As no method existed at that time for precise determinations of the perturbations, there was considerable uncertainty as to when to look for the next return, but Halley estimated that the general result would be a retardation, and that it would not be visible much before the beginning of 1759. This, as we shall see, proved to be a very fortunate guess.
Naturally, the old chronicles were now diligently searched for records of earlier appearances. Beginning with 240 b.c., a continuous series of supposed appearances is found at an average interval of 76.8 years, but with the very considerable range of a little more than five years between the longest and shortest of these periods. With such a range, naturally, the period alone furnishes a very slight foundation for establishing identity, and, unfortunately, in many cases, the accounts given are so meager and unsatisfactory as to leave the matter in grave doubt. In a number of instances, where the comet has been connected with natural events, such as conjunctions of planets, eclipses, earthquakes and the like, or with important matters of history, the accounts may contain, in connection with much rubbish, sufficient material of value to leave no doubt as to identity. It has been thought by many that this is the comet mentioned by Josephus in connection with the destruction of Jerusalem. This would, perhaps, not be impossible, though a considerable stretching of the period would be required. The appearance in A.D. 451 is the first record which seems open to no doubt on the score of identity. In the autumn of that year the Huns, under Attila, were defeated by the Roman armies. Numer-
