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'but in spite of the strictest watching nothing was seen of it. It is believed that, like Lexell's comet, it has been diverted from its course by some member of our system, and that in this case the November meteors may have been the disturbing cause.

296. It has been estimated that there may be many millions of comets belonging to our system, and perhaps passing between this and other systems. We see but few of them, because those only are visible to us which are well placed for observation when they pass the Earth in their journey to or from perihelion, while there may be thousands which at their nearest approach to the Sun are beyond the orbit of Neptune.

297. In the case of a comet without a nucleus, we have reason to believe that the coma is a mass of white-hot gas, similar in composition to that of which the nebulæ are composed; but we do not yet know that when we see a bright comet with a nucleus it is composed of similar material; one thing is certain, that as the tail indicates the waste, so to speak, of the head, each return to the Sun must reduce the mass of the comet. A reduction of speed would in time be enough to reduce the most refractory comet into a quiet member of the solar family, as the orbit would become less elliptical, or more circular, at each return to perihelion. This effect has, in fact, been observed in some of the short-period comets. Encke's comet, for instance, now performs its revolution round the Sun in three days less than it did eighty years ago. It has been affirmed that this effect is due to the friction offered by the ethereal medium-an effect we do not perceive in the case of the planets, as their mass is so much larger--as the resistance of the air stops the flight of a feather sooner than it does that of a stone. Sir Isaac Newton has calculated that a cubic inch of air at the Earth's surface-that is, as much as is contained in a

K

good-sized pill-box--if reduced to the density of the air 4,000 miles above the surface, would be sufficient to fill a sphere the circumference of which would be as large as the orbit of Neptune. The tail of the largest comet, if it be gas, may therefore weigh but a few ounces or pounds; and the same argument may be applied to the comet itself, if it be not solid. We can understand, then, that with such a small supply there is not much room for waste, and with such a small mass the resistance offered to it may easily become noticeable.

LESSON XXIV.-LUMINOUS

METEORS.

SHOOTING

RADIANT POINTS.

STARS. NOVEMBER SHOWERS.

298. There are very few nights in the year in which, if we watch for some time, we shall not see one of those appearances which are called, according to their brilliancy, meteors, bolides, or falling or shooting stars. On some nights we may even see a shower of falling stars, and the shower in certain years is so dense that in some places the number seen at once equals the apparent number of the fixed stars seen at a glance ;* indeed, it has been calculated that the average number of meteors which traverse the atmosphere daily, and which are large enough to be visible to the naked eye on a dark clear night, is no less than 7,500,000; and if we include meteors which would be visible in a telescope, this number will have to be increased to 400,000,000! so that, in the mean, in each volume as large as the Earth, of the space which the Earth traverses in its orbit about the Sun, there are as many as 13,000 small bodies, each body such as would furnish a shooting

* Baxendell.

star, visible under favourable circumstances to the naked eye. If telescopic meteors be counted, this number should be increased at least forty-fold.

299. It is now generally held that these little bodies are not scattered uniformly in the space comprised by the Solar System, but are collected into several groups, some of which travel like comets, in elliptic orbits round the Sun; and that what we call a shower of meteors is due to the Earth breaking through one of these groups. Two such groups are well defined, and we break through them in August and November in each year. The exquisitely beautiful star-shower which was witnessed during the year 1866 has placed the truth of this explanation beyond all doubt. Let us consider how the appearances observed are connected with the theory, and what the theory actually is in its details.

300. Here again we must fall back upon our imaginary ocean (Art. 105) to represent the plane of the ecliptic. Let us further suppose that the Earth's path is marked out by buoys placed at every degree of longitude, beginning from the place occupied by the Earth at the autumnal equinox, and numbered from right to left from that point. Now if it were possible to buoy space in this way, we should see the November group of meteors rising from the plane at the point occupied by our Earth on the 14th of November.

301. But why do we not have star-showers every November? Because the orbit of the meteors has the principal mass of the little bodies in one part of it, its extent along the elliptic orbit being such that it requires two or three years to make its passage round the Sun. So that to get a star-shower we must not only go through the orbit, but through that exact part of it where the mass is collected. Hence we do not go through the group every year, because the mass of little bodies performs its revolu

tion like a comet, in 33 years. So that if we go through the mass one year, it will have passed the node the next year, and we shall not have a shower again until the mass happens to be at the node again thirty-three years after.

302. Now what will happen when the Earth, sailing along in its path, reaches the node and encounters the mass of meteoric dust, the particles of which travel, as we know, in the opposite direction?

303. Let us in imagination connect the Earth and Sun by a straight line: at any moment the direction of the Earth's motion will be at right angles to that line (or, as it is called, a tangent to its orbit): therefore, as longitudes are reckoned, as we have seen, from right to left, the motion will be directed to a point 90° of longitude behind the Sun. The Sun's longitude at noon on the 14th November, 1866, was 232° within a few minutes; 90° from this gives us 142°.

304. As therefore the meteors, as we meet them in our journey, should seem to come from the point of space towards which the Earth is travelling, and not from any side street as it were, we ought to see them coming from a point situated in longitude 142°, or thereabouts. Now what was actually seen?

305. One of the most salient facts, noticed by those even who did not see the significance of it, was that all the meteors seen in the late display really did seem to come from one part of the sky. In fact, there was a region in which the meteors appeared trainless, and shone out for a moment like so many stars, because they were directly approaching us. Near this spot they were so numerous, and all so foreshortened, and for the most part faint, that the sky at times put on almost a phosphorescent appear ance. As the eye travelled from this region, the trains became longer, those being longest as a rule which first made their appearance over head, or which trended west

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