8 THE POPULAR SCIENCE MONTHLY 



as he did that they moved in straight lines. Finally Dorfel, a clergy- 

 man 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 



Si i; Isaac Xewtox. 



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. Er- 

 rors, however, die hard, and this particular form manifested the usual 

 vitality, in fact, it has not even now entirely disappeared. 



