Page:The story of the comets.djvu/46

24 Curvature of the tail is a very common feature, especially in the case of large naked-eye comets. Sometimes the appearance is that of a tail originally straight, which has become bent into the form of a cavalry sabre; at others the bending is accompanied by a lateral swelling out at the extremity, after the fashion of a Turkish scymitar. The Comets of 1844 (iii.) and Donati's Comet of 1858 are good examples of comets with curved tails, whilst the great Comet of 1882 was a notable example of a scymitar tail at one period of its visibility—but of that comet more hereafter, for other reasons.

After a tailed comet has passed round the Sun at the epoch of perihelion and starts on its way back into Space the tail usually more or less precedes the head instead of following it. This fact opens up a difficulty which can be stated more easily than it can be solved. Whilst the doctrine of Gravitation assuredly applies to comets which come within the reach of the Sun and are thus drawn towards the Sun, yet even before as well as after they have reached their least distance from the Sun they mysteriously become subject to a repulsive solar action of some sort which it is difficult to define or explain, which has been truly said to have "no known counterpart in any other observed fact of nature", and weakens the theory of Gravitation.

The Comet of 1769 had a double curved tail thus ∽ according to La Nux, who observed it at the Isle of Bourbon. The great Comet of 1882 exhibited a striking and uncommon form of tail, some account of which will be given in a later chapter.

Fig. 10 illustrates the changes in the direction of the tail of a comet as it comes up to the Sun, passes its perihelion, or point of nearest approach to the Sun, and then goes away from the Sun. It is intended to show that a tail which, when the comet is still far oft' from the Sun, is straight becomes curved as the comet's motion becomes more rapid. That as the curvature of the orbit gets sharper so will the tail exhibit itself as curved will be seen from inspection of the diagram to