2G8 



THE ATMOSPHERE AND METEOEOLOGY. 



globe, and in the same space^ it is now the polar wind, and now the 

 tropical wind which prevails. But a compensation never fails to be 

 effected between these atmospheric currents, and the wind neutralized 

 or repulsed in one part of the hemisphere soon makes itself felt at 

 another point. While the strife exists between two masses of air 

 animated by contrary movements, the vicissitudes of the conflict and 

 the gradual preponderance of one of the winds result in temporarily 

 modifying the direction of the air, and making the weathercock 

 turn successively to the various points of the horizon. It is from 

 the meeting of two regular winds that the apparent irregularity of all 

 the atmospheric system results. 



Though the strife between the two aerial streams, now at one point, 

 now at another, does not cease, they are not, however, equal in force, 

 and one of them always finishes by gaining the victory after a longer 

 or shorter period of resistance. This wind of superior force is the 

 returning current, descending from the heights of space to reach the 

 level of the ground beyond the zone of the trade-winds. In fact, it 

 is evident that in its circuit round the planet any one stratum of air 

 must be much more expanded when it repairs from the torrid zone to 

 the frozen regions, than it is on its return from the pole, after having 

 been condensed by the cold ; it occupies thus, in consequence of its 

 temperature, a much greater space in the first journey. This is 

 not all ; the vapours with which the air of the equatorial zone is 

 loaded, contribute to expand it still more, while the polar winds are 

 relatively dry, and, consequently, much more dense. Thus the winds 

 which come from the tropical zone, that is to say, the south-west 

 winds in the northern hemisphere, and those from the north-west in 

 the southern hemisphere, must have the preponderance, and blow 

 during a more considerable space of time. It is thus at least in the 

 temperate zone of the north, where the winds which are directed 



30, 



Fig. 108.. 



-General direction of winds in England and North America. The total duration of the 

 atmospheric currents for the year is represented by 100. 



