Newton's methods, which even in the seventeenth century were archaic, became in time quite out of date. This reason, the growth of the subject, and the development of analysis made it desirable to expound dynamical astronomy afresh. Towards the end of the eighteenth century the task was undertaken by Laplace in his _Mécanique Céleste_. This is far more than the translation of the _Principia_ into the language of modern analysis, for it greatly extends the theory of some branches of the subject which had been left incomplete by Newton, either on account of his not having the requisite analysis at his command or because the necessary facts were not available. Laplace acknowledged his debt to Newton, and expressed his deliberate opinion that the _Principia_ was pre-eminent over every previous production of human genius--"so near the gods, man cannot nearer go." A century later a fresh exposition of the subject embodying the discoveries of the nineteenth century was given by F.F. Tisserand in his _Mécanique Céleste_; this presents the subject in its modern form. Newton had applied his theory to the solar system as it existed, and had not investigated its origin. We owe to Laplace the enunciation of a hypothesis as to its evolution. According to this conjecture, the solar system originated in a quantity of incandescent gas rotating round an axis through its centre of mass. Laplace assumed that as this gas cooled, it would contract, and that successive rings would break off from its outer edge; these rings in their turn would cool, and finally condense into the planets with their satellites; while the sun represents the central core which would be left. Recent investigations show that this cannot be taken as correct without numerous modifications. Moreover every extension of our knowledge requires the introduction of alterations in the hypothesis, and this clearly suggests that the conjecture is untenable. It played, however, a useful part in its day, as suggesting a common origin for all members of the system. Perhaps I ought to add that a nebular origin had been previously outlined by Kant, who had also suggested meteoric aggregations and tidal friction as agents concerned, but these were little more than vague conjectures.