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VIII. that, allowing for planetary perturbations, a great comet which appeared in 975 followed a path which might be very closely represented by a comet with the elements of the comet of 1556. Hind even went one step further, and concluded that "a comet observed in China in the summer of 683, and one seen in the circumpolar heavens 104, present some indications of identity with the grand comet of 1264 and 1556, but the accounts we possess are too vague to admit of anything more than conjecture ".

It can well be imagined that this comet left its mark on history. In 1264 it was considered to have announced the death of Pope Urban IV; whilst, having regard to the particular time when it appeared in 1556, it was considered to have brought about the abdication of the Emperor Charles V. This, however, must be regarded as an exploded romance, because the Emperor abdicated in 1555, but a thrilling story was long in circulation based on materials gathered (I will not say invented) by the great French cometographer Pingré.

There are some comets moving in elliptic orbits which cannot conveniently be grouped, but which should be mentioned. One of these is the Comet of 1661, which has been supposed to be identical with the Comet of 1532. Halley and Méchain both computed the orbit of the latter comet, as did Méchain that of the Comet of 1661. The orbits obtained for the Comet of 1532 differed materially, and Olbers, who made an independent calculation, upheld Halley's results, and assigned to the comet a period of 129 years. If this conclusion was well founded the comet should have returned about 1789 or 1790, but it is not known to have done so. However, to this negative evidence too much importance must not be attached. It remains to be seen whether the comet reappears in or about the year 1918. Sir J. Herschel picked out a number of other comets as supplying coincidences of interval backwards, but it does not appear that he carried out any rigorous investigations respecting them.