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168 a straight line directed towards the Sun's centre. Another small body, likewise at a distance practically infinite, has a slight motion of its own, but is not moving directly toward the Sun; urged on by the Sun's imperious attraction, its velocity will continually increase; however, as it is not going directly toward the Sun, it will not strike it, but as it goes past, the pull of the Sun will cause its path to be violently curved; whirling around the Sun, it will return toward the infinite depths of space from which it came, its orbit becoming a parabola. A body which has originally a very considerable velocity of its own will come down to the sun in an hyperbolic orbit, and then retreat, never again to visit us. A body moving in a parabola may have its velocity checked, as it approaches the Sun, by the attraction of some planet; its orbit will thus be changed to an ellipse. Were the movement of the body accelerated by the planet's action, the orbit would become an hyperbola."

Scarcely less interesting than the question "What is a comet?" are the cognate questions "Why do comets come to us?" and "Where do they come from?" It is obvious that these questions can only be answered in a hypothetical and inadequate fashion, but still it is possible to say something. Two provisional answers suggest themselves: either (1) comets are chance visitors wandering through space and now and again casually caught up by the Sun, or by some of the major planets acting like the old naval press-gang and compelling them to attach themselves to the Sun and by taking elliptic orbits to become permanent members of the solar system; or (2) they are aggregations of primæval matter not formed by the Creator into substantial planets, but left lying about in space to be picked up and gathered into entities as circumstances permit.

C. L. Poor has graphically summarised the situation. At the risk of a little repetition. I will transcribe what he says :—"They have been considered as true wanderers, travelling through space, drifting hither and thither, just as the Sun, with its attendant retinue of planets, is moving onward in some unknown path. When the paths of the Sun and such