CHAPTER II.

OTHER METEORIC RINGS.

II. The Meteors of August 6th-11th.

Muschenbroek, in his _Introduction to Natural Philosophy_, published in 1762, called attention to the fact that shooting-stars are more abundant in August than in any other part of the year. The annual periodicity of the maximum on the 9th or 10th of the month was first shown, however, by Quetelet, shortly after the discovery of the yearly return of the November phenomenon. Since that time an extraordinary number of meteors has been regularly observed, both in Europe and America, from the 7th to the 11th of the month; the greatest number being generally seen on the 10th. In 1839, Edward Heis, of Aix-la-Chapelle, saw 160 meteors in one hour on the night of the 10th. In 1842, he saw 34 in ten minutes at the time of the maximum. In 1861, on the night of the 10th, four observers, watching together at New Haven, saw in three hours--from ten to one o'clock--289 meteors. On the same night, at Natick, Massachusetts, two observers saw 397 in about seven hours. At London, Mercer County, Pennsylvania, on the night of August 9th, 1866, Samuel S. Gilson, Esq., watching alone, saw 72 meteors in forty minutes, and, with an assistant, 117 in one hour and fifteen minutes. Generally, the number observed per hour, at the time of the August maximum, is about nine times as great as on ordinary nights. Like the November meteors, they have a common "radiant;" that is, their tracks, when produced backward, meet, or nearly meet, in a particular point in the constellation Perseus.

Of the 315 meteoric displays given in Quetelet's "Catalogue des principales apparitions d'étoiles filantes," 63 seem to have been derived from the August ring. The first 11 of these, with one exception, were observed in China during the last days of July, as follows:

1 A.D. 811, July 25th. 2 820, " 25th-30th. 3 824, " 26th-28th. 4 830, " 26th. 5 833, " 27th. 6 835, " 26th. 7 841, " 25th-30th. 8 924, " 27th-30th. 9 925, " 27th-30th. 10 926, " 27th-30th. 11 933, " 25th-30th.

The next dates are 1243, August 2d, and 1451, August 7th. A comparison of these dates indicates a forward motion of the node of the ring along the ecliptic. This was pointed out several years since by Boguslawski. A similar motion of the node has also been found in the case of the November ring. That these points should be stationary is, indeed, altogether improbable. The nodes of all the planetary orbits, it is well known, have a secular variation.

On the evening of August 10th, 1861, at about 11h. 30m., a meteor was seen by Mr. E. C. Herrick and Prof. A. C. Twining, at New Haven, Connecticut, which "was much more splendid than Venus, and left a train of sparks which remained luminous for twenty seconds after the meteor disappeared." The same meteor was also accurately observed at Burlington, New Jersey, by Mr. Benjamin V. Marsh. It was "conformable,"--that is, its track produced backward passed through the common radiant--and it was undoubtedly a member of the August group. The observations were discussed by Professor H. A. Newton, of Yale College, who deduced from them the following approximate elements of the ring:[8]

Semi-axis major 0·84 Eccentricity 0·28 Perihelion distance 0·60 Inclination 84° Period 281 days. Motion, retrograde.

The earth moving at the rate of 68,000 miles per hour, is at least five days in passing entirely through the ring. This gives a thickness of more than 8,000,000 miles.

The result of Professor Newton's researches on the orbit of this ring, though undertaken with inadequate data, and hence, in some respects, probably far from correct, is nevertheless highly interesting as being the first attempt to determine the orbit of shooting-stars. More recent investigations have shown a remarkable resemblance between the elements of these meteors and those of the third comet of 1862. The former, by Schiaparelli, and the latter, by Oppolzer, are as follows:

Meteors of August 10th. Comet III., 1862.

Longitude of perihelion 343° 38´ 344° 41 Ascending node 138 16 137 27 Inclination 63 3 66 25 Perihelion distance 0·9643 0·9626 Period 105 years(?). 123 years(?). Motion Retrograde. Retrograde.

This similarity is too great to be accidental. _The August meteors and the third comet of 1862 probably belong to the same ring._

III. The Meteors of April 18th-26th.

The following dates of the April meteoric showers are extracted from Quetelet's table previously referred to:

1 A.D. 401, April 9th. 2 538, " 7th. 3 839, " 17th. 4 927, " 17th. 5 934, " 18th. 6 1009, " 16th. 7 1094, " 10th. 8 1096, " 10th. 9 1122, " 11th. 10 1123, " 11th. 11 1803, " 20th. 12 1838, " 20th. 13 1841, " 19th. 14 1850, " 11th-17th.

The display of 401 was witnessed in China, and is described as "very remarkable." That of 1803 was best observed in Virginia, and was at its maximum between one and three o'clock. The alarm of fire had called many of the inhabitants of Richmond from their houses, so that the phenomenon was generally witnessed. The meteors "seemed to fall from every point in the heavens, in such numbers as to resemble a shower of sky-rockets." Some were of extraordinary magnitude. "One in particular, appeared to fall from the zenith, of the apparent size of a ball 18 inches in diameter, that lighted the whole hemisphere for several seconds."

The probability that the meteoric falls about the 20th of April are derived from a ring which intersects the earth's orbit, was first suggested by Arago, in 1836. The preceding list indicates a forward motion of the node. The radiant, according to Mr. Greg, is about _Corona_. The number of meteors observed in 1838, 1841, and 1850, was not very extraordinary. Recent observations indicate April 9th-12th as another epoch. The radiant is in Virgo.

