342 PHYSICAL SCIENCE 



bodies, and more especially of Comets which he is to discuss in 

 this Book. He proposes at the outset to endeavour to ascertain 

 whether the earth stands still while the universe revolves round 

 it, or if it is the universe that remains at rest while the earth 

 revolves. But he is led on from one topic to another, without 

 having arrived at a definite solution of this problem when the 

 volume comes to an end. From various expressions, however, 

 it may be inferred that he adhered to the primitive belief that it 

 is the universe which goes round the earth. Thus in Chapter IX. 

 he speaks of the movement that drags the universe along and 

 asks what is swifter than that revolution (281). The heavenly 

 bodies may not stand or turn aside, they all move onward with 

 the irrevocable movement of this eternal creation (299). 



The opinions of various philosophers on comets are quoted 

 and criticised. He is especially severe in his comments on 

 Epigenes, who thought that comets are produced somewhat as 

 fires are excited by whirlwinds. The Greek threefold classification 

 of comets is cited, and then Artemidorus comes in for his share 

 of vituperation ; to disprove his theory of the firmament being a 

 solid roof to the world would be, in the author's opinion, nothing 

 but beating the air. Ephorus fares no better, being briefly dis- 

 missed as often deceiving and often deceived. Apollonius of 

 Myndus held that many comets are distinct planetary bodies, 

 which wax and wane like the planets, being brightest when they 

 are nearest us and growing dimmer as they recede to a greater 

 distance. But Seneca refuses to admit that they have the 

 character of true planets ; in his view they are insubstantial 

 irregular fire (291). But he does not agree with his Stoic 

 brethren in regarding them as generated in dense air and pursuing 

 their course according to where they can find fuel to sustain 

 them (292-4). He conceives that they are not mere sudden 

 and transient fires, but belong to the eternal works of nature. 

 They steadily traverse their course, and he can understand that 

 they may have such wide orbits as to carry them far beyond the 

 limits of the Zodiac (296). He can see no reason why the five 

 planets then known should be the only stars that move across 

 the sky, though others had not yet been discovered. Astronomy, 

 he remarks, is in its infancy, many mysteries of nature remain 

 still to be discovered, and the day will come when posterity will 

 marvel at our ignorance of things which will then appear to be 

 so evident. Some future observer will demonstrate the paths of 

 the comets, why they wander so far from the other stars, and 

 what is their size and constitution. We may be content with 

 what we have found out, and leave something for posterity to 

 discover. 



