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 1787 was made memorable by Laplace's announcement that the lunar acceleration depended upon the secular changes in the eccentricity of the earth's orbit. This removed all doubt then existing as to the stability of the solar system. The universal validity of the law of gravitation to explain all motion in the solar system was established. That system, as then known, was at last found to be a complete machine.

In 1796 Laplace published his Exposition du système du monde, a non-mathematical popular treatise on astronomy, ending with a sketch of the history of the science. In this work he enunciates for the first time his celebrated nebular hypothesis. A similar theory had been previously proposed by Kant in 1755, and by Swedenborg; but Laplace does not appear to have been aware of this.

Laplace conceived the idea of writing a work which should contain a complete analytical solution of the mechanical problem presented by the solar system, without deriving from observation any but indispensable data. The result was the Mécanique Céleste, which is a systematic presentation embracing all the discoveries of Newton, Clairaut, D'Alembert, Euler, Lagrange, and of Laplace himself, on celestial mechanics. The first and second volumes of this work were published in 1799; the third appeared in 1802, the fourth in 1805. Of the fifth volume, Books XI. and XII. were published in 1823; Books XIII., XIV., XV. in 1824, and Book XVI. in 1825. The first two volumes contain the general theory of the motions and figure of celestial bodies. The third and fourth volumes give special theories of celestial motions,—treating particularly of motions of comets, of our moon, and of other satellites. The fifth volume opens with a brief history of celestial mechanics, and then gives in appendices the results of the author's later researches. The Mécanique Céleste was such a master-piece, and so complete, that Laplace's successors have