TERRESTRIAL PHENOMENA. 33 



and (2) an essentially dry, smoke-like exhalation from dry 

 earth. This ascended higher than the watery vapour, and 

 was the one which Aristotle believed to be ignited. The 

 modes of arrangement and the sizes of the ignited exhalations 

 varied, and various names were given them. Aristotle 

 describes some of them. His descriptions are neither full 

 nor clear, but he refers to falling stars and, apparently, fiery 

 meteors to which he gives the name " aix," i.e., something 

 which rushes impetuously, and to blood-red and other flame- 

 like appearances, which seem to include auroras.* 



In another passage Aristotle modifies his views on the 

 mode of production of falling stars. He was met by the 

 difficulty that the downward motions of these bodies were 

 not such as might be expected from an ignited exhalation, 

 for the natural motion of fire is in an upward direction. To 

 overcome this difficulty, he says, in a passage difficult to 

 understand, that hot matter is violently pressed out down- 

 wards, in consequence of the air being compressed by the 

 cold, and thus the motion is more like that of a falling body 

 than that of flame, t 



After commenting on the views of Anaxagoras, Demo- 

 critus, and others on the nature of comets, Aristotle says that 

 the dry and hot exhalations, referred to already, beneath the 

 moving parts of the heavens, together with the underlying 

 air, are whirled round the Earth, and that whatever they 

 meet is ignited, provided it is of the proper constitution, 

 a falling star being thereby produced.! Under conditions 

 such that the resulting ignited matter becomes compressed 

 and burning proceeds for a long time at a steady rate, and 

 simultaneously an exhalation of suitable constitution rises 

 from below and meets with the burning matter, the falling 

 star becomes a comet. § When the ignition occurs in a 

 lower region of the Kosmos, a comet appears as a separate 

 phenomenon, but if the ignition occurs beneath some star or 

 planet, then this becomes a comet. li The first kind of comet 

 is probably meant to be one with a conspicuous tail, and 

 the second one with a conspicuous nucleus and a tail less 

 distinct. 



In order to explain the appearance of the Milky Way, 

 Aristotle again made use of his theory of ignited exhalations. 

 He believed that if ignition of a dry exhalation beneath a 



* Meteorol. i. c. 4, ss. 5 and 6, and c. 5, s. 1. 



f Ibid. i. c. 4, ss. 7-10. | Ibid. i. c. 7, ss. 1 and 2. 



§ Ibid. i. c. 7, s. 3. || Ibid. i. c. 7, s. 5. 



D 



