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has already been stated, but without much explanation, that all comets move in orbits which are either elliptic, parabolic, or hyperbolic—3 of the 4 possible sections of a cone.

Fig. 91 represents the various sections of a right cone, as it is called, and will convey a better idea of Cometary Orbits than a verbal description would do. When a right cone is cut at right angles to its axis the resulting section A B, will be a Circle: no comet, however, is known to revolve in a circular, or even in anything like a circular orbit, though, on the other hand, all the planetary orbits may be said to be nearly circular. When a cone is cut obliquely, so that the inclination of the cutting plane to the axis of the cone is greater than the constant angle formed by the generating line of the cone and the axis, as C D, the resulting section will be an Ellipse, the shape of which will vary from almost a circle on the one hand to almost