[Illustration: Fig. 20.—Wells’s Comet of 1882, seen in full daylight near the Sun on Sept. 18.]
[Illustration: Fig. 21.—Quenisset’s Comet, July 9, 1893 (Quenisset).]
The tails of comets exhibit very great varieties not only of size but of form; some are long and slender; some are long and much spread out towards their ends, like quill pens, for instance; some are short and stumpy, mere tufts or excrescences rather than tails. Not unfrequently a tail seems to consist of two parallel rays with no cometary matter, or it may be only a very slight amount of cometary matter traceable in the interspace; some have one main tail consisting of a pair of rays such as just described, together with one or more subsidiary or off-shoot tails. The comet of 1825 had five tails and the comet of 1744 had six tails. It might be inferred from all this that the tails of comets are so exceedingly irregular, uncertain and casual as to be amenable to no laws. This was long considered to be the case; but a Russian observer named Bredichin, as the result of much study and research, has arrived at the conclusion that all comet tails may be brought under one or other of three types; and that each type is indicative of certain distinct differences of origin and condition which he considers himself able to define. The first type comprises tails which are long and straight; “they are formed” (to quote Young’s statement of Bredichin’s views) “of matter upon which the Sun’s repulsive action is from twelve to fifteen times as great as the gravitational attraction, so that the particles leave the comet with a relative velocity of at least four or five miles a second; and this velocity is continually increased as they recede, until at last it becomes enormous, the particles travelling several millions of miles in a day. The straight rays which are seen in the figure of the tail of Donati’s Comet, tangential to the tail, are streamers of this first type; as also was the enormous tail of the comet of 1861. The second type is the curved plume-like train, like the principal tail of Donati’s Comet. In this type the repulsive force varies from 2.2 times gravity (for the particles on the convex edge of the tail) to half that amount for those which form the inner edge. This is by far the most common type of cometary train. A few comets show tails of the third type—short, stubby, brushes violently curved, and due to matter of which the repulsive force is only a fraction of gravity—from 1/10 to ½.”