Page:The story of the comets.djvu/44

CHAPTER III.

THE TAILS OF COMETS.

Tails usually a prolongation of the Radius Vector.—Occasionally the tail faces the Sun.—Then called a "beard".—Comets with several tails.—The Comet of 1825.—The Comet of 1744 with 6 tails.—Curvature of Tails.—Repulsive Action of the Sun on Tails of Comets.—Changes of Direction of Tails.—Tails probably hollow cones or hollow cylinders. Vibration of Tails.—Jets of Light in the heads of Comets.—Formation of Envelopes.—Fans of Light.—Abnormal Changes in the Tails of certain recent Comets.—Swift's Comet of 1892 (i.).—Brooks's Comet of 1893 (iv.).—Observations by Barnard.—Morehouse's Comet of 1908 (iii.).—Speculations as to the formation of Tails.—Bredichin's classification of Tails.—(1) Long straight Rays.—(2) Curved plume-like Trains.—(3) Short, stubby, and sharply curved brushes of light.—What is the material of which Tails are made?—Speculation on the subject not very profitable.—Electricity and Light-pressure probably co-operating influences.—Summary by Maunder.

Some of the more usual and prominent features connected with the tails of comets from the standpoint of recorded facts will now be dealt with, leaving more or less on one side the vast mass of theory and speculation which surrounds the subject.

It was observed by Peter Apian that the trains of 5 comets seen by him between the years 1531 and 1539 were turned from the Sun, forming more or less a prolongation of the radius vector, which is the name given to an imaginary line joining the centre of the Sun and the centre of the head of a comet.^[1] This may be regarded as a general rule, although exceptions do occur. Thus the tail of the Comet of 1577 deviated 21° from the line of the radius vector. Valz stated that the tails of the Comets of 1863 (iv. and v.) deviated from the planes of the orbits, and that only 2 other comets are known the tails of which did the same. In some few instances, where a comet has had more than one tail, the second

1. Comptes Rendus, vol. lviii, p. 853. 1864.