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 Path—Absorption 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 Stone-falls—The Meteorite at Ensisheim—Collections of Meteorites—The Rowton Siderite—Relative Frequency of Iron and Stony Meteorites—Fragmentary Character of Meteorites—Tschermak's Hypothesis—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 often 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 an interesting and sometimes in a striking manner.
These small bodies compensate in some degree for their minute size by the 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 sun does not disdain to undertake their control. Each particle, whether it be as small as the mote in a sunbeam or as mighty as the planet Jupiter, must perforce trace out its path around the sun in conformity with the laws of Kepler.
Who does not know that beautiful occurrence which we call a shooting star, or which, in its more splendid forms, is sometimes called a meteor or fireball? It is to objects of this class that we are now to direct our attention.
A small body is moving round the sun. Just as a mighty planet revolves in an ellipse, so even a small object will be guided 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 never see them except under extraordinary circumstances.
When the meteor flashes into view it is moving with such enormous velocity that it often traverses 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 impede its flight. It may have been moving round and round the sun for thousands, perhaps for millions of years, without suffering any interference; but the supreme moment arrives, and the meteor perishes in a streak of splendour.