Periodic Comets of between 70 and 80 years.—Westphal's (1852, iv.).—Pons's (1812).—Di Vico's (1845, iv.).—Olbers's (1815).—Brorsen's (1847, v.).—Halley's.—Particulars of each of these Comets.—Return of Pons's Comet in 1883.—Observations of it by Trépied and others.—Many comets no doubt revolving in elliptic orbits, but with periods of hundreds or thousands of years.—Selected List of some of these.—The Comets of 1264 and 1556.—The Comets of 1532 and 1661.
Besides the periodic comets mentioned in the last chapter there are a large number of comets to which periods of far greater length have been assigned, amounting in many cases to thousands of years. It is obviously futile to talk about these as recognised members of the solar system, but there is a small group of 6 comets which may fairly be regarded as members of our system, although it must be confessed that at present only 3 out of the 6 have vindicated their claim by having appeared a second time. These comets are the following:—
