THE LAW OF THEIR VEERING. 225 



which again gradually passes through north-east 

 into east. In like manner the west wind will be 

 reconverted by the calm into a south wind, which 

 again will gradually turn round to west. 



If, therefore, polar and equatorial currents are 

 alternating with each other in the northern hemi- 

 sphere, the mean direction of the wind will travel 

 round the compass through south, west, north, east, 

 south, and the changes will take place oftener 

 between south and west, or between north and 

 east, than between west and north, or east and 

 south \ since the first changes show only a con- 

 tinuance of the exciting cause, while the latter 

 point to a change of it. 



By a like course of reasoning, we are led at 

 once to the conclusion, that, when in the southern 

 hemisphere polar currents alternate with equa- 

 torial, the mean direction of the wind will be suc- 

 cessively south, east, north, west, south, and that 

 the changes ensue oftener between north and west, 

 or south and east, than between west and south, 

 or east and north. 



The trade-winds form but a particular and, 

 indeed, most simple case of this general behaviour. 

 The cause to which they owe their origin is most 

 active in the belt of the calms, where the influence 

 of the tropical sