I. Phenomena, however wonderful, are not noted and admired unless they are uncommon. The sun and moon and starry heavens have no observers, but a Comet at once sets the whole world agog. The nature of the stars is a sublime and likewise a profitable study 271 II. The nature of _Comets_ has not been hitherto fully investigated. They are so rare that one wants a record of the movements of all ever observed 273 III. Democritus, Eudoxus, Conon, Epigenes, and Apollonius of Myndus all fail to give any satisfactory account of the matter. Nor had the Egyptians or Chaldaeans investigated them 274 IV. Epigenes explains the Comet as due to a conjunction of Saturn with Mars or the Sun: it is akin to whirlwind and “beam” meteors 275 V. But there are essential differences between whirlwind, which is terrestrial, and beams and torches, which are above the clouds. There is a difference of duration also. Beams and Comets, it is true, have been mistaken for one another. It was a Comet, according to Aristotle, that appeared before the destruction of Buris and Helice. The character of the flame differs in the two forms 276 VI. There are two kinds of Comets, according to Epigenes. They are produced by air driven up and setting on fire suitable material above, which takes place every day at the same hour 277 VII. But Comets are not concomitants of winds; there is no parallelism in the phenomena. The higher ones, which have an orbit, he attributes to the north wind, but the facts do not square here either 278 VIII. The course and altitude of Comets render the whirlwind explanation impossible 279 IX. The force and duration of whirlwinds are similarly inadequate 280 X. The slowness and steadiness of the Comet could not be accounted for on this assumption, nor its general behaviour and shape 281 XI. We must look for some other explanation. Now Comets, it must be premised, appear in all quarters of the sky. Whatever the divisions of them made by the Greeks, they are all of one origin. Some of the ancients thought they were due to the union of two planets 283