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174 comets move either on a higher or lower level, just as travellers are made to do at railway stations when they wish to cross the rails. One comet, Biela's, crosses the orbit of the earth, as if to remind us of the danger we would run were the crowd of comets allowed the same privilege.

Comets may be regarded as the scouts or pioneers of the solar system. The ancients supposed we were encased in crystal spheres; but the comets in their course shivered them to atoms. Descartes then encircled us with vortices, but the comets in the hands of Newton stopped their action for ever. They have told us also of a resisting medium, and hinted at the probable doom of the solar system; and, no doubt, there are other high functions assigned them. We may regard a comet as a plumb-line let down into the depths of space, to explore the nature of the currents, and the objects that may exist far beyond the reach of vision. The change in their constitution, and the disturbance of their orbits, may tell us of the existence of worlds which the telescope may never reach. As the lead of the mariner, with its adhesive surface, brings up unmistakable evidence of the nature of the bottom, so comets, when we understand them better, may bring interesting news of regions hitherto unexplored. Comets rush forth from the starting-post of the sun with the speed of lightning, but they soon slacken their pace as if to feel their way; and, by the time they reach the end of their