Page:The evolution of worlds - Lowell.djvu/87

Rh whole, and even penetrating within its orderly precincts, a third class would be visible which might be described for size as cosmic dust, and for display as heavenly pyrotechnics. Coming from all parts of space indifferently they would seem to seek the Sun in almost straight lines, bow to him in circuit, and then depart whence they came. For in such long ellipses do they journey that these seem to be parabolas. These visitants are the comets and their associates the meteor streams.

Although for purposes of discrimination we have labelled the several classes apart, an essential fact about the whole company is to be noted: that no hard and fast line can be drawn separating the several constituents from one another. In size the members of the one class merge insensibly into the other. Some of the planets are hardly larger than some of the satellites; some of the satellites than some of the asteroids; some of the asteroids than comets and shooting stars. In path, too, we find every gradation from almost perfect circularity like the orbits of lo and Europa to the very threshold of where one step more would cease to leave the body a member of the Sun's family by turning its ellipse into an hyperbola. Finally, in inclination we have every angle of departure from orthodox platitude to unconforming uprightness. This point, that heavenly bodies, like terrestrial ones, show