ZONE or VAEIABLE WINDS. 2G7 



successive oscillations now to the right, now to the left of the axis of 

 their movement. In consequence, when we find ourselves placed on 

 a point in the mountains which commands a view of the highest 

 peaksj we must, according to the various directions which the aerial 

 current takes, be by turns exposed to the fury of the tempest, or 

 protected by some high summit on which the force of the wind is 

 broken.* Even in countries but slightly varied in surface, or over 

 plains covered with houses and woods, the wind does not blow in 

 equal manner like the trade-wind of the seas. It advances by 

 a series of gusts and blasts, each one of which represents a vic- 

 tory of the atmospheric current over an obstacle on the plain. Close 

 to the ground the wind is always intermittent, while in the heights 

 of the air it proceeds almost always with an equal and majestic move- 

 ment like the current of a river. 



The abrupt gusts of the lower strata of this ocean are thus only 

 secondary phenomena, and in all the sudden turns of the winds 

 which one might easily believe to have occurred by chance, the dis- 

 order is more apparent than real. Though the wind makes itself felt 

 by turns from every part of the horizon, there, nevertheless, exist 

 only two atmospheric currents in each of the temperate zones : that 

 which comes from the pole to replace the expanded air of tropical 

 regions, and that which flows back from the equator after being 

 raised in the heights of space above the stratum of the trade-winds. 

 In the northern hemisphere these two winds set out, one from 

 the north, the other from the south ; but in consequence of the 

 rotatory movement of the earth, their direction is gradually changed, 

 like that of the trade-winds. The wind from the north changes into 

 a wind from the north-east, while the wind from the south ends by 

 blowing from the south-east. Thus, as Dove remarks, the greater 

 part of the aerial currents deceive the observer, because they do not 

 come from the regions whence they appear to blow. The wind from 

 the north-east is in reality much more the wind from the north than, 

 the mass of air whose direction is truly southern ; in the same way, 

 the wind from the south-east is truly the south wind, and that which 

 seems to come from the south has the south-east as a starting-point. 



Two great aerial currents thus dispute the extent of each terrestrial 

 hemisphere from the pole to one of the tropics. Generall}^ all this 

 space is divided into vast oblique bands, composed of masses of air 

 flowing in opposite ways, some from the pole, and others from the 

 equatorial regions. The bands move over the circumference of the 



* H, de Saussure, Voyages dans Les Alpes. 



