[Illustration: FIG. 72.--How the Comet’s Tail is disposed.]
We have so often seen a stream of sparks stretching out along the track of a sky-rocket, that we might naturally suppose the tail of a comet streamed out along its path in a somewhat similar manner. This would be quite wrong. You see from Fig. 72 that the tail does not lie along the comet’s path, but is always directed outwards from the sun. If you will draw a line from the sun to the head of the comet and follow the direction of the line, it shows the way in which the tail is arranged. You will also notice how the tail of the comet seems to grow in length as it approaches the sun. When the comet is first seen, the tail is often a very insignificant affair, but it shoots out with enormous rapidity until it becomes many millions of miles long by the time the comet is whirling round the sun. Those glories soon begin to wane as the comet flies outward; the tail gradually vanishes, and the wanderer retreats again to the depths of space in the same undecorated condition as that in which it first approached.
When a comet appears, it is always a matter of interest to see whether it is an entirely new object, or whether it may not be only another return of a comet which has paid us one or more previous visits. The question then arises as to how they are to be identified. Here we see a wide contrast between unsubstantial bodies like comets and the weighty and stately planets. Sketches of the various planets or of the face of the sun, though they might show slight differences from time to time, are still always sufficiently characteristic, just as a photographic portrait will identify the individual, even though the lapse of years will bring some changes in his appearance. But the drawing of a comet is almost useless for identification. You might as well try to identify a cloud or a puff of smoke by making a picture of it. Make a drawing of a comet at one appearance, and sketch particularly the ample tail with which it is provided. The next time the comet comes round it may very possibly have two tails, or possibly no tail at all. We are therefore unable to place any reliance on the comet’s personal appearance in our efforts to identify it. The highway which it follows through the sky affords the only means of recognition; for the comet, if undisturbed by other objects, will never change its actual orbit. But even this method of identification often fails, for it not unfrequently happens that during its erratic movements the comet gets into fearful trouble with other heavenly bodies. In such cases the poor comet is sometimes driven so completely out of its road that it has to make for itself an entirely new path, and our efforts to identify it are plunged in confusion. It has happened that a second comet or even a third will be found in nearly the same track, but whether these are wholly different, or whether they are merely parts of the same original object, it is often impossible to determine.