| From A.D. 401 to | A.D. 538, | 20 | periods of | 6.85 | years. |
| 538 to | 839, | 44 | " | 6.84 | " |
| 839 to | 927, | 13 | " | 6.77 | " |
| 927 to | 934, | 1 | " | 7.00 | " |
| 934 to | 1009, | 11 | " | 6.82 | " |
This period nearly corresponds to those of several comets whose aphelion distances are somewhat greater than the mean distance of Jupiter. So long as the cluster occupied but a small arc of the orbit the displays would evidently be separated by considerable intervals. The comparative paucity of meteors in modern times may be explained by the fact that the ring has been subject to frequent perturbations by Jupiter.
Groups in which the Meteoroids are sparsely scattered.
By the labors of Heis, Greg, Herschel, Schiaparelli, and others, the radiants of more than fifty sparsely strewn meteor-systems have been determined. Of these the following, which are well defined, seem worthy of special study:
| DATE. | POSITION OF RADIANT. | |
| R. A. | N. Decl. | |
| January 1-4 | 234° | 51° |
| January 18 | 232° | 36° |
| April 25 | 142° | 53° |
The orbits and periods, except in the few cases previously considered, are entirely unknown. Some of the observed clusters are probably the débris of ancient comets whose aphelia were in the vicinity of Jupiter's orbit.
CHAPTER X.
THE ORIGIN OF COMETS AND METEORS.
The fact that comets and meteors, or at least a large proportion of such bodies, have entered the solar system from stellar space, is now admitted by all astronomers. The question, however, in regard to the origin and nature of these cosmical clouds still remains undecided. The theory that they consist of matter expelled with great velocity from the fixed stars appears to harmonize the greatest number of facts, and is accordingly entitled to respectful consideration. The evidence by which it is sustained may be briefly stated as follows: