in the Air and in the Sea. 31 



there the thermometer not seldom shows 50° C. in the shade, 

 while in the equatorial regions of the ocean the temperature of 

 the air never rises above 30° C. The trade-wind, however, at 

 the north-west coast of Africa blows constantly from the desert, 

 carrying the fine sand far out to sea. 



Further, in summer the temperature of the 20th and 30th 

 degrees of latitude is not lower, indeed it is higher than that of 

 the equator, and yet the shifting of the trade-zones is inconsi- 

 derable, aud the wind keeps its usual direction. 



Likewise, according to Hadley's theory an ascending or a de- 

 scending current should prevail in the calm-zones. Now this 

 current must be very considerable, if it brings forth the fresh- 

 blowing trades and anti-trades; and the ascent of the air in the 

 equatorial calms, and its descent in the tropical calms, would 

 make themselves perceptible, even if the motion were very slow. 

 This, however, is not the case : a particle of dust loosened from 

 the sails falls, both in the equatorial and the tropical calm-zone, 

 quietly to the deck, without exhibiting the slightest tendency to 

 be impelled upward or downward. From this we conclude that 

 the upward aud downward currents of air in the zones of cairns^ 

 if they really exist, must be so slight that the generation of the 

 trade-winds and anti-trades cannot possibly be ascribed to 

 them. 



Let it further be remembered that in the middle latitudes of 

 the northern hemisphere the anti-trade often blows from the 

 north-west instead of south-west, and in the southern hemisphere 

 from south-west instead of north-west — which could not be, if, 

 as required by Hadley's theory, it formed a current directed 

 toward the poles. 



Lastly, in Central Europe these constant west winds blow in 

 summer very moderately, while in winter their force is much in- 

 creased — which again does not correspond with the theory ; for 

 in summer the eastern steppes are strongly heated and should 

 attract the west wind, while the cold which prevails in Eastern 

 Europe in winter would, on the contrary, contribute to the 

 weakening of the west wind. 



All this and many other reasons'* show that the existing 

 theory of the trade-winds is not sufficient to account for the 

 phenomenon, and that another must be sought. 



We do not on this account dispute that heated air must 

 ascend; we only believe that, since the heating and expansion 

 proceed very gradually, the ascent must also be very slow and 

 hence cannot constitute the principal cause of the trade-winds. 

 Their main motive cause appears therefore to lie in other forces, 

 of which we will speak subsequently. 



* Laughton, 'Physical Geography.' pp. 120-12/. 



