described, and having got a sample, he readily deduces the shape, dimensions, and position of the whole orbit. He proceeds just as one would do in cutting out figures in the sward of a garden. When a portion of a regular figure is given, it is easy to cut out the whole figure. In this way the astronomer can readily tell how near it will approach the sun, and at w^hat time. As the comet approaches the sun, internal changes can be detected. The globular mass is drawn out, the nucleus or brightest point being at the end nearest the sun. The other end soon assumes the appearance of a tail.
The forms of most large comets conform to one general type. The tail is slightly convex on one side, and has very much the appearance of a scimitar, the nucleus representing the hand with which it is grasped. The Chinese call comets "broom-stars;" and the appearance of many is very much that of a broom, bent and worn with much use. The outline of the convex, is generally better defined than that of the concave side. A comparatively dark band runs lengthways through the middle of the comet, so that it has often the appearance of two tails; the two bright streaks on each side of the tail being taken for two distinct tails.
This is the appearance to the naked eye. When the nucleus is watched with the telescope, very remarkable changes are witnessed. The luminous matter of the comet rushes forth from the nucleus,
