Comets fall into two classes. There are those whose orbits follow curves that are not closed, like the circle or the ellipse, but appear to extend indefinitely into space. A comet following such an orbit (parabolic or hyperbolic) seems to come wandering in from the depths of space, passes round the sun, and then gradually recedes into the space from which it came, never again to be seen of human eye. It is now becoming questionable, however, whether any comet can really be said to come in from infinite space; and the view is being more generally held that orbits which to us appear portions of unclosed curves may in reality be only portions of immensely elongated ellipses, and that all comets are really members of the solar system, travelling away, indeed, to distances that are immense compared with even the largest planetary orbit, but yet infinitely small compared with the distances of the fixed stars. Second, there are those comets whose orbits form ellipses with a greater or less departure from the circular form. Such comets must always return again, sooner or later, to the neighbourhood of the sun, which occupies one of the foci of the ellipse, and they are known as Periodic Comets. The orbits which they follow may have any degree of departure from the circular form, from one which does not differ very notably from that of such a planet as Eros, up to one which may be scarcely distinguishable from a parabola. Thus we have Periodic Comets again divided into comets of short and comets of long period. In the former class, the period ranges from that of Encke's comet which never travels beyond the orbit of Jupiter, and only takes 3·29 years to complete its journey, up to that of the famous comet whose periodicity was first discovered by Halley, whose extreme distance from the sun is upwards of 3,200,000,000 miles, and whose period is 76·78 years. Comets of long period range from bodies which only require a paltry two or three centuries to complete their revolution, up to others whose journey has to be timed by thousands of years. In the case of these latter bodies, there is scarcely any distinction to be made between them and those comets which are not supposed to be periodic; the ellipse of a comet which takes three or four thousand years to complete its orbit is scarcely to be distinguished, in the small portion of it that can be traced, from a parabola.