Page:Popular Science Monthly Volume 13.djvu/592

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not fatal, to the views which it has been so frequently adduced to sustain. The superficial character of the examination which Laplace has given to this subject is betrayed by two statements which he makes in regard to it in two different parts of his writings. In setting forth the nebular hypothesis in his "Système du Monde," he asserts that the matter separated from a contracting nebula would take and maintain an annular figure, if there were a complete uniformity in its entire circuit and in its rate of cooling; but in the "Mécanique Céleste," in treating on Saturn's rings, he concludes that their preservation would be impossible without some decided irregularities in their structure. It is scarcely necessary to say that the annular appendage could not be of long duration if the conditions necessary for its existence or security in one age were fatal to it at another. On examining the alleged history of its birth, also, we feel at a loss for some cause of intermission in the work of detaching matter from the cooling nebula. It is difficult to imagine why, after the outer ring was completed, the separation of matter from Saturn, after a long continuance, should have ceased for a while; and why, after the completion of the inner ring, the centrifugal force again became weak, and that it has declined steadily until the present time, when, at the planet's equator, it is scarcely one-sixth of the force of gravity. Since the period when Saturn is supposed to have launched forth the zone of matter circulating nearest to him, his movements could be but little retarded by tidal action; and there seems to be no cause which could reduce his angular velocity of rotation during a contraction from the loss of primitive heat.

Kant, who regarded the rings as composed of aëriform matter separated from Saturn, was led to the natural inference that the time in which the planet turns once on his axis must be equal to that which the nearest annular zone requires to make a circuit around him. From such considerations, the eminent savant was induced to assign for the rotation of the planet the period of six hours, twenty-three minutes, and fifty-three seconds. But this theoretical or predicted length of Saturn's day is only about three-fifths of the actual value which was first revealed by the observations of Sir William Herschel, and lately determined with more precision by Prof. A. Hall. From the difficulties which the facts present in this case, Laplace endeavors to extricate his doctrine of planetary evolution by maintaining that it requires only that Saturn's day should be shorter than the period of revolution due to the inner ring, supposed to be one unbroken solid mass. But the basis on which this conclusion is founded has been exploded by modern researches, which show the impossibility of the existence of such vast solid annular structures; and Prof. Kirkwood, though long a supporter of the views of the great French astronomer, has lately pronounced the evidence obtained from the Saturnian system and from the inner moon of Mars as adverse to the nebular hypothesis.

While all scientific researches are exposed to uncertainty in propor-