Page:Philosophical Transactions - Volume 003.djvu/186

 Why they do not alwayes move equal spaces in equal times; nor alwayes in a Line precisely streight, (there being almost none such in Nature but sometimes with an inclination of its Diske, so as to make a parabola, yet never the Segment of a Circle? How they observe two motions, an External, and Internal or Natural? How one of the flat sides of the Cometick Diske respecteth the Sun, as the Magnetick Needle does the North-Pole? Why the Motion of Comets is swifteft, where the Sun-beams fall perpendicularly on it? Why the motion of a recent Cometick matter is made spirally? Why that Spiral is not the same in all Comets? Why one Comet moves much swifter than another? Why the motion oi one and the same Comet increaseth and decreaseth? How a new Comet issues out of its Atmosphere? That the densest Comets move swiftest. That, though the motion of Comets be unequal, yet 'tis not irregular. Whether Comets alwayes are most vigorous in the middle of their way? When Comets describe a Semi-parabola only? Why Comets are almost never perceiv'd' by Us, when they first emerge out of their Atmosphere? That, the motion of Comets hath a conformity with that of Ships. That the Sun does the same in the deviation of Comets, what a Pilot or Ship-master doth by the meanes of Cables in the direction of Sayling. That Comets, like Ships, describe sometimes an entire, sometimes a Semi-parabola. That it appears by the inflexion of the Trajectory line, whether the way of the Cornet be Circular or Elliptick or Parabolical. Why this motion is rather Parabolical, than Hyperbolical? That the greatest deviation of Comets from the line of direction is scarce of two degrees; though 'tis not altogether improbable, but that in some it may be a little greater, especially in those, that traverse through a much vaster space of the Heavens, than others, and are of a very long duration: That the matter and figure of a Comet, and the impetus at first impressed, make much to its motion: That Comets, which come out of larger Atmospheres, and from about their Equator, and are of a compact body, are swiftest, and especially those, that have their rise from the Sun, How the motion of the Spots of the Sun may be compared with that of Comets? How the greatest velocity and tardity of the motion of Comets may be limited? That there is a Libratory motion in Comets as well as in the Moon, &c. Rh