Page:Popular Science Monthly Volume 76.djvu/12

8 as he did that they moved in straight lines. Finally Dörfel, a clergyman of Plauen, upper Saxony, eighty years after the death of Tycho, proved by a graphic process that the orbit was parabolic. He knew nothing, however, of the physical causes underlying this motion, but shortly after this appeared the immortal "Principia" of Sir Isaac

Newton with his remarkable demonstration relating to the motion of bodies under the action of gravity, viz., that the orbit may be an ellipse, parabola or hyperbola. Thus comets took their true place as orderly members of the system, and the last reason, if any such ever existed, for the superstitious dread of previous times disappeared. Errors, however, die hard, and this particular form manifested the usual vitality, in fact, it has not even now entirely disappeared.