| 14 | periods of | Jupiter | are nearly equal to | 5 | of the ring. | |
| 9 | " | Saturn | " | " | 8 | " |
| 23 | " | Uranus | " | " | 58 | " |
This group or stream has its perihelion at the orbit of the earth; its aphelion, at that of Uranus. (See diagram, p. 24.) It must therefore produce star-showers at the latter as well as at the former. Our planet, moreover, at each encounter appropriates a portion of the meteoric matter; while at the remote apsis of the stream Uranus in all probability does the same. The matter of the ring will thus by slow degrees be gathered up by the two planets.
B.
Comets and Meteors.
The recent researches and speculations of European astronomers in regard to the origin of comets and of meteoric streams, have suggested to the author the propriety of reproducing the following extracts from an article written by himself, in July, 1861, and published in the Danville Quarterly Review for December of that year:
"Different views are entertained by astronomers in regard to the origin of comets; some believing them to enter the solar system ab extra; others supposing them to have originated within its limits. The former is the hypothesis of Laplace, and is regarded with favor by many eminent astronomers. It seems to afford a plausible explanation of the paucity of large comets during certain long intervals of time. In one hundred and fifty years, from 1600 to 1750, sixteen comets were visible to the naked eye; of which eight appeared in the twenty-five years from 1664 to 1689. Again, during sixty years from 1750 to 1810, only five comets were visible to the naked eye, while in the next fifty years there were double that number. Now, according to Laplace's hypothesis, patches of nebulous matter have been left nearly in equilibrium in the interstellar spaces. As the sun, in his progressive motion, approaches such clusters, they must, by virtue of his attraction, move toward the center of our system; the nearer portions with greater velocity than the more remote. The nebulous fragments thus introduced into our system would constitute comets; those of the same cluster would enter the solar domain at periods not very distant from each other; the forms of their orbits depending upon their original relative positions with reference to the sun's course, and also on planetary perturbations. On the other hand, the passage of the system through a region of space destitute of this chaotic vapor would be followed by a corresponding paucity of comets.
"Before the invention of the telescope, the appearance of a comet was a comparatively rare occurrence. The whole number visible to the naked eye during the last three hundred and sixty years has been fifty-five; or a mean of fifteen per century. The recent rate of telescopic discovery, however, has been about four or five annually. As many of these are extremely faint, it seems probable that an indefinite number, too small for detection, may be constantly traversing the solar domain. If we adopt Laplace's hypothesis of the origin of comets, we may suppose an almost continuous fall of primitive nebular matter toward the center of the system—the drops of which, penetrating the earth's atmosphere, produce sporadic meteors; the larger aggregations forming comets. The disturbing influence of the planets may have transformed the original orbits of many of the former, as well as of the latter, into ellipses. It is an interesting fact that the motions of some luminous meteors—or cometoids, as perhaps they might be called—have been decidedly indicative of an origin beyond the limits of the planetary system.
"But how are the phenomena of periodic meteors to be accounted for, in accordance with this theory?
"The division of Biela's comet into two distinct parts suggests several interesting questions in cometary physics. The nature of the separating force remains to be discovered; 'but it is impossible to doubt that it arose from the divellent action of the sun, whatever may have been the mode of operation.'
"'A signal manifestation of the influence of the sun,' says a distinguished writer, 'is sometimes afforded by the breaking up of a comet into two or more separate parts, on the occasion of its approach to the perihelion. Seneca relates that Ephoras, an ancient Greek author, makes mention of a comet which before vanishing was seen to divide itself into two distinct bodies. The Roman philosopher appears to doubt the possibility of such a fact; but Keppler, with characteristic sagacity, has remarked that its actual occurrence was exceedingly probable. The latter astronomer further remarked that there were some grounds for supposing that two comets, which appeared in the same region of the heavens in the year 1618, were the fragments of a comet that had experienced a similar dissolution. Hevelius states that Cysatus perceived in the head of the great comet of 1618 unequivocal symptoms of a breaking up of the body into distinct fragments. The comet when first seen in the month of November, appeared like a round mass of concentrated light. On the 8th of December it seemed to be divided into several parts. On the 20th of the same month it resembled a multitude of small stars. Hevelius states that he himself witnessed a similar appearance in the head of the comet of 1661.'[34] Edward Biot, moreover, in his researches among the Chinese records, found an account of 'three dome-formed comets' that were visible simultaneously in 896, and pursued very nearly the same apparent path.