LETTER XIV. 



THE WINDS, CONTINUED: THEIR INFLUENCE 

 ON CLIMATE. 



If two winds are blowing in opposite directions 

 one above the other, or near to each other, there 

 must lie between them a layer of air at rest, which 

 passes gradually on each side into the motion of 

 the two currents."* If one wind is stronger than 

 the other, it will by degrees draw the air that was 

 at rest into its own movement ; part of the oppo- 

 site weaker stream will come to rest, and will then 

 in like manner be dragged into the motion of the 

 stronger current, and thus the weaker will gradu- 

 ally give place to the stronger. Thus, for instance, 

 the south-west wind of the upper regions that is 

 retreating from the tropics, displaces, where its 

 current is strongest, the north-east current that 

 blows along the surface, and thus gradually comes 

 down to the earth as the prevailing wind of the 

 lower regions; and just in the same manner, a 



* You will remember the experiment of the open door, 

 and the behaviour of the candle-flames, 



Q 3 



