the same process of cooling and contracting, and might therefore throw off, under the operation of the same mechanical laws, zones of vapor more or less dense, which might consolidate into moons or satellites, and which should also revolve, like the planets, round their primary. Thus, Uranus has six satellites, and Saturn seven; while the latter has also thrown off two zones so perfectly uniform in their internal structure that they remain unbroken, and constitute a double ring around the planet. In this _first_ form of the theory, which assumes the existence of the sun and its atmosphere, and the rotation of both round an axis, La Place sought to give a scientific form to the speculations of Sir William Herschell on the condensation of Nebulae, by proving simply the _dynamical possibility_ of the formation of a planetary system by such means, according to the known laws of matter and motion; but he did not affirm the scientific certainty of his conjecture, and far less the actual production of the solar system in this way. He has been followed by M. Comte, who has attempted to furnish, if not a complete demonstration, at least a plausible mathematical verification, of the hypothesis.[29] Utterly excluding all supernatural agency in the work of creation, he equally excludes from the problem which he attempts to solve, the origin of the sun and its atmosphere; and confining himself to the task of accounting, in the way not of demonstrative certainty, but merely of plausible hypothesis, for the formation of the planets and satellites of our solar system, he conceives the theory of La Place to be susceptible of such a numerical verification as is sufficient to give it a high degree of verisimilitude. Assuming that the periodic time of each planet must be equal to that of the portion of the solar atmosphere of which it was formed at the era when it was thrown off, and combining the theorems of Huygens on the measure of centrifugal forces with Newton's law of gravitation, he establishes a simple equation between the time of the rotation of each zone or section of the solar atmosphere, and the distance of the corresponding planets. On applying this equation to the various bodies of our system, he found that the periodic time of the moon agrees, at least within the tenth of a day, with the duration of the earth's revolution, when her atmosphere is supposed to have extended to the moon; and that the periodic times of the planets maintain a similar correspondence with what must have been the duration of the solar revolution when they were severally thrown off from its atmosphere. It is the less necessary, however, to enter on a detailed exposition of his argument, because he admits that it can afford at the utmost only a probable proof of an hypothesis; and further, because it is expressly limited to the production of the planets and their satellites, while not only is the existence of the