PHYSICAL SCIENCE BK. v 



IX 



1 BUT why, again, are winds of this nature stronger 

 in spring and summer ? For during the remainder 

 of the year they are very light, never rising 

 sufficiently to fill the sails of a boat. The reason is 

 that spring is a wetter season. There is at that 

 time more evaporation going on, both from the 

 abundance of water lying about, and from the 

 saturation of the ground to overflowing through 

 the moist character of the sky. And the reason 

 why this wind is equally prevalent in summer is 

 that the heat of the day remaining after sundown 

 and lasting during a great part of the night draws 

 out exhalations, and attracts more forcibly any of 

 them that are wont to be given off spontaneously 

 by the ground. But subsequently the heat has not 



2 sufficient strength to use up what it drew out. This is 

 the reason, I say, why the soil and its moisture give off 

 for a longer period [at certain seasons] the particles 

 derived from the earth's wonted emanations and ex- 

 halations. The sunrise produces wind by its stroke 

 as well as by its warmth. For, as I have already 

 said, the light which precedes the sun does not as yet 

 heat up the atmosphere, but merely smites upon it ; 

 being smitten the air retires to one side. And yet I 

 cannot go so far as to admit that the light is quite 

 devoid of heat, inasmuch as it is derived from heat. 

 Probably it does not contain as great an amount as 

 would appear from its effect. Still, it accomplishes 

 its own task by separating and rarefying the dense 



3 exhalations. Moreover, places which through some 

 disservice of nature are so shut in that they cannot 

 receive the direct rays of the sun, even they, I say, 



