CIECULATION OF THE WINDS. 239 



CHAPTER lY. 



GENERAL LAW OF THE CIECULATION OF WINDS. — TRADE-WINDS FROM THE NORTH- 

 BAST AND SOUTH-EAST. — EQUATORIAL CALMS. — OSCILLATION OF THE SYSTEM 

 OF WINDS. 



In the continental regions, and principally in tliose of the tem- 

 perate zone, it would often be difficult to recognize at first the general 

 law which presides over the movements of the atmosphere, for these 

 various oscillations may be modified by a crowd of local circum- 

 stances, such as the direction and height of mountain -chains, the 

 extent of plains, the contours of the shores, the abundance or scarcity 

 of vegetation. Even in one day the winds will sometimes blow 

 successively from all points of space, and among these rapid changes 

 to which the atmospheric currents are subject it is not always possible 

 to ascertain with certainty the normal direction of the mass of air in 

 movement. To understand the laws of the atmosphere in their sim- 

 plicity we must transport ourselves to the equatorial regions of the 

 ocean, above which the sun describes each day its immense semi-circle 

 in the space of twelve hours, and where all the movements of nature, 

 regulated by the uniform march of the sun, have something of the 

 rhythm of the celestial cj^cles. It is there that we may seek the 

 first displacement of the atmosphere, which travels as an immense 

 sheet of air all round the globe. We are there present at the birth 

 of the winds. It is there that Eolus would be seated if the gods still 

 lived. 



During the days of summer we perceive from afar a vibratory mo- 

 tion of the air over the heated earth, a kind of vaporous trembling, 

 doubtless rendered visible by the incessantly changing mirage of 

 objects lying beyond.* This is because the strata of the atmo- 

 sphere reposing on the ground have gradually expanded, and rise in 

 spirals through the colder and denser medium which weighs upon 

 them. In the same way the rarefied air of furnaces mounts rapidly 

 towards the upper regions whither its relative lightness carries it. 



♦ See above, p. 227. 