IV. The Meteors of December 6th-13th.

On the 13th of December, 1795, a large meteoric stone fell in England. On the night, between the 6th and 7th of December, 1798, Professor Brandes, then a student in Göttingen, saw 2000 shooting-stars. On the 11th of the month, 1836, a fall of meteoric stones, described by Humboldt as "enormous," occurred near the village of Macao, in Brazil. During the last few years unusual numbers of shooting-stars have been noticed by different observers from the 10th to the 13th; the maximum occurring about the 11th. From A.D. 848, December 2d, to 1847, December 8th-10th, we find 14 star showers in Quetelet's catalogue, derived, probably, from this meteoric stream. As in other cases, the dates seem to show a progressive motion of the node. The position of the radiant, as determined by Benjamin V. Marsh, Esq., of Philadelphia, from observations in 1861 and 1862, and also by R. P. Greg, Esq., of Manchester, England, is at a point midway between Castor and Pollux.

V. The Meteors of January 2d-3d.

About the middle of the present century, Mr. Julius Schmidt, of Bonn, a distinguished and accurate observer, designated the 2d of January as a meteoric epoch; characterizing it, however, as "probably somewhat doubtful." Recent observations, especially those of R. P. Greg, Esq., have fully confirmed it. The meteors for several hours are said to be as numerous as at the August maximum. The radiant is near the star _Beta_ of the constellation Böotes.

Quetelet's list contains at least five exhibitions which belong to this epoch. Two or three others may also be referred to it with more or less probability.

* * * * *

Several other meteoric epochs have been indicated; some of which, however, must yet be regarded as doubtful. In thirty years, from 1809 to 1839, 12 falls of bolides and meteoric stones occurred from the 27th to the 29th of November. Such coincidences can hardly be accidental. Unusual numbers of shooting-stars have also been seen about the 27th of July; from the 15th to the 19th of October, and about the middle of February. The radiant, for the last-mentioned epoch, is in _Leo Minor_. The numbers observed in October are said to be at present increasing. At least seven of the exhibitions in Quetelet's catalogue are referable to this epoch. It is worthy of remark, moreover, that three of the dates specified by Mr. Greg as _aerolite_ epochs are coincident with those of shooting-stars; viz., February 15th-19th, July 26th, and December 13th. The whole number of exhibitions enumerated in Quetelet's catalogue is 315. In eighty-two instances the day of the month on which the phenomenon occurred is not specified. Nearly two-thirds of the remainder, as we have seen, belong to established epochs, and the periodicity of others will perhaps yet be discovered. But reasons are not wanting for believing that our system is traversed by numerous meteoric streams besides those which actually intersect the earth's orbit. The asteroid region between Mars and Jupiter is probably occupied by such an annulus. The number of these asteroids increases as their magnitudes diminish; and this doubtless continues to be the case far below the limit of telescopic discovery. The zodiacal light is probably a dense meteoric ring, or rather, perhaps, a number of rings. We speak of it as _dense_ in comparison with others, which are invisible except by the ignition of their particles in passing through the atmosphere. From a discussion of the motions of the perihelia of Mercury and Mars, Leverrier has inferred the existence of two rings of minute asteroids; one within the orbit of Mercury, whose mass is nearly equal to that of Mercury himself; the other at the mean distance of the earth, whose mass cannot _exceed_ the tenth part of the mass of the earth.

Within the last few years a distinguished European savant, Buys-Ballot, of Utrecht, has discovered a short period of variation in the amount of solar heat received by the earth: the time from one maximum to another exceeding the period of the sun's apparent rotation by about twelve hours. The variation cannot therefore be due to any inequality in the heating power of the different portions of the sun's surface. The discoverer has suggested that it may be produced by a meteoric ring, whose period slightly exceeds that of the sun's rotation. Such a zone might influence our temperature by partially intercepting the solar heat.

GENERAL REMARKS.

1. The average number of shooting-stars seen in a clear, moonless night by a single observer, is about 8 per hour. _One_ observer, however, sees only about one-fourth of those visible from his point of observation. About 30 per hour might therefore be seen by watching the entire hemisphere. In other words, 720 shooting-stars per day could be seen by the naked eye at any one point of the earth's surface, did the sun, moon, and clouds permit.

2. The mean altitude of shooting-stars above the earth's surface is about 60 miles.

3. The number visible over the whole earth is about 10,460 times the number to be seen at any one point. Hence the average number of those daily entering the atmosphere and having sufficient magnitude to be seen by the naked eye, is about 7,532,600.

4. The observations of Pape and Winnecke indicate that the number of meteors visible through the telescope, employed by the latter, is about 53 times the number visible to the naked eye, or about 400,000,000 per day.[9] This is two per day, or 73,000 per century, for every square mile of the earth's surface. By increasing the optical power, this number would probably be indefinitely increased. At special times, moreover, such as the epochs of the great meteoric showers, the addition of foreign matter to our atmosphere is much greater than ordinary. It becomes, therefore, an interesting question whether sensible changes may not thus be produced in the atmosphere of our planet.

5. In August, 1863, 20 shooting-stars were doubly observed in England; that is, they were seen at two different stations. The average weight of these meteors, estimated--in accordance with the mechanical theory of heat--from the quantity of light emitted, was a little more than two ounces.

6. A meteoric mass exterior to the atmosphere, and consequently non-luminous, was observed on the evening of October 4th, 1864, by Edward Heis, a distinguished European astronomer. It entered the field of view as he was observing the milky way, and he was enabled to follow it over 11 or 12 degrees of its path. It eclipsed, while in view, a number of the fixed stars.