round also, pointing all the time right away from the great luminary. Then, on reaching the other side of the Sun, he once more begins his retreat, wandering away and away for another two hundred thousand years,

ORBIT OF A COMET, WHICH WANDERS OFF NEVER TO RETURN

into realms of utter darkness and icy cold, going ever more and more slowly, as the Sun's pull lessens with distance. And while he thus travels, his tail now goes first and the head follows after, and gradually the tail itself disappears.

Such, in brief, is the story of certain comets belonging

UNING OF CALIFORNIA

to our System. There are others which perhaps do not so belong; or, if they once did, they have broken loose from the Sun's control. So great is their speed that not all his drawing power can hold them. They pay a passing visit, and wander off, never to return.

It is believed that the Sun exercises a curious repulsive power over the tails of comets, driving them always outward and away from himself; and it has been suggested that possibly this repulsion may reside in light.

"That very law which moulds a tear
And bids it trickle from its source,
That law preserves the earth and sphere
And guides the planets in their course.

SAMUEL ROGERS.

A word of explanation here as to the curious" streaks" on the comet-photograph. They really are stars. The telescope, travelling with the surface of our Earth, and moved by clockwork to counteract that motion, is steadily fixed on the comet, thus taking its "likeness during a more or less long exposure. But the far-off stars, lying at great distances beyond, are left behind by the moving telescope as it follows the comet, and are "as it were dragged across the field of view, leaving each one a trail of light on the photographic plate." This is not seen in a photograph of a nebula; for the nebulæ are, roughly and as concerns the telescope, on the same plane as the stars; whereas the comet, when photographed, is enormously nearer, probably either within, or not very far from, the limits of the Solar System.

III. ROUGH ORE OF THE UNIVERSE

Some little way back a connection between comets and meteorites was mentioned. We have now to think about this connection.

And first-what are meteorites?

If present theories hold good, they are the rough ore of the Solar System, and indeed of the Universe; the raw material out of which suns and worlds, satellites and comets are fashioned.

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A meteorite, such as those that have been found to have fallen on the Earth, may be of any size, from a mass of iron or other solid substance as big as a house, down to balls no bigger than a cricket-ball, or a marble, or a pea, or even a grain of sand-though by that time we are getting down to meteoric dust.

The head of a comet is believed to contain, or to consist of, masses of meteoric iron, large or small. The tail probably contains or consists of nothing more substantial than the finest meteoric dust; specks of matter floating widely apart, each reflecting the Sun's rays. In addition to this, comets very likely shine in part by their own light.

Meteorites in enormous numbers-numbers beyond calculation, beyond imagination-exist throughout the Solar System; and hundreds of them are perpetually falling hither. When one happens to wander so near as to be captured by Earth's pull, then, as the small body rushes earthward, the friction of our atmosphere heats it intensely, making it glow with a brilliant light.

If we happen just then to glance in the direction