Before we consider the orbs which compose our own solar system, their motions and relationships, which we shall do in another chapter, let us consider for a little the theory of the origin of systems, taking our own for a sample of the rest. Tennyson the poet, puts the story of system-building thus:-- “This world was once a fluid haze of light Till toward the centre set the starry tides, And eddied into suns that wheeling cast The planets.” We saw in a previous lesson that our world was once a vast ball of fiery vapor. The great astronomer Laplace, after many years of study, published a theory about system-making, called the “Nebular Hypothesis,” which, as simply as we can put it, is this: First, there is a great cloud of glowing vapor, the various particles of which are by the law of gravitation drawn toward a single centre. As the particles press equally to the centre from all sides, it is evident that the first result will be a ball of constantly increasing solidity. This nebulous mass possesses also a motion of rotation, or turning over on its own axis. As the ball grows smaller and more dense, it will spin round faster and faster. The gaseous matter of the sphere having become fluid or partly fluid, the ball still whirls on, and we must now notice a second motion, called centrifugal or tangental, which has become more apparent as the rotation increases in velocity. While by the force of gravity all atoms seek the centre, by this centrifugal force atoms are driven from the centre. This tangental motion is familiarly seen in the case of mud on a wheel-tire, the mud being flung off from the wheel by a motion created by rotation. So from the spinning globe a ring of matter will be detached and fly off into space. If this ring were equally hard and thick in all its parts, and exactly poised about the globe from which it sprung, it might keep the ring shape. But in nearly all cases the ring when flung off would be irregular, and would consequently break up. As it broke, the largest fragments, keeping the wheeling motion and made spherical by gravity, would draw into themselves the smaller near fragments. Then, after a while, following the example of the globe from which they spun off, these new globes would cast off rings of matter which would contract and harden into globes, and become their satellites or attendants, as they, held by the force of gravity, remain attendants upon the first great globe.