Pirebulls and Shooting-stars.—From their height and apparent diameter, the actual diam ete• of the largest fireballs was estimated by Humboldt to vary from 500 to 2800 feet ; others allow a diameter of about a mile. ln most cases of luminous meteors. a train of light many miles in length is left behind. One or two in stances are on record where the train of the fire ball continued shining for half an after the body disappeared. This remarkable phe110111C11011 is as yet unexplained; it cannot be attributed reasonably to incandescence due to heat alone. The heights of shooting-stars are found to aver age from 74 to 50 miles at the points at which they begin and cease to be visible. Their veloc ities vary from 18 to 3d miles in a second.
One of the most remarkable facts connected with shooting-stars is, that certain appearances of them are periodic. On most. occasions they are sporadic—that is, they appear singly, and traverse the sky in all directions. At other times they appear in swarms of thousands, mov ing parallel; and these swarms are periodic, or recur on the same days of the year. Attention was first directed to this fact on occasion of the prodigious swarm which appeared in North America between November 12 and 13, 1833, de scribed by Professor Olmsted, of New Haven. The stars fell on this occasion like flakes of snow, to the number, as was estimated, of 240,000, in the space of nine hours, varying in size from a moving point or phosphorescent line to globes of the moon's diameter. The most heperout observation made was that they all appeared to proceed from the same quarter of the heavens, the vicinity, namely, of the star y. in the con stellation Leo; and although that star had changed greatly its height and direction during the time that the phenomenon lasted, they con tinued to issue from the same point. It was afterward computed by Encke that this point was the very direction in which the earth was moving in her orbit at the time. Attention being directed to recorded appearances of the same kind, it was observed with surprise that several of the most remarkable had occurred on the same day of November, especially that seen by Humboldt at Cumana in 1799, and by other observers over a great extent of the earth. The November stream was again observed in the United States in 1831, between November 13 and 14, though less intense. Though often vague, and in some years altogether absent, this phenomenon has recurred with such regularity, both ill America and Europe, as to establish its periodic character.
Another periodic swarm of considerable regu larity is that appearing between August 9 and 14, and noticed in ancient legends as the "fiery tears" of St. Lawrence, whose festival is on the tenth of that month. There are other periodic appearances, and the following epochs arc espe cially worthy of remark: April 20, July 28, August 10, November 14, November 24, December 11.
It remains to notice brielly the various opin ions that have been advanced as to the origin of abrolites and the theory of meteors in general. The hypotheses that have been formed in answer to the question, Whence come those solid masses that fall upon the earth? are of two kinds: sonic ascribing to them a telluric origin, and others making them alien to the earth. Of the first
kind is the conjecture that they may be stones ejected from terrestrial volcanoes, revolving for time along with the earth, and at last returning to it. Another theory, which at one time found considerable favor, supposed that the mat ter of which are composed existed in the atmosphere in the form of vapor, and was by some unknown cause suddenly aggregated and precipitated to the earth. These conjec tures are untenable in the face of the phenom ena stated above, and are now eompletely given up.
In seeking a source beyond the earth, the moon readily presented itself. Others was the first to investigate (1795) the initial velocity necessary to bring to the earth masses projected from the moon. This "ballistic problem," as Humboldt calls it, occupied during ten or twelve years the geometricians Laplace, Riot, 13randes, and Poisson. It was calculated that, setting aside the resistance of air, an initial velocity of about S000 feet in a second, which is about three or four times that of a cannon hall, would sutlice to bring the stones to the earth with a velocity of 35,000 feet. But Others showed that to account for the actual measured velocity of mete oric stones the original velocity of projection must be fourteen times greater than the above.
The dismission of hypotheses as to the genesis of the recognized planets out of portions of the gradually cmitraeting vaporous mass of the sun: the continued discovery of hitherto unobserved planets between the orbits of .Mors and Jupiter: the countless multitudes of comets that are observed traversing our system in all directions, and undergoing appreciable alteration both of consistency and orbit—all prepare us for the idea. that matter may exist in the interplanetary spaces in every variety of form and condition. To account for the phenomena of meteors as above described, we must suppose that there are both detached masses, each revolving in an inde pendent orbit, and giving rise to sporadic mete ors, and also connected systems, forming rings or zones around the sun. The intersection of the earth's orbit by such zones or streams would account for the periodic swarms of meteors: and if we suppose the asteroids composing it to be irregularly grouped, we see a reason why the same stream should not be always of equal intensity. There might even be periodicity in this respect too.
What causes the luminous and ignited condi tion of aerolites? Terrestrial magnetism was at one time suggested as the exciting cause. It is now recognized, however, that the atmosphere extends to a very great height, and the ignition is believed to be caused by friction between the rapidly moving body and the air. As to mete orites that do not fall on the earth, we may suppose that some are merely deflected from their path by the proximity of the earth, are rendered luminous through a short arc, and continue their course with altered orbit, while the greater part arc soon burnt up and fall to the earth in impalpable dust. See METEORS.
AltROMAN'CY, See SUPERSTITION.