334 PHYSICAL SCIENCE 



rivers and lakes, and where the moist air naturally gives rise to 

 blasts of wind. Second ; Long-continued evaporation carries the 

 terrestrial emanations aloft, where the intermingling of the breath 

 results in wind. Third ; Much more important is the fact that the 

 air in its very constitution possesses an innate power of motion ; 

 we cannot imagine that while we ourselves are endowed with a 

 capacity of movement and water has this power also the atmo- 

 sphere should be left inert and immovable (197). Fourth ; Some- 

 times the sun is itself the cause of wind, when he loosens and 

 expands the thick air (198). 



In this enumeration allusion is made to one or two features 

 of natural history which the author appears to accept as fact. 

 He thinks there must be some vital force in water, otherwise it 

 could not bring forth animals and plants, as we know it does. 

 But not only water; fire, too, which devours everything, possesses 

 this generative capacity, for, unlikely as it might be thought, it 

 is nevertheless true that fire gives birth to some animals. The 

 air, too, has some vital energy, as it alternately thickens, con- 

 tracts, and expands, and rids itself of its impurities. The portion 

 of it contained within the earth is asserted in a later part of the 

 volume to be the source of the life of the vegetation at the surface 

 (244). 



The local winds, now known as "land and sea breezes," are 

 next discussed (198). Instead of the simple explanation which 

 in our own day has shown these aerial currents to be beautiful 

 examples of the results of diurnal variations of atmospheric pres- 

 sure, the ancient theory represented that during the day the 

 exhalations from the land are borne on high to supply the sun 

 with nourishment, while at night, as they are not needed for that 

 purpose, they accumulate until they have filled up a given space 

 enclosed by mountains. When in such a space there is no more 

 room, they move towards the quarter to which they can most 

 easily escape ; hence the wind. It is curious, however, to note 

 that Seneca only describes the land breeze, which falls away as 

 the morning advances. He does not specially refer to the 

 equally characteristic sea breeze, which springs up after the other 

 dies down, and continues during the day, until in the evening it 

 is again replaced by the land breeze. 



The important Etesian or northerly winds, with all their 

 important local modifications in the Mediterranean basin, must 

 have been a subject of constant observation to the Greeks and 

 Romans. There was a general belief that as these winds reap- 

 peared regularly in summer, they were in some way connected 

 with the position of the sun in the firmament. Seneca, after 

 briefly stating this opinion, dissents from it on the ground that, 



