of August, the radiant having a daily motion of about 1° R.A. to E.N.E. The Lyrids also vary in the position of their radiant, but the Orionids form a stationary position from about the 9th to the 24th of October. A large proportion of the ordinary feeble showers also appear to be stationary.
Solid bodies (chiefly stone or stone and iron) enter the atmosphere from without at all conceivable angles and at a velocity of about 26 m. per second, while the earth’s orbital velocity is about 1814 m. per second. In thus rapidly penetrating the air heat is generated, the meteor becomes incandescent, and the phenomena of the streak or train is produced. Before the object can pierce the dense lower strata of air its material is usually exhausted, but on rare occasions it withstands the fiery ordeal, and fragments of the original mass fall upon the earth.
Multitudes of meteors infest space. On a clear moonless night one person may count eight or ten shooting stars in an hour. But there are more than twice as many visible in the early morning hours as in the evenings, and during the last half of the year there are also more than twice as many visible as during the first half. It is computed that twenty millions of meteors enter the atmosphere every day and would be visible to unassisted vision in the absence of sunlight, moonlight and clouds, while if telescopic meteors are included the number will be increased twentyfold. Ordinary meteors, in the region of the earth’s orbit, appear to be separated by intervals of about 250 m. In special showers, however, they are much closer. In the rich display of the 12th of November 1833, the average distance of the particles was computed as about 15 m., in that of the 27th of November 1885 as about 20 m., and in that of the 27th of November 1872 as about 35 m.
The meteors, whatever their dimensions, must have motions around the sun in obedience to the law of gravitation in the same manner as planets and comets—that is, in conic sections of which the sun is always at one focus. The great variety in the apparent motions of meteors proves that they are not directed from the plane of the ecliptic; hence their orbits are not like the orbits of planets and short-period comets, which are little inclined, but like the orbits of parabolic comets, which often have great inclinations.
Historical records supply the following dates of abundant meteoric displays:—
| 902, Oct. 13. | 1101 Oct. 17. | 1602, Oct. 28. | 1833 Nov. 13. |
| 931, Oct. 14. | 1202 Oct. 19. | 1698, Nov. 9. | 1866 Nov. 14. |
| 934, Oct. 14. | 1366 Oct. 19. | 1698, Nov. 12. | 1867 Nov. 14. |
| 1002, Oct. 15. | 1533 Oct. 23. | 1799, Nov. 12. | 1868 Nov. 14. |
These showers occurred at intervals of about one-third of a century, while the day moved along the calendar at the rate of one month in a thousand years. The change of style is, however, responsible for a part of the alteration in date. The explanation of these recurring phenomena is that a great cloud or distended stream of meteors revolves around the sun in a period of 3312 years, and that one portion of the elliptical orbit intersects that of the earth. As the meteors have been numerously visible in five or six successive years it follows they must be pretty densely distributed along a considerable arc of their orbit. It also follows that, as some of the meteors are seen annually, they must be scattered around the whole orbit. Travelling at the rate of 26 m. per second, they encounter the earth moving 1814 m. per second in an opposite direction, so that the apparent velocity of the meteors is about 44 m. per second. They radiate from a point within the Sickle of Leo and are termed Leonids. In 1867 the remarkable discovery was made that Tempel’s comet (1866 : I.) revolved in an orbit identical with that of the Leonids. That the comet and meteors have a close physical association seems certain. The disintegrated and widely dispersed material of the comet forms the meteors which embellish our skies on mid-November nights.
Fine meteoric showers occurred in 1798 (Dec. 7), 1838 (Dec. 7), 1872 (Nov. 27), 1885 (Nov. 27), 1892 (Nov. 23) and 1899 (Nov. 23 and 24), and the dates indicate an average period of 6·7 years for fifteen returns. The meteors move very slowly, as they have to overtake the earth, and their apparent velocity is only about 9 m. per second. They are directed from a point in the sky near the star γ Andromedae. Biela’s comet of 1826, which had a period of 6·7 years, presented a significant resemblance of orbit with that of the meteors, but the comet has not been seen since 1852 and has probably been resolved into the meteoric stream of Andromedids.
Rich annual displays of meteors have often been remarked on about the 10th of August, directed from Perseus, but they do not appear to have exhibited periodical maxima of great strength. They are probably dispersed pretty evenly along a very extended ellipse agreeing closely in its elements with comet 1862 : III. But the times of revolution are doubtful; the probable period of the comet is 121 years and that of the meteors 10512 years. This shower of Perseids is notable for its long duration in the months of July and August and for its moving radiant.
There was a brilliant exhibition of meteors on the 20th of April 1803, and in other years meteors have been very abundant on about the 19th to the 21st of April, shooting from a radiant a few degrees south-west of α Lyrae. The display is apparently an annual one, though with considerable differences in intensity, and the cycle of its more abundant returns has not yet been determined. A comet which appeared in 1861 had a very suggestive agreement of orbit when compared with that of the meteors, and the period computed for it was 415 years.
Apart from the instances alluded to there seem few coincidences between the orbital elements of comets and meteors. Halley’s comet conforms very well, however, with a meteoric shower directed from Aquarius early in May. But there are really few comets which pass sufficiently near the earth to give rise to a meteoric shower. Of 80 comets seen during, the 20 years ending 1893, Professor Herschel found that only two, viz. Denning’s comet of 1881 and Finlay’s of 1886, approached comparatively near to the earth’s path, the former within 3,000,000 m. and the latter within 4,600,000 m.
Radiants of Principal Showers.—The following is a list of the chief radiant points visible during the year:—
| Date | Radiant | Date | Radiant | ||||
| R. A. | Dec. | R. A. | Dec. | ||||
| Jan. 2–3 | 230° | + | 53° | July–Sept. | 47° | + | 43° |
| Feb. 10–15 | 75° | + | 41° | Sept. 5–15 | 62° | + | 37° |
| March 1–4 | 166° | + | 4° | Sept. 3–22 | 74° | + | 41° |
| March 24 | 161° | + | 58° | Oct. 2 | 230° | + | 52° |
| April 19–22 | 271° | + | 33° | Oct. 4 | 310° | + | 79° |
| April–May | 193° | + | 58° | Oct. 15–24 | 92° | + | 15° |
| May 1–6 | 338° | − | 2° | Oct. 20–25 | 100° | + | 13° |
| May 11–18 | 231° | + | 27° | Oct. 30–Nov. 1 | 43° | + | 22° |
| May–July | 252° | − | 21° | Nov. 2 | 58° | + | 9° |
| June 13 | 310° | + | 61° | Nov. 14–16 | 151° | + | 22° |
| July 15–19 | 314° | + | 48° | Nov. 16–28 | 154° | + | 40° |
| July 28–30 | 339° | − | 11° | Nov. 20–23 | 63° | + | 22° |
| Aug 9–13 | 45° | + | 57° | Nov. 17–23 | 25° | + | 43° |
| Aug 10–15 | 290° | + | 53° | Dec. 4 | 162° | + | 58° |
| Aug 21–25 | 291° | + | 60° | Dec. 9–12 | 108° | + | 33° |
Many meteors exhibit the green line of magnesium as a principal constituent. Professor N. von Konkoly remarked in the fireball of 1873 (July 26) the lines of magnesium and sodium. Other lines in the red and green have been detected and found by comparison with the lines of marsh gas. Bright meteors often emit the bluish-white light suggestive of burning magnesium. In addition to magnesium and sodium the lines of potassium, lithium and also the carbon flutings exhibited in cometary spectra, have been seen.
Meteoric observation has depended upon rough and hurried eye estimates in past years, but the importance of attaining greater accuracy by means of photography has been recognized. At several American observatories, and at Vienna, fairly successful attempts were made in November 1898 to photograph a sufficient number of meteor-trails to derive the Leonid radiant, and the mean position was at R.A. 151° 33′, Dec. + 22° 12′. But the materials obtained were few, the shower having proved inconspicuous. The photographic method, appears to have practically failed during recent years, since there has
