Page:The story of the comets.djvu/45

III. tail has extended more or less towards the Sun. Such a tail has been sometimes spoken of as a "beard". Amongst the recent comets which have had such an appendage may be mentioned those of 1823, 1848 (ii.), 1851 (iv.), 1877 (ii.), and 1880 (vii.).

Although the credit of noticing that the tails of comets are usually turned away from the Sun is ascribed to P. Apian, the researches of E. Biot shows that this fact was noted by the Chinese long before the time of Apian, to wit, in the year 837 A.D., when a brilliant comet was visible.

Although comets usually have but one tail, 2 are not uncommon, whilst even that number is often increased by the presence of slender streamers, which are virtually independent tails. The great Comet of 1825 seen by Dunlop in Australia had 5 tails, and that of 1744 had as many as 6. This last statement depending as it did, for a long time, on the unconfirmed testimony of a Swiss astronomer named De Chéseaux, used not to be believed, but there is now no doubt as to its authenticity. The 3rd Comet of 1903 (Borelly's) was photographed at Greenwich showing 9 tails, all told, but they required some looking for. It seems certain now that photography often reveals tails of which telescopes and naked eyes take no account.

When a comet has 2 tails it may happen that both are of about the same size and length; or that the second tail is not so much to be regarded as a second independent tail as a little offshoot of one main tail. In this case the secondary tail is usually less bright and much shorter than the main tail. For instance, Pons's long-period Comet of 1812 at its reappearance in 1886 had on December 29 a principal tail 8° long and a secondary one very faint and only 3° long; but the secondary tail is not always the shorter of the 2. Swift noted the secondary tail of the Comet of 1881 (ii.) to have been 55° long—the longest secondary tail on record.