This account of Halley's C. h been given at length, to illustrate the principles on which the calculations are made. There are three other comets whose periodicity is established, and whose paths are accurately known: 1. That of Encke, with a short period of 1204 days. Its orbit does not extend so far as the orbit of Jupiter. and a slight acceleration in its periodic times of return has suggested the possibility of the space, within our solar system at least, being occupied by a resisting medium, though of extreme rarity.
2. That of Biela or Gambart, having a period of six years and three quarters. Dur ing the visit of this C., in 1846, it was seen to separate into two distinct comets, which kept moving side by side till they disappeared. On the return of the C. in the autumn of 1852, the distance between the two nuclei had much increased. Since then, although twice due, it has not again been seen.
3. That of Faye,' with a period of seven years and a half.
The orbits of both these last extend beyond the orbit of Jupiter, but not so far as that of Saturn.
The above are the only comets whose orbits are considered by astronomers as estab lished, although there are others whose orbits are ascertained with a less degree of accu racy and certainty, requiring the test of future returns to fix their periodicity. One other deserves mention from the great peculiarities of its course.
In the month of June, 1770, Messier discovered a C. 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 corresponded to a periodic revolu tion of 5} years. This result suggested grave difficulties. It had been found impossi ble to identify this C. with any previously observed, and yet it was difficult to conceive that a bright C.. with so short a period of return, should have previously escaped observation. What was still more remarkable, it was never seen again, though anx iously looked for in the places where Lexell's orbit would have brought it. It became popularly called Lexell's lost C., tind.gaire occasion to many sarcaSmsby the wits of the day at the expense of astronomers, wholiad so much boasted of having found the key to the cometary movements. In the present day, the explanation is complete. The C. was never seen before 1770, 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 this planet entirely changed its orbit. But why has the C. not since been seen? Its passage to the point of perihelion in 17 76 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 not have been recognized as identical with Lexell's comet.
The celebrated C. 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 same 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. This C. is supposed to be identical with the one that appeared about the time of CEesar's death (44 n.c.), 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 revohition, 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 Encke giving it a period of 8,813 years. This C. approached nearer to the sun than any known, except perhaps the C. of 1843, the calculation of whose perihelion distance, from the paucity of observations, has little certainty. The C. of 1660 approached the sun within the 163d of the semi diameter of the earth's orbit. The tail of a comet is nearly always away from the snn, 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 degree. These 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 C. is exposed iu its approach to the sun.