Periodical comets exterior to the solar system; the type of this class is Halley's comet, which is tile only comet of mean period whose return has been verified by observation–Enumeration of comets with periods between 69 and 200 years– Periods; aphelion and perihelion distances.

Of the comets belonging to this class Halley's comet is the type; but it is the only one of which we have several undisputed apparitions. When a comet is suspected to be identical with some other comet that has been previously observed, from the similarity of the parabolic elements, its return is probable; but as a rule great uncertainty attaches to the length of the period, even if, assuming the identity of the two comets, the perturbations be left out of the question. A third apparition is, therefore, generally necessary before the identity and real periodicity of a comet can be affirmed. And this third element up to the present time is wanting in the comets we are now engaged upon. But it will evidently suffice to prove a second apparition, when the elliptic elements have been calculated solely from observations of the first apparition.

The following, in the order of their discovery, are the nine comets of mean period which we have to mention:–

The first on the list is the comet of 1532, observed by Apian and by Fracastoro, ‘ whose head,’ says the latter observer, ‘ was three times larger than Jupiter, with a beard two fathoms long.’