had no history, or were only adverted to in vague tradition. Of this kind is the immense mass seen by Pallas in Siberia, now in the Imperial Museum in St. Petersburg. The largest known is one in Brazil, estimated at 14,000 pounds.
One constant characteristic of meteoric stones is the fused black crust, like varnish, with which the surface is coated. From the circumstance of this coat being very thin, and separated from the inner mass by a sharply defined line, it is thought to indicate some rapid action of heat which has not had time to penetrate into the substance of the stone. This view is favored by the fact that the stones are found in a strongly heated but not incandescent state when they fall. Their specific gravity ranges from two to seven or eight times that of water. Chemically, the meteoric stones have the same constitution as our earth, the chief constituent being nickel-iron, which occurs in variable proportions. No new element has been found in them, and only about twenty-five of those already known. These old elements are often combined in a different manner to form new minerals not yet known in the earth.
Besides these solid masses of considerable size, numerous instances are on record of showers of dust over large tracts of land: and it is remarkable that such dust has generally been found to contain small, hard, angular grains resembling augite. Stories of the fall of gelatinous masses from the sky are ranked by Humboldt among the mythical fables of meteorology. It has been supposed that such fables may have originated in the very rapid growth of gelatinous algæ, as Nostoc.
Fireballs and Shooting-stars. — From their height and apparent diameter, the actual diameter of the largest fireballs was estimated by Humboldt to vary from 500 to 2800 feet; others allow a diameter of about a mile. In most cases of luminous meteors, a train of light many miles in length is left behind. One or two instances are on record where the train of the fireball continued shining for half an hour after the body disappeared. This remarkable phenomenon is as yet unexplained: it cannot be attributed reasonably to incandescence due to heat alone. The heights of shooting-stars are found to average from 74 to 50 miles at the points at which they begin and cease to be visible. Their velocities vary from 18 to 36 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, moving 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, described 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 important observation made was that they all appeared to proceed from the same quarter of the heavens, the vicinity, namely, of the star γ, in the constellation Leo: and although that star had changed greatly its height and direction during the time that the phenomenon lasted, they continued 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 1834, 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 in America and Europe, as to establish its periodic character.
Another periodic swarm of considerable regularity 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 are especially worthy of remark: April 20, July 28, August 10, November 14, November 24, December 11.
It remains to notice briefly the various opinions that have been advanced as to the origin of aërolites 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: some 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 a time along with the earth, and at last returning to it. Another theory, which at one time found considerable favor, supposed that the matter of which aërolites 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 conjectures are untenable in the face of the phenomena stated above, and are now completely given up.
In seeking a source beyond the earth, the moon readily presented itself. Olbers 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, Biot, Brandes, and Poisson, It was calculated that, setting aside the resistance of air, an initial velocity of about 8000 feet in a second, which is about three or four times that of a cannon ball, would suffice to bring the stones to the earth with a velocity of 35,000 feet. But Olbers showed that to account for the actual measured velocity of meteoric stones the original velocity of projection must be fourteen times greater than the above.
The discussion of hypotheses as to the genesis of the recognized planets out of portions of the gradually contracting vaporous mass of the sun; the continued discovery of hitherto unobserved planets between the orbits of Mars 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
