motions, though approaching near enough to be themselves so nr_.ch affected as to change the entire character of the orbit. (3) The curvature of the tails, and the acceleration of the periodic time in the case of Eneke's comet. indicate the possibility of their being af fected by a resisting medium, which has never been observed to have the slightest influence on the planetary periods, though so long observed. Even the nuclei of comets appear to be of ex tremely small density. This may be inferred, though with less force than regards the tails, from the two last considerations above men tioned; and, moreover, there are reliable accounts of stars of a very low order of magnitude being seen through the nuclei themselves.