288 PHYSICAL SCIENCE BK. vn 



Well, what argument 1 then justifies the assertion 

 that it is not merely the five planets that move, 

 but that there are many such in many quarters 

 of the universe ? Or if there is no probable 

 proof of this, one may rejoin : What is there to 

 prevent one from saying either that all the stars 

 4 move or that none of them does ? Besides, your 

 argument is in no way helped by that crowd of stars 

 which you assume to be everywhere roaming about ! 

 For the more there are of them, the oftener will 

 they meet with others ; whereas comets are rare, 

 and for that reason marvellous. And will not 

 every age give evidence against you by noting and 

 recording for the use of posterity the emergence of 

 such stars ? 



XV 



1 AFTER the death of Demetrius, king of Syria, 

 whose kingdom was divided by his sons Demetrius 

 and Antiochus, a little before the Achaean War, a 

 comet blazed forth not inferior to the sun in size. 

 Its orb was at first fiery red, and emitted a bright 

 light sufficient to dispel the darkness of night. By 

 and by its size was gradually reduced and its 

 brightness waned. Finally it went completely out. 

 How many stars, suppose you, would require to 



2 combine to make up such a huge mass ? You 

 might collect in one a thousand of them without 

 ever matching the size of the sun. In the reign of 

 Attalus a comet appeared, moderately small in size 

 to begin with. By and by it mounted up and spread 

 out and moved as far as the equator, equalling in 



1 The argument is resumed from the beginning of XIII. after the 

 digression about the &quot;firmament.&quot; 



