There are other small comets that are also regular inhabitants of the solar system; but, as with Halley's comet, so with these, two circumstances are to be borne in mind. First, that each successive revolution round the sun involves an increasing degradation of their brightness, since there is a manifest waste of their material at each near approach to the sun; until at length the comet is seen no more, not because it has left the warm precincts of the sun for the outer darkness, but because it has spent its substance. Halley's comet was not as brilliant or as impressive in 1835 as it was in 1759: in 1910 it may have become degraded to an appearance of quite the second order. Next, we have no knowledge, no evidence, that any of these comets have always been members of the solar family. Some of them, indeed, we know were adopted into it by the influence of one or other of the greater planets: Uranus we know is responsible for the introduction of one, Jupiter of a considerable number. The vast majority of comets, great or small, seem to blunder into the solar system anyhow, anywhere, from any direction: they come within the attractive influence of the sun; obey his laws whilst within that influence; make one close approach to him, passing rapidly across our sky; and then depart in an orbit which will never bring them to his neighbourhood again. Some chance of direction, some compelling influence of a planet that it may have approached, so modified the path of Halley's comet when it first entered the solar system, that it has remained a member ever since, and may so remain until it has ceased to be a comet at all. It follows, therefore, that, as to the number of great comets that may be seen in any age, we can scarcely even apply the laws of probability. During the last couple of thousand years, since chronicles have been abundant, we know that many great comets have been seen. We may suppose, therefore, that during the preceding age, that in which the Scriptures were written, there were also many great comets seen, but we do not know. And most emphatically we are not able to say, from our knowledge of comets themselves and of their motions, that in the days of this or that writer a comet was flaming in the sky. If a comet had been observed in those ages we might not recognize the description of it. Thus in the fourth year of the 101st Olympiad, the Greeks were startled by a celestial portent. They did not draw fine distinctions, and posterity might have remained ignorant that the terrifying object was possibly a comet, had not Aristotle, who saw it as a boy at Stagira, left a rather more scientifically worded description of it. It flared up from the sunset sky with a narrow definite tail running "like a road through the constellations." In recent times the great comet of 1843 may be mentioned as having exactly such an appearance.