314 THE CHEMISTRY OF CREATION. 



evening, and during the night, the land being 

 a better radiator of heat than the water, it be- 

 comes colder than the sea. The heated column, 

 therefore, now rises from the sea, and the cold 

 horizontal flow is from the land, the sensible 

 result being a fresh breeze to seawards. When 

 the heated column, in either case, reaches a 

 certain height in the air, it turns over, blows 

 along the upper regions, and then comes down 

 to supply the place of the descending current. 

 This will be readily understood by reference to 

 the figures, which rudely represent the state of 

 things during the day and at night. 



A modification of the "same phenomenon which 

 has been frequently observed in mountainous 

 regions, is the hill and valley breeze, and it 

 arises precisely from the same cause, namely, 

 inequality of temperature setting in motion an 

 ascending and compensating current. Mr. Dar- 

 win makes particular mention of a powerful one 

 observed by him in his travels in Mexico. 



Winds, due to a similar cause, whose peri- 

 odical occurrence was familiar to the ancients 

 under the title of the Etesian winds, take place 

 on land, when one district being more heated 

 by the solar rays than another, the cold current 

 of air flows from and across others, to supply 

 the place of the ascending hot current. 



Incomparably the most important and grand- 



