CHAPTER XVII.

SHOOTING STARS.

Small Bodies of our System-Their Numbers-How they are Observed-The Shooting Star-The Theory of Heat-A great Shooting Star-The November Meteors-Their Ancient History-The Route followed by the Shoal-Diagram of the Shoal of Meteors-How the Shoal becomes Spread out along its PathAbsorption of Meteors by the Earth-The Discovery of the Relation between Meteors and Comets-The remarkable Investigations concerning the November Meteors-Two Showers in Successive Years-No Particles have ever been Identified from the great Shooting Star Showers-Meteoric Stones-Chladni's Researches Early Cases of Stonefalls-The Meteorite at Ensisheim-Collections of Meteorites-The Rowton Siderite-Relative frequency of Iron and Stony Meteorites-Fragmentary Character of Meteorites-No Reason to connect Meteorites with Comets-Tschermak's Theory-Effects of Gravitation on a Missile ejected from a Volcano-Can they have come from the Moon?-The Claims of the Minor Planets to the Parentage of Meteorites-Possible Terrestrial Origin-The Ovifak Iron.

In the preceding chapters we have dealt with the gigantic bodies which form the chief objects in what we know as the solar system. We have studied mighty planets measuring thousands of miles in diameter, and we have followed the movements of comets, whose dimensions are to be told by millions of miles. Once, indeed, in a previous chapter, we have made a descent to objects much lower in the scale of magnitude, and we have examined that numerous class of small bodies which we call the minor planets. It is now, however, our duty to make a still further, and this time a very long step, downwards in the scale of magnitude. Even the minor planets must be regarded as colossal objects, when compared with those little bodies whose presence is revealed to us in a most interesting, and sometimes in a most striking manner.

These small bodies compensate in some degree for their minute size, by the enormous profusion in which they exist. No attempt, indeed, could be made to tell in figures the myriads in which they swarm throughout space. They are probably of very varied dimensions, some of them being many pounds or perhaps tons in

weight, while others seem to be not larger than pebbles, or even than grains of sand. Yet, insignificant as these bodies may seem, the great sun himself does not disdain to accept their control. Each particle, whether it be as small as the mote in a sunbeam or as mighty as the planet Jupiter, will perform its path around the sun in conformity with the laws of Kepler.

Who does not know that very beautiful occurrence which we call a shooting star, or which, in its more splendid forms, is sometimes called a meteor or fire-ball? It is to objects of this class that we are now to to direct our attention.

A small body, perhaps as large as a paving-stone or larger, more often perhaps not as large as a marble, is moving round the sun. Just as a mighty planet revolves in an ellipse, so this small object will move round and round in an ellipse, with the sun in the focus. There are, at the present moment, inconceivable myriads of such meteors moving in this manner. They are too small and too distant for our telescopes, and we can never see them except under extraordinary circumstances.

At the time we see the meteor it is usually moving with enormous velocity, so that it often traverses a distance of more than twenty miles in a second of time. Such a velocity is almost impossible near the earth's surface: the resistance of the air would prevent it. Aloft, in the emptiness of space, there is no air to resist the meteor. It may have been moving round and round the sun. for thousands, perhaps for millions of years, without let or hindrance; but the supreme moment arrives, and the meteor perishes in a streak of splendour.

In the course of its wanderings the body comes near the earth, and within a few hundred miles of its surface of course begins to encounter the upper surface of the atmosphere with which the earth is enclosed. To a body moving with the appalling velocity of a meteor, a plunge into the atmosphere is usually fatal. Even though the upper layers of air are excessively attenuated, yet they suddenly check the velocity, almost as a rifle bullet would be checked when fired into water. As the meteor rushes through the atmosphere the friction warms the surface of the meteor; gradually

it becomes red-hot, becomes white-hot, and is usually driven off into vapour with a brilliant light, while we on the earth, one or two hundred miles below, exclaim: "Oh, look! there is a shooting star."

We have here on a very striking scale an experiment illustrating the mechanical theory of heat. It may seem incredible that mere friction should be sufficient to generate heat enough to produce so brilliant a display, but we must recollect two facts: first, that the velocity of the meteor is, perhaps, one hundred times that of a rifle bullet; and second, that the efficiency of friction in developing heat is proportioned to the square of the velocity. The meteor in passing through the air must therefore be heated by the friction of the air about ten thousand times as much as the rifle bullet is heated. We do not make an exaggerated estimate in supposing that by its rush through the air the rifle bullet becomes heated ten degrees; yet if this be admitted, we must grant that there would be such an enormous development of heat attending the flight of the meteor that even a fraction of it would be sufficient to drive the object into vapour.

Let us first consider the circumstances under which these external bodies are manifested to us, and for the sake of illustration, we may take a remarkable fire-ball which occurred on November 6th, 1869. This fire-ball was extensively seen from different parts of England; and by combining and comparing these observations, we obtain accurate information as to the height of the object and the velocity with which it travelled.

It appears that this meteor commenced to be visible at a point ninety miles above Frome, in Somersetshire, and that it disappeared at a point twenty-seven miles over the sea, near St. Ives, in Cornwall, both the path, its height, and the principal localities from which it was observed being shown in the map (Fig. 62). The whole length of its course was about 170 miles, which was performed in a period of five seconds, thus giving an average velocity of thirty-four miles per second. A remarkable feature in the appearance which this fire-ball presented was the long persistent streak of luminous cloud, about fifty miles long and four

miles wide, which remained in sight for fully fifty minutes. We have in this example an illustration of the chief features of the

Fig. 62.-The Path of the Fireball of November 6th, 1869.

phenomena of a shooting star presented on a very grand scale. It is, however, to be observed that the persistent luminous streak is not a universal, nor, indeed, a very common characteristic of a shooting star.

The small objects which occasionally flash across the field of

the telescope show us that there are innumerable telescopic shooting stars, too small and too faint to be visible to the unaided eye. These objects are all dissipated in the way we have described; it is, in fact, only at the moment, and during the process of their dissolution, that we become aware of their existence. Small as these missiles probably are, their velocity is so prodigious, that they would render the earth uninhabitable were they permitted to rain down unimpeded on its surface. We must, therefore, among the other good qualities of our atmosphere, not forget that it constitutes a kindly screen, which shields us from a tempest of missiles, the velocity of which no artillery could equal. It is, in fact, the very fury of these missiles which is the cause of their utter destruction. Their anxiety to strike us is so great, that friction dissolves them into harmless vapour.

Next to the splendid event of a mighty meteor such as that we have just described, the most striking occurrence in connection with shooting stars is what is known as a shower. These showers have, within the last century, attracted a great deal of attention, and they have abundantly rewarded the labour devoted to them by affording some of the most interesting astronomical discoveries of modern times.

The showers of shooting stars do not occur very frequently. No doubt the quickened perception of those who especially attend to meteors will detect a shower when others see only a few straggling shooting stars; but speaking generally, we may say that the present generation can hardly have witnessed more than two or three such occurrences. I have myself seen two great showers, one of which, in November, 1866, has impressed itself on my memory as a most glorious astronomical spectacle.

To commence the story of the November meteors it is necessary to look back for nearly a thousand years. In the year 902 occurred the death of a Moorish king, and in connection with this event an old chronicler relates how "that night there were seen, as it were lances, an infinite number of stars, which scattered themselves like rain to right and left, and that year was called the Year of the Stars."