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20 one and the same comet. So far as regards the Comets of 1668 and 1843, considered alone, there is nothing absolutely forbidding the idea of their identity: Perturbations might account for the differences between their 2 orbits. But the comets of 1880 and 1882 cannot possibly be one and the same; they were both observed for a considerable time and accurately, and the observations of both are absolutely inconsistent with a period of 2 years, or anything like it. In fact for the Comet of 1882 all of the different computers found periods ranging between 600 and 900 years."

Hoek has suggested a considerable number of other comet groups besides those already named.

The immense mass of material ejected from the heads of comets and added to the tails has suggested that comets in time must perish from the exhaustion of their material. The idea seems startling, but it cannot be said to be primâ facie unsound, and there certainly are facts to support it. Miss Clerke's reflections on this subject are to the point:—"Kepler's remark that comets are consumed by their own emissions, has undoubtedly a measure of truth in it. The substance ejected into the tail must, in overwhelmingly large proportion, be for ever lost to the central mass from which it issues. True, it is of a, nature inconceivably tenuous; but unrepaired waste, however small in amount, cannot be persisted in with impunity. The incitement to such self spoliation proceeds from the Sun; it accordingly progresses more rapidly the more numerous are the returns to the Solar vicinity. Comets of short period may thus be expected to wear out quickly."

In the light of all that has been said on the subject in these pages and elsewhere, can any summary statement be made in answer to the question, "What are comets made of?"

I give, under great reserve, the answer: that probably the heads are a mixture of solid and gaseous matter, and that