The importance of this event can be better understood when it is remembered that it was only the second instance of the recognised return of a comet (that of Halley's, sixty-three years previously, having, as already stated, been the first); and that it, moreover, established the existence of a new class of celestial objects, somewhat loosely distinguished as "comets of short period." These bodies (of which about thirty have been found to circulate within the orbit of Saturn) are remarkable as showing certain planetary affinities in the manners of their motions not at all perceptible in the wider travelling members of their order. They revolve, without exception, in the same direction as the planets--from west to east; they exhibit a marked tendency to conform to the zodiacal track which limits planetary excursions north and south; and their paths round the sun, although much more eccentric than the approximately circular planetary orbits, are far less so than the extravagantly long ellipses in which comets comparatively untrained (as it were) in the habits of the solar system ordinarily perform their revolutions. No _great_ comet is of the "planetary" kind. These are, indeed, only by exception visible to the naked eye; they possess extremely feeble tail-producing powers, and give small signs of central condensation. Thin wisps of cosmical cloud, they flit across the telescopic field of view without sensibly obscuring the smallest star. Their appearance, in short, suggests--what some notable facts in their history will presently be shown to confirm--that they are bodies already effete, and verging towards dissolution. If it be asked what possible connection can be shown to exist between the shortness of period by which they are essentially characterised, and what we may call their _superannuated_ condition, we are not altogether at a loss for an answer. Kepler's remark,[243] 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 reasonably be expected to _wear out_ quickly.