accumulated layers, or strata, of matter becoming visible. But with this we come to a standstill; we cannot tell where the matter comes from, and still less, where it goes to; we can only record what our eyes, assisted by telescopes, tell us. There can, however, I think, be no doubt that the matter of a comet becomes displayed to our senses as the result of a process of expulsion, or repulsion, from the nucleus; and then, having become launched into space, it comes under the influence, also repulsive, of the Sun. All these things are visible facts. As to causes, we suggest little, because we know so little. Anyone who has seen a comet and has watched the displays of jets and luminous envelopes, such as I have endeavoured to set forth, will realise at once how impossible it is to describe these things in words. They must be seen either in actual being or in picture. Some further allusions to this branch of the subject may perhaps be more advantageously made after we have considered the movements and orbits of comets. [Illustration: Fig. 24.—Comet III. of 1862, on Aug 22, showing jet of luminous matter (Challis).] There is often a slight general resemblance between a planet and a comet, as regards the path which each class of body pursues. Probably the least reflective person likely to be following me here understands the bare fact, that all the planets revolve round the Sun, and are held to defined orbits by the Sun’s influence, or attraction, as it is called. Perhaps, it is not equally realised, that in a somewhat similar, but not quite the same way, comets are influenced and controlled by the Sun. Comets must be considered as regards their motions to be divisible into two classes:—(1) Those which belong to the Solar System; and (2) those which do not. Each of these two classes must again be sub-divided, if we would really obtain a just conception of how things stand. By the Comets which belong to the Sun, I mean those which revolve round the Sun in closed orbits;[5] and are, or may be, seen again and again at recurring intervals. There are 2 or 3 dozen comets which present themselves to our gaze at stated intervals, varying from about 3 to 70 years. There are again other comets which without any doubt (mathematically) are revolving round the Sun in closed orbits, but in orbits so large and with periods of revolution so long (often many centuries), that though they will return again to the sight of the inhabitants of the earth some day, yet no second return having been actually recorded, the astronomer’s prediction that they will return, remains at present a prediction based on mathematics but nothing more.