The work of the Marquis de Laplace (1749-1827) was enormously influential on the development of mathematical physics, astronomy and statistics. His Exposition du syst me du monde (first published in 1796) is often regarded as the most important book on mechanics after Newton's Principia Mathematica, and the elegance and clarity of its style won Laplace a seat in the Acad mie Fran aise. The book, which was translated into English in 1809, was intended to 'offer a complete solution of the great mechanical problem presented by the solar system'. It was in this work that Laplace offered his nebular hypothesis, which proposed that the solar system originated from the contraction and cooling of a cloud of incandescent gas. The book, here in its second edition of 1799, is an introduction to Laplace's multi-volume masterpiece, the Trait de M canique C leste, of which Mary Somerville's English version is also reissued in this series.