In seeking a source beyond the earth, the moon readily presented itself. Olbers was the first to investigate (1793) the initial velocity necessary to bring to the earth masses projected from the moon. This "ballistic problem," as Humboldt calls it, occupied, during ten er 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 ft. in a secoud,which is about five or six times that of a cannon-ball, would suffice to bring the stones to the earth with a velocityof 83,000 ft. But Olbers has shown that to account for the actual measured velocity of meteoric stones, the original velocity of projection must be fourteen times greater than the above. It is this lunar theory, that we have no proof of active volcanoes now existing in the moon; and with the improvement of the telescope, the probability of the contrary is increasing. It is, accordingly, giving place to the planetary theory, which we noticed at the outset—a theory which harmonizes bet ter with the tendency of physical research and of speculation generally.
:The discussion of hypotheses as to the genesis of the recognized planets out of por tions 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 multi tudes 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 mat ter may exist in the inter-planetary 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 independent orbit, and givinn. rise to epo radie meteors; and also connected systems, forming rings or zones round 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 sec a reason why the same stream should not be always of equal intensity. There may even be periodicity in this respect too. Between 1799 and 1833—two of the most brilliant manifestations of the November stream on record—there elapsed 34 years; and the next brilliant appearances were in 1866 and 1807, as Olbers had predicted.
What causes the luminous and ignited condition of Orates? Terrestrial magnetism was at one time suggested as the exciting cause. It is now recognized, however, that the atmosphere extends, although in a very rare condition, to at least a height of 200 in., and the ignition is believed to be caused by friction between the rapidly moving body and the air. As to meteors unattended by aerolites, we may suppose that some are merely de flected 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 are soon burnt up and fall to the earth in impalpable dust. See MErEons.