It may also be difficult to understand why the sun does not keep the comet when at last it has arrived. Why should the wandering body be in such a hurry to recede? Surely it might be expected that the attraction of the sun ought to hold it. If something were to check the pace of the comet in its terrific dash round the sun, then, no doubt, the object would simply tumble down into the sun and be lost. The sun has, however, not time to pull in the comet when it comes up with a speed 20,000 times that of an express train. But the sun does succeed in altering the direction of the motion of the comet, and the attraction has shown itself in that way.
I can illustrate what happens in this manner. Here is a heavy weight suspended from the ceiling by a wire; it hangs straight down, of course, and there it is kept by the pull of the earth. Supposing I draw the weight aside and allow it to swing to and fro, then the motion continues like the beat of a pendulum. The weight is always pulled down as near to the earth as possible, but when it gets to the lowest point, it does not stay there, it goes through that point, and rises up at the other side. The reason is that the weight has acquired speed by the time it reaches the lowest point; and that, in virtue of its speed, it passes through the position in which it would naturally rest, and actually ascends the other side in opposition to the earth’s pull, which is dragging it back all the time. This will illustrate how the comet can pass by and even recede from the body which is continually attracting it.
Just a few words of caution must be added. Suppose you had an ellipse so long that the comet would take thousands and thousands of years to complete a circuit, then the part of the ellipse in which the comet moves during the time when we can see it is so like a parabola, that we might possibly be mistaken in the matter. In fact, a geometer will tell us that if one end of an ellipse was to go further and further away, the end that stayed with us would gradually become more and more like this curve. Therefore, some of those comets which seem to move in parabolas may really be moving in extremely elongated ellipses, and thus, after excessively long periods of time, may come back to revisit us.
THE MATERIALS OF A COMET.
A comet is made of very unsubstantial material. This we can show in a very interesting manner, when we see it moving over the sky between the earth and the stars. Sometimes a comet will pass over a cluster of very small stars, so faint that the very lightest cloud that is ever in the sky would be quite sufficient to hide them. Yet the stars are distinctly visible right through the comet, notwithstanding that it may be hundreds of thousands of miles thick. This shows us how excessively flimsy is the substance of a comet, for while a few feet of haze or mist suffice to extinguish the brightest of stars, this immense curtain of comet stuff, whatever it may be made of, is practically transparent.
I have often told you that we are able to weigh the heavenly bodies, but a comet gives us a great deal of trouble. You see that the weighing machine must be of a very delicate kind if you are going to weigh a very light object. Take, for example, a little lock of golden hair, which no doubt has generally a value quite independent of the number of grains that it contains. Suppose, however, that we are so curious as to desire to know its weight, then one of those beautiful balances in our laboratories will tell us. In fact, if you snipped a little fragment from a single hair, the balance would be sensitive enough to weigh it. If, however, you were only provided with a common pair of scales like those which are suited for the parcel post, then you could never weigh anything so light as a lock of hair. You have not small enough weights to begin with, and even if you had they would be of no use, for the scale is too coarse to estimate such a trifle. This is precisely the sort of difficulty we experience when we try to weigh a comet. The body, though so big, is very light, and our scales are so cumbersome that we are in a position of one who would try to weigh a lock of hair with a parcel-post balance. We cannot always find suitable scales for weighing celestial bodies. We have to use for the purpose whatever methods of discovering the weights happen to be available. So far, the methods I have mentioned are of the rudest description; they serve well enough for weighing heavy masses like planets, but they will not do for such unsubstantial bodies as comets.
But, though we fail in this endeavor, i.e. to weigh comets, yet skilful astronomers have succeeded in something which at first you might think to be almost impossible. They have actually been able to discover some of the ingredients of which a comet is made. This is so important a subject that I must explain it fully.
The most instructive comet which we have seen in modern days is that which appeared in the year 1882. It was an object so great that its tail alone was double as long as from the earth to the sun. It was discovered at the observatories in the southern hemisphere early in September of that year. A little later it was observed in the northern hemisphere in extraordinary circumstances. It must be remembered that a comet is generally a faint object, and that even those comets which are large enough and bright enough to form glorious spectacles in the sky at night are usually invisible during the brightness of day. For a comet to be seen in daylight was indeed an unusual occurrence; but on the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great comet close to the sun. Unfortunately clouds intervened, and he was prevented from observing the critical occurrence just approaching. An astronomer at the Cape of Good Hope—Mr. Finlay—who had also been one of the earliest discoverers of the comet, was watching the body on the same day. He followed it as it advanced close up to the sun; bright indeed must that comet have been which permitted such a wonderful observation. At the sun’s edge the comet disappeared instantly; in fact, the observers thought that it must have gone behind the sun. They could not otherwise account for the suddenness with which it vanished. This was not what really happened. It was afterwards ascertained that the comet had not passed behind the sun; it had, indeed, come between us and our luminary. In its further progress this body showed in a striking degree the incoherent nature of the materials of which a comet is composed. It seemed to throw off portions of its mass along its track, each of which continued an independent journey. Even the central part in the head of the comet—the nucleus, as it is called—showed itself to be of a widely different nature from a substantial planetary body. The nucleus divided into two, three, four, or even five distinct parts, which seemed, in the words of one observer, to be connected together like pearls on a string.
The comet of 1882 was also very instructive with regard to the actual materials from which such bodies are made. Astronomers have a beautiful method by which they find out the substances present in a heavenly body, even though they never can get a specimen of the body into their hands. We know at least three materials which were present in this comet. The first of them is an ingredient which is very commonly found in comets—a chemist calls it carbon. It is an extremely familiar material on the earth; for instance, coal is chiefly composed of carbon. Charcoal when burned leaves only a few ashes. All the substance that has vanished during combustion is carbon; in fact, it is not too much to say that carbon is found abundantly not only in wood, but in almost every form of vegetable matter. The food we eat contains abundant carbon, and it is an important constituent in the building up of our own bodies. Generally speaking, carbon is not found in a pure state—it is associated with other substances. Soot and lampblack are largely composed of it; but the purest form of this element carbon that we know is the diamond.
It is interesting to note that carbon is certainly found as a frequent constituent of comets. The great comet of 1882 undoubtedly contained it, as well as certain other substances. Of these we know two: the first is the element sodium, an extremely abundant material on earth, inasmuch as the salt in the sea is mainly composed of it. It was also discovered that the same great comet contained another substance very common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse at Dunecht showed that iron was present in this body which had come in to visit us from the depths of space.