them, are as well accounted for by this supposed discontinuity of their material as by assuming them to be gaseous. So are the sudden changes in shape and size of the tail. It will not be visible except when the comet is in such a position as to turn a sort of flat edge towards us, so that we can look at once through a great depth of its mass. For the reflection from each elementary fragment will be so slight, that it will be only when an enormous number are ranged along the same line of vision that their aggregate light will be sufficient to affect the eye. To borrow a felicitous illustration from Professor Tait, we may see the same thing represented in miniature by the flight of a flock of sea-birds. Great numbers of them often fly about, approximately in one plane; and if they are at such a distance as not to be discernible singly, they will be equally invisible when their plane has its face turned towards us. But when a sudden sweep brings them into the plane of our vision, so that we get a number of them in one line, they start into sight at once, as a black streak against the face of the sky.

The nucleus is believed, from recent spectroscopic observations, to consist wholly or mainly of incandescent gas. But the explanation of this necessitates no addition to the meteorolite theory. In the head

of a comet, where its component fragments are crowded most closely together, there must be very frequent and violent collisions between them; and the heat generated by these impacts will convert them into vapour, just as we know to be the case when they strike our own atmosphere. Unfortunately

no large and bright comet, which could be observed in a thoroughly satisfactory manner, has appeared since the invention of the spectroscope. We may hope that the next one which visits our skies will settle the vexed question of the constitution of the tail. The polariscope tells us that it shines by reflected sunlight, but whether partially or wholly it is unable to decide. If there be any self-luminosity, the spectroscope will reveal it at once, and will show also whether it emanates from solid or gaseous matter. And even if the light is found to be all sunlight, there is reason to hope that if the cometary matter be gaseous it will modify the spectrum sufficiently to make us aware of that fact. On the other hand, if the tail consist only of solid particles, the spectrum will be purely and simply an enfeebled solar one, with all its lines absolutely unmodified. Meanwhile the scale of evidence seems to incline in favour of the newer theory. Professor Tait maintains that certain facts which have never been satisfactorily explained on

the old hypothesis can be successfully grappled with by the new one.

The paths pursued by the comets are very various indeed. Many of them, like the planets, move in ellipses round the sun, some traversing their orbits in three or four years, while others roam so far away that many centuries elapse before they again revisit our neighbourhood. A great number, however, only circle once round the sun, and never return to it again. The orbits into which, in accordance with the law of gravitation, they are bent, are so inconceivably long, that before they can reach the farther parts of them they come within the attracting influence of other stars, and are drawn off to pursue new orbits around new centres; and in this way a comet may wander through the universe for countless ages, seeking rest and finding none, till at last some star seizes it with a firmer grasp than the rest, compels it into a smaller orbit, and thus secures it as a perpetual attendant upon itself.

The question has often been asked, whether there is any likelihood of one of these nomadic bodies coming into collision with our own earth, and if this event did take place, what its effect would be upon us. On the meteoric theory the answer to this question is a startling one. The earth does come

into collision with a comet regularly twice every year, and the result is simply a shower of shooting stars, more or less numerous and brilliant, according to the density of the portion of the tail which we encounter. But we must by no means conclude that every such collision would be attended by equally harmless consequences. Fortunately the comets which we encounter are composed of small and widelyscattered fragments, but many will probably consist of far larger masses more densely crowded together. Numbers of meteoric stones are too large to be converted into vapour during their passage through our atmosphere, and reach the ground in a solid, but red-hot, state. Several of these are on record which weighed more than a hundred pounds, and one, which fell in Spain in 1810, measured thirty inches in length and weighed three-quarters of a ton. An encounter with a comet composed of such masses as this would be a frightful ordeal for the earth to pass through. Its whole surface would be bombarded for some minutes or hours with great lumps of red-hot rock, which would burn and destroy everything upon which they fell. The only chance. of the human race surviving such a catastrophe would lie in the fact that astronomers would probably foresee its advent, and warn everyone to take

LAPLACE'S NEBULAR HYPOTHESIS.

We have now completed our survey of the great system of which we ourselves form a part. Sun, planets, satellites, and comets-all the elements of the solar system-have successively passed before us; and the only heavenly orbs we have left to consider are the more distant stellar ones, which are so far removed from our own immediate ken. But before we proceed to visit those distant realms, we must glance for a moment at Laplace's great theory of the origin of our system, one of the grandest and most magnificent speculations which it has ever entered into the heart of man to conceive. It is true that Newton's discovery of the law of gravitation furnished us with a key to much that was dark and inexplicable before; it reduced the motions of the planets to harmonious symmetry, and replaced the elaborate eccentrics, cycles, and epicycles of the ancients by simpler and more familiar curves. That law, as all our readers are aware, explains why the