For, on account of their infrequency, their orbit cannot as yet be discovered or examined in detail, to see whether they observe periodic laws, and whether some fixed order causes their reappearance at the appointed day. Such a development of astronomy is recent, having been lately introduced into Greece. III Democritus, the most acute of all the ancient philosophers, says he 1 suspects there are several stars whose orbits are erratic. But he has given neither their number nor their names, as the motions of the five planets were not in his time understood. Eudoxus was, in fact, the first to import from Egypt into Greece the knowledge of these motions, though he says nothing about comets. From this it becomes plain that, even among the Egyptians, the people that bestowed most care on observation of the sky, the portion of astronomy that relates to comets had not been worked out. Subsequently Conon, who was himself a 2 careful investigator, made a record of the sun’s eclipses that had been observed by the Egyptians; but he made no mention of comets, though he would certainly not have omitted anything definite on the subject that he had learned in Egypt. So much is certain; two authors, Epigenes and Apollonius of Myndus, the latter highly skilled in casting horoscopes, who say that they studied among the Chaldaeans, are at variance in their accounts. The latter asserts that comets are placed by the 3 Chaldaeans among the number of the wandering stars (_i.e._ planets), and that their orbits have been determined. Epigenes, on the contrary, asserts that the Chaldaeans have ascertained nothing regarding comets, which are thought by them to be fires produced by a kind of eddy of violently rotating air. IV In the first place, if it like you, let us set down the views of the 1 last-mentioned author and refute them. He supposes that the planet Saturn has most influence in determining all motions of the heavenly bodies. When it presses upon the constellations next Mars, or crosses to the neighbourhood of the moon, or encounters the rays of the sun, being naturally cold and windy, it contracts and masses the atmosphere at more than one place. By and by, if Saturn absorb all the sun’s rays, there is thunder and lightning. If he has Mars in agreement, the 2 lightning is forked. Moreover, he continues, forked and sheet lightning contain different materials. Evaporation from water or other moisture produces only gleams that threaten but stop short of striking. The hotter and drier exhalation of the earth forges the bolts of forked lightning. Beam meteors and torches, which differ from one another 3 only in size, are produced in this same way. When any ball of air--what we call a whirlwind--encloses moist earthy matter, wherever it rushes it presents the appearance of an extended line of fire, which lasts just so long as the mass of air remains, which carries within it the supply of moist earthy matter. V