Page:The evolution of worlds - Lowell.djvu/59

Rh ellipse, and those which do not, the hyperbolæ. If a body travel in the first or closed class about the Sun, it is clearly a member of his family; if in the second, it is a visitor who bows to him only in passing and never returns. Which orbit it shall pursue depends at a given distance solely upon the speed of the body; if that speed be one the Sun can control, the body will move in an ellipse; if greater, in an hyperbola. Obviously the Sun can control just the speed he can impart. Now a comet entering the system from without would already possess a motion of its own which, when compounded with the solar-acquired speed, would make one greater than the Sun could master. Comets, therefore, if visitors from space, should all move in hyperbolæ. None for certain do; and only six out of four hundred even hint at it. Comets, then, are all members of the solar family, excentric ones, but not to be denied recognition of kinship for such behavior.

Still, admittance to the solar family circle was denied to meteorites and shooting-stars. Thus Professor Kirkwood, in 1861, had considered "that the motions of some luminous meteors (or cometoids, as perhaps they might be called) have been decidedly indicative of an origin beyond the limits of the solar system." Here cometoid was an apt coinage, but when comets were later shown not to be of extra-solar origin, the reasoning