These discoveries are especially interesting because they seem to show the uniformity of material composing our system. We already knew that sodium and iron abounded in the sun, and now we have learned that these bodies and carbon as well are present in the comets. In the next chapter we shall learn that the very same materials—sodium and iron—are met with in bodies far more remote from us than any bodies of our own system.
Comets have such a capricious habit of dashing into the solar system at any time and from any direction, that it is worth while asking whether a comet might not sometimes happen to come into collision with the earth. There is nothing impossible in such an occurrence. There is, however, no reason to apprehend that any disastrous consequences would ensue to the earth. Man has lived on this globe for many, many thousands of years, and the rocks are full of the remains of fossil animals which have flourished during past ages; indeed, we cannot possibly estimate the number of millions of years that have elapsed since living things first crawled about this globe. There has never been any complete break in the succession of life, consequently during all those millions of years we are certain that no such dire calamity has happened to the earth as a frightful collision would have produced, and we need not apprehend any such catastrophe in the future.
I do not mean, however, that harmless collisions with comets may not have occasionally happened; in fact, there is good reason for knowing that they have actually taken place. In the year 1861 a fine comet appeared; and it is not so well remembered as its merits deserve, because it happened, unfortunately for its own renown, to appear just three years after the comet of 1858, which was one of the most gorgeous objects of this kind in modern times. But in 1861 we had a novel experience. On a Sunday evening in midsummer of that year, we dashed into the comet, or it dashed into us. We were not, it is true, in collision with its densest part; it was rather the end of the tail which we encountered. There were, fortunately, no very serious results. Indeed, most of us never knew that anything had happened at all, and the rest only learned of the accident long after it was all over. For a couple of hours that night it would seem that we were actually in the tail of the comet, but so far as I know no one was injured or experienced any alarming inconvenience. Indeed, I have only heard of one calamity arising from the collision. A clergyman tells us that at midsummer he was always able in ordinary years to read his sermon at evening service without artificial light. On this particular occasion, however, the sky was overcast with a peculiar glow, while the ordinary light was so much interfered with that the sexton had to provide a pair of candles to enable him to get through the sermon. The expense of those candles was, I believe, the only loss to the earth in consequence of its collision with the comet of 1861.
Fig. 76.—How the Tail of a Comet arises.
The tail of a comet appears to develop under the influence of the sun. As the wandering body approaches the source of central heat it grows warm, and as it gets closer and closer to the sun, the fervor becomes greater and greater, until sometimes the comet experiences a heat more violent than any we could produce in our furnaces. The most infusible substances, such as stones or earth, would be heated white-hot and melted, and even driven off into vapor, under the intense heat to which a comet is sometimes exposed. Comets, indeed, have been known to sweep round the sun so closely as to pass within a seventh part of its radius from the surface. It seems that certain materials present in the comet, when heated to this extraordinary temperature, are driven away from the head, and thus form the tail ([Fig. 76]). Hence we see that the tail consists of a stream of vaporous particles, dashing away from the sun as if the heat which had called them into being was a torment from which they were endeavoring to escape.
The tail of a comet is, therefore, not a permanent part of the body. It is more like the smoke from a great chimney. The smoke is being incessantly renewed at one end as the column gets dispersed into the air at the other. As the comet retreats, the sun’s heat loses its power. In the chills of space there is, therefore, no tail-making in progress, while the small mass of the comet renders it unable to gather back again by its attraction the materials which have been expelled. Should it happen that the comet moves in an elliptic orbit, and thus comes back time after time to be invigorated by a good roasting from the sun, it will, of course, endeavor to manufacture a tail each time that it approaches the source of heat. The quantity of material available for the formation of tails is limited to the amount with which the comet originally started; no fresh supply can be added. If, therefore, the comet expends a portion of this every time it comes round, an inevitable consequence seems to follow. Suppose a boy receives a sovereign when he goes back to school, and that every time he passes the pastry-cook’s shop some of his money disappears in a manner that I dare say you can conjecture, I need not tell you that before long the sovereign will have totally vanished. In a similar way comets cannot escape the natural consequences of their extravagance; their store of tail-making substance must, therefore, gradually diminish. At each successive return the tails produced must generally decline in size and magnificence, until at last the necessary materials have been all squandered, and we have the pitiful spectacle of a comet without any tail at all.
The gigantic size of comets must excite our astonishment. A pebble tossed into a river would not be more completely engulfed than is our whole earth when it enters the tail of one of these bodies. But we now pass by a sudden transition to speak of the very smallest bodies, of little objects so minute that you could carry them in your waistcoat pocket. You will perhaps be surprised that such things can play an important part in our system and have a momentous connection with mighty comets.
METEORS.
If you look out from your window at the midnight sky, or take a walk on a fine clear night, you will occasionally see a streak of light dash over the heavens, thus forming what is called a falling, or a shooting, star ([Fig. 77]). It is not really one of the regular stars that has darted from its place. The objects we are now talking of are quite different from stars proper. To begin with, the shooting stars are comparatively close to us when we see them, and they are very small, whereas the stars themselves are enormous globes, far bigger than our earth, or often even bigger than the sun. Sometimes a great shooting star is seen which makes a tremendous blaze of light as bright as the moon, or even brighter still. These objects we call meteors, and you will be very fortunate if you can ever see a really fine one. Astronomers cannot predict these things as they predict the appearance of the planets. Bright meteors consequently appear quite unexpectedly, and it is a matter of chance as to who shall enjoy the privilege of beholding them. But it is not about the great meteors that we are now going to speak particularly; they are often not so interesting as the small ones.