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 115. Passing from stars to comets, it must be admitted that the want of precise regularity renders them of less account. Still the years in which they have been specially prevalent, or in which a notable one has recurred, are known as the 'Comet Years,' and it is not uncommon to hear observant folk refer to some event—its years having escaped them—as having taken place in the year, say Hally's comet, or of Donati's comet. 'It may be taken as a fact (though in no proper sense a rule) that a bright and conspicuous comet comes about once in ten years, and a very remarkable comet every thirty years. Thus we have had during the present century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874, and 1882, whereof these of 1811, 1843, and 1858 were specially celebrated. Tested then by either standard of words, "bright and conspicuous," or "specially celebrated," it may be affirmed that a good comet is now due.' 'It is fortunate,' Sir John Herschell remarked, 'for astronomy that the confusion of dates and the irreconcilable contradictions which historical statements too often exhibit when confronted with the best knowledge we possess of the ancient reckonings of time, affect recorded observations but little. An astronomical observation of any striking and wellmarked phenomenon carries with it, in most cases, abundant means of recovering its exact date.'

116. The true place of astronomy in chronology is neither so high as that which Herschell claims for it, nor so insignificant as to be neglected with safety; but midway between these two points. Chronology, without the aid of astronomy, is only an abstract science; and astronomy, severed from