2G0 THE ATMOSPHERE AND METEOEOLOGY. 



along in tlie general circuit. Instead of being at right angles to 

 the coast, they more often form with it an acute angle ; they blow 

 cross- ways, as Captain Dampier said. Nevertheless, it is not 

 only in the domain of the trade-winds or along the borders of the 

 ocean that the littoral breezes occur ; they blow everywhere where a 

 considerable difference of temperature exists between the land and the 

 water, wherever the fresh air of the sea or of a lake goes to fill the 

 vacuum left on the coast-line by an ascending current of warm air. A 

 remarkable example of it is seen in the narrow Adriatic Sea. There, 

 during each fine day, the breeze rises in the centre of the gulf, and 

 takes its direction at the same time in two contrary ways ; on one 

 side towards the shores of Italy, on the other towards the islands 

 and mountains of Istria and Dalmatia. During the night the 

 coasts that surround the waters of the Adriatic send back to the 

 sea, as to a common centre, the fresh air which they have received ; 

 to the divergent currents of the day succeeds a wave of convergent 

 breezes. 



In the same way the mountains have their own system of breezes 

 alternating with a regularity similar to that of the land and sea 

 breeze on the coasts of the ocean. In the day, especially in summer, 

 when the summits of the mountains are exposed to all the intensity 

 of the solar rays, and receive a considerable quantity of heat which 

 causes their temperature to approach that of the valleys, the air 

 reposing on the summits expands and rises. At the same time the 

 air of the plains which lie at the foot of the mountains is itself expand- 

 ed in greater proportions, so that an ascending current is produced 

 from the base to the summit of the peaks, in all the valleys, and over 

 all the escarpments. The atmospheric strata of the plain move in the 

 direction of the heights with all the more impetuosity the more 

 strongly heated the summits have been by the sun. In certain valleys, 

 especially those of the Stura, and other Alpine rivers, which water 

 the plains of Piedmont, the ascending wind has such force that the 

 greater part of the trees are uniformly inclined towards the mountains. 

 Pollen, remains of plants, insects and butterflies, are carried away by 

 the current of air, and by their debris soil the whiteness of the snow. 

 In the night, phenomena of an opposite kind are produced, but with 

 less intensity ; the high mountains whose summits rise far into the 

 sky, lose their heat bj^ nocturnal radiation more rapidly than the val- 

 leys, the sheets of air which surround them are chilled and descend 

 again, in part towards the plains from which they had ascended a 

 few hours before. Thus an exchange between the two zones is estab- 



