2G6 THE ATMOSPIIEEE AND METEOEOLOGY. 



CHAPTER VIII. 



ZONE OF VARIABLE WINDS.— STRUGGLE OF OPPOSING WINDS. — SIEAl? DIRECTION 

 OF THE ATMOSPHERIC CURRENTS. — LAW OF GYRATION. 



Beyo:sd the changing limits where the trade-winds of the two 

 hemispheres blow, commence the zones of variable winds. There the 

 masses of air flow now in one direction, now in another, and appar- 

 ently in a very irregular fashion. Sometimes a single wind directs 

 itself incessantly during whole weeks towards one point of the 

 horizon ; sometimes the atmospheric currents which succeed each 

 other make the tour of the compass in a few hours ; at other times, 

 again, the air remains calm between two meteorological regions where 

 the winds move in. opposite directions. Indeed the word iceatJiercock 

 has become a synonym of all that is unstable and versatile. 



That which contributes to this disorder of the air in Europe, and 

 in the other lands which are outside the zone of the trade-winds, 

 is the inequality of the ground. The general currents which 

 pass above a chain of mountains do not blow with the same regu- 

 larity as in the plain. In fact, the winds must be all the more un- 

 equal in their successive gusts, the less even the surface is over 

 which they blow. The same wind which moves over the seas with 

 the uniformity of an immense river, departs from its regular pace as 

 soon as it is interrupted in its course by inequalities of the soil. At 

 the foot of the grand mountains of Switzerland, and especially in the 

 environs of Geneva, where the surface relief is already very varied, 

 the alternations which are produced in the force of the wind are such 

 that the anemometer sometimes indicates a variation of intensity from 

 single to triple. In the high gorges of the Alps it often happens, 

 even during violent tempests, that the atmosphere presents at intervals 

 the most perfect calm. To all the furies of the tempest there succeed for 

 an instant silence and repose, then the hurricane recommences to blow 

 with great violence. This is because the atmospheric currents, 

 similar in this respect to the rivers of the ocean, do not direct them- 

 selves invariably towards the same point of the horizon, and move by 



