MUTUAL ACTION OF TWO CURRENTS. 220 



itself to the far west of Europe. We have not as 

 yet learned enough of the conditions concerned to 

 enable us to account satisfactorily for this frequent 

 change of the direction of the wind over all parts 

 of Europe. 



During the strife of two winds blowing near each 

 other in contrary directions, the layer of air be- 

 tween them cannot be drawn into the motion of 

 the more powerful current, without first suffering 

 a certain amount of condensation. Hence there 

 arises, together with the onward motion in the 

 direction of the current, a sideward movement of 

 the condensed air in a direction at right angles to 

 the former ; in other words, the air that is just 

 outside the current, and that was before either 

 actually or relatively at rest, or perhaps even 

 moving in the opposite direction, flows into the 

 current with a speed that is greater the more it 

 had been condensed before, that is, the greater the 

 resistance it offered before it could be swept away 

 into the stream. But now the side-current, thus 

 set up, itself exerts a like action. It condenses 

 the air that lies before it, and that was already 

 engaged in the main-stream, though at rest, rela- 

 tively to the side-currents ; it thus becomes con- 

 tinually turned from its course, and always in the 

 same direction, and must, therefore, take a twisting 

 or whirling motion. Thus ivhirlwinds always 



