TEBRESTKIAL PHENOMENA. 45 



seems to say that, during the formation of a cloud by the 

 coming together of the ascending vaporous and dry 

 exhalations, the upper part of the cloud, being cooled more 

 rapidly than the rest, is thicker or heavier. Wherefore, he 

 concludes, thunderbolts, lightning, and tempests, and every 

 thing of this kind, travel downwards, although heat naturally 

 travels upwards.* Whatever amount of the dry exhalation, 

 he says, is enclosed within the clouds, during the process of 

 ascent and cooling, is separated when the clouds meet and, 

 being carried along and striking violently against neigh 

 bouring clouds, this exhalation gives rise to a shock, the 

 noise of which is called thunder, t Aristotle proceeds to say 

 that the spirituous vapour itself, which has thus been 

 pressed out or separated, is generally burnt with a slight or 

 weak burning and is what is called lightning, t Here again 

 he fails to appreciate the intensely hot nature of a white-hot 

 body, compared with that of a red-hot body. Aristotle con 

 cluded that lightning was produced after both the shock and 

 the accompanying thunder. He correctly states, however, 

 that the lightning is seen before the thunder is heard, because 

 sight is quicker than hearing, just as can be seen in the row 

 ing of triremes, for at the moment when the oars are raised 

 the sound of the preceding splash of the oars is heard. 



Aristotle sums up his views on the causes of winds, 

 earthquakes, lightning, and thunder towards the end of 

 Meteorol. ii. c. 9, where he says that they are all essentially 

 the same, viz., a dry exhalation which produces earthquakes 

 when operating within the Earth, winds when operating 

 about the surface of the Earth, and lightning and thunder 

 when operating among the clouds. || 



He discusses at great length the saltness of the sea. 

 Some philosophers, he says, believed that the sea was pro 

 duced originally in the following manner : The whole space 

 about the earth was water which, being dried by the heat of 

 the Sun, gave off vapours from which winds were generated, 

 the residual water forming the sea. They believed, there 

 fore, that the sea was becoming less and less, and would 

 ultimately become quite dry. They were led to this con 

 clusion, according to Aristotle, by observing that many 

 places were drier in their time than they formerly were. IT 

 He treats with contempt the opinion of Empedocles and 



* Meteorol. ii. c. 9, s. 4. f Ibid. ii. c. 9, s. 5. 



J Ibid. ii. c. 9, s. 8. Ibid. ii. c. 9, s. 9. 



|| Ibid. ii. c. 9. s. 21. IF Ibid. ii. c. 1, s. 3, ii. c. 3, s. 7. 



