Page:The story of the comets.djvu/39

II. pursuing a hyperbola of greater eccentricity than that affected by the larger nucleus. In the memoir from which the foregoing facts are gathered, Bredichin argues that parabolic orbits may be converted into elliptical ones not only by planetary perturbations, but by the action of the sun causing disruption of nuclei, some portions of which will be driven into elliptical orbits whilst others fall into hyperbolic orbits.

Supposing a comet to become split up into 2 or more portions, it is conceivable that each might travel round the Sun in an orbit of its own, with an independent period of revolution, and become in all senses an independent body. Undoubtedly there would be a family resemblance between the orbits as regards size, shape, and position relatively to the Sun, and the term "a family of comets" has come into use in this connection. This certainly has often led astronomers engaged in computing orbits to draw (or jump at) conclusions of identity. It cannot be said that any case yet put forward of a broken-up comet has yielded satisfactory evidence of the identity of any parts of such a comet; but a German astronomer, Berberich, some 15 years ago, offered some suggestions on this subject which I give for what they are worth. Speculation on the subject has been rife since 1770, when Lexell's Comet was discovered, and found, as was supposed, to have a period of only 5 years. It was therefore expected that, allowing for planetary perturbations, it would be seen again in 1779, but it was not seen, and never has been seen since, though not a few comets which have been visible during the last 140 years have been suggested to be identical with Lexell's.

The German astronomer just named is responsible for the assertion that the great Comet of September 1882 (iii.), was divided into 4 parts, each of which became a comet revolving round the Sun; the respective periods being 670, 770, 880, and 960 years. He went on to suggest that the Comets of 1668, 1689, 1835 (sic), 1880 (i.), and 1887 (i.) had a similar