Laplace assumed as his starting-point the existence of a nebula formed of incandescent gas, and extending beyond the limits of the outermost planet of our system. It was in rotation about a central axis, and possessed in consequence a disc-like or lenticular form. Radiating its heat away in all directions through surrounding space, it grew continually colder, and in cooling diminished in bulk. As a consequence of this contraction its rate of rotation increased, till at length the centrifugal force of the outermost part became so great that this could no longer continue to follow the contracting mass within, and thus remained behind as a great rotating ring. The continued contraction of the internal mass, and the resulting increase in the velocity of rotation, again brought about the same condition of things, and a fresh ring was left behind. [Sidenote: Cooling of the Nebula] This process was repeated time after time, till as many rings were formed as there are planets in the solar system; the central mass which survived within the innermost ring condensed to form the sun. The rings were highly unstable--that is to say, a slight disturbing force was sufficient to destroy their continuity; they broke across and rolled up into great nebulous globes, which revolved round the sun in the same direction as the original nebula, and rotated on their axes in the same direction as that in which they revolved. Most of them repeated the behaviour of the original nebulæ, leaving behind rings as they contracted, and these rings either rolled up to form moons or satellites, or, in the solitary instance of Saturn’s rings, retained their annular form. The rings are now known to consist of a multitude of solid bodies, as proved by Clerk-Maxwell. [Sidenote: The Temperature of the Earth]