Page:Popular Science Monthly Volume 15.djvu/17

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ing if the planes in which they moved differed one degree in their inclinations to the ecliptic. A slight difference in the size or in the shape of their orbits would also be an unsurmountable barrier to their union. If a collision should occur between two asteroids, they would be only shattered into fragments, and a coalescence into one mass would be rendered more hopeless. But on the extreme verge of a solar system the numerous meteors consigned to large circular orbits lying in the same plane would have very nearly the same velocity in contiguous zones, and would be ready for the work of aggregation when their numbers were sufficiently increased by a long-continued electro-magnetic action.

In such an innumerable group of small and light bodies in symmetrical array, a large meteorite or the nucleus of a comet might become the embryo of a future world which may require many thousand years to attain the mass of one of the average asteroids. But its attraction after a time must become powerful enough to clear a large tract of space of matter, and thus to divide into two zones the great ultra-planetary ring of floating matter, while it must gradually make the paths of the small bodies deviate from true circles. From the outer zone it receives the meteors, which are in the perihelia of their orbits, and have their velocity most rapid; but the meteoric bodies from the internal zone unite with the growing mass near the points at which their motion is reduced to the lowest rate. Accordingly, the rotation of the new world must be in the same direction in which the constituents of the great ring were moving, and in which the parent orbs moved around their common center of gravity. The same direction of motion would also be exhibited by meteors which, instead of incorporating at once with the growing world, only described ellipses around it in accordance with the law of gravity.

In this early stage of its existence a world would be able to acquire a large train of meteors revolving permanently around it chiefly in consequence of two circumstances: The rapid increase in the mass and the attraction of the growing planet will make the velocity gained by bodies in approaching it always less than that lost while they are retiring; and orbits, even when slightly hyperbolic, would be changed into ellipses. Besides this, the vast atmosphere of nebulous matter around the new-born sphere would be more effective for the same end, as it would check the velocity of the passing meteors and cause them to revolve around the growing mass long before they incorporate with it. It is in consequence of these meteoric falls, and not the mere process of cooling, that the abundance of cometary and nebulous matter surrounding a young world is brought into a more dense condition. A planetary atmosphere of oxygen and hydrogen would maintain a gaseous form in spite of the refrigerating influence of many ages; but it would be quickly converted into aqueous vapor by the chemical forces awakened on the fall of a meteoric stone, and in the course of time might become liquid or solid as it parted with heat.