DIEECTION OF HOT AND COLD WINDS. 



249 



trade- winds, determined by tlie rotatory movement of the earth. At 

 its return from the equator, each particle of air in movement turns 

 towards the east, instead of deviating to the west, as in its voyage 

 from the polar to the torrid zone. After having sojourned in the 

 equatorial regions it traverses successively countries whose speed 

 around the axis of the earth is less than its own. In proportion as 

 it retreats from the zone of calms, it thus finds itself in advance of 

 all the subjacent points of the planet, and changes into a wind from 

 the south-west. Below it, glides the north-east trade-wind, gener- 

 ally in an opposite direction ; but in consequence of the friction of 

 the aerial particles, a stratum of calm air is formed between the two 

 atmospheric currents, where all the meteoric phenomena due to the 

 contact of the two masses, unlike in heat, moisture, and electric tension, 

 are manifested. According to Dove, the counter trade-wind would 

 bend more and more to the east, because of the increasing curve of 



KE. 



KE. 



Fig. 105.— Theory of Dove. Theory of Muhry. 



the earth in the direction of the pole. According to Miihry, on the 

 contrary, the direction of this wind would be exactly parallel to that 

 of the lower current, and would curve gradually towards the north, 

 in consequence of the attraction exercised in the polar regions by the 

 wind which descends towards the equator. This last theory appears 

 the most probable ; but it is for direct observation to decide in a 

 definite manner.* 



One might believe at first that the upper counter-current flows 

 towards the pole, maintaining itself in the high regions of the atmo- 

 sphere, and that the polar wind, on its side, more compact In its particles, 

 because of the cold, always glides over the surface of the globe. It 

 is only rarely that it is so. In a somewhat undecided region 

 which, for the North Atlantic, oscillates alternately, according to the 

 seasons, from the 21st to the 25th degree of latitude, the returning 

 wind commences to descend from the heights of the sky to the sur- 

 face of the sea, and strikes against the aerial masses which flow from 

 * Mittheilungen von Petermann, ix. 1866. 



