The movements of the atmosphere form the subject of discussion in this part of the treatise. In the first chapter the author seeks for an exact definition of the term “Wind” (_ventus_), and ends by adopting one which is obviously inaccurate--“wind is air flowing in one direction,”--for as he afterwards speaks of whirlwinds he was well aware that the movement may be in every direction, or vorticose. Dismissing the opinion of Democritus as to the origin of wind, he states that in his judgment wind may arise from four different causes. First; The earth itself breathes forth a vast amount of air from its interior, where there are large 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 atmosphere should be left inert and immovable (197). Fourth; Sometimes 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, contracts, 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 pressure, 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.