198 PHYSICAL SCIENCE BK. v 



sometimes contracts, at others opens up and dis 

 perses. There is thus the same difference between 

 air and wind as between lake and river. 1 There are 

 occasions when the sun is the sole cause of wind, as 

 he rarefies the stiff atmosphere and opens it out 

 from its thick contracted state. 



VII 



HAVING spoken of the winds in general, let us now 

 proceed to the discussion of individual winds. 

 Perchance the discovery of the time and place of 

 their origin will conduce to the discovery of their 

 manner of formation. First, then, let us look at 

 breezes before dawn, which are borne either from 

 rivers or hollow valleys or from some bay. None 

 of these winds lasts long, but falls when the sun has 

 got stronger ; nor is it carried up out of sight of the 

 earth. This class of wind sets in in spring, and 

 does not last beyond summer. It comes chiefly 

 from a quarter where there are spaces of water 

 and mountains. Plains, for instance, may have 

 abundance of water, and yet they have no breeze ; 

 I mean a breeze strong enough to be called wind. 



VIII 



How, then, is a blast of this kind, which is called 

 by the Greeks a gulf breeze (ev/co\7rias), formed ? 

 This is the theory of them : All the exhalations of 

 marshes and rivers and they are abundant and 

 constant form by day the sun s nourishment. By 

 night, however, there is no drain on them, and they 



1 This remark would have been more apposite in Chap. L, above; 

 possibly that is its correct place. 



