Page:Popular Science Monthly Volume 13.djvu/591

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THE ASTRONOMICAL HISTORY OF WORLDS.

modern science, matter and motion were all required for calling a world into existence; but it was soon found that, unless, in the beginning, the materials which formed the solar system moved with a certain order and regularity, they could never have risen from the chaotic to the cosmical condition. As all the planets move around the sun in the same direction, Laplace was led to believe that in remote times all must have been connected together; and such a primitive connection might be afforded if the sun and his attendants were originally a vast fire-mist, their matter being so much attenuated by heat that it extended far beyond its boundaries of the solar domain. He supposed that such an immense rarefied mass, on being set in motion by some cause which he does not specify, would ultimately be compelled by its own friction and by gravity to rotate with a uniform angular velocity in all its parts and around a common centre. In accordance with the principles of physical astronomy, he concluded that this rotation would become rapid as the immense solar nebula cooled and contracted, until at last the centrifugal force became great enough to overpower gravity and to throw off matter from the equator of the whirling mass. Laplace considered that, under the most probable circumstances, the nebulous matter thus thrown off, or abandoned by the shrinking spheroid, would all collect together to form a planet; but that, in some unusual cases, it would assume the expanded figure of a vast solar ring; and that, under certain conditions, it might break up into a number of asteroids. The singular group of bodies revolving between Mars and Jupiter is supposed to have come into existence in consequence of some rare accident, which made the great solar ring a prey to many centres of aggregation, instead of allowing it to coalesce around a single one. In all other cases, the cooling and contraction are said to have been successful in giving birth to a great planet, whenever the centrifugal force became sufficient to separate the equatorial portions of the rotating solar nebula. According to the views of Laplace, Neptune must be regarded as the first-born world of those already known; while Uranus is next in age, and the other planets were launched into being in a succession depending on their distances from the sun; so that Mercury is the youngest member of the solar family. It has been also concluded that from the condition of its birth each planet must have commenced its career as a rotating nebula; and that many of the larger ones, by subsequent cooling and contraction, were at certain periods enabled to throw off their equatorial matter, which in all but two instances was converted into a satellite. Of these minor worlds or moons, Saturn has succeeded in obtaining eight, in addition to the double ring, which in the eyes of Laplace appeared as two embryonic satellites, and which has been so often appealed to for proof of the world-making doctrine under consideration.

Yet, when examined with care and impartiality, the evidence derived from the condition of the Saturnian girdle will be found unfavorable, if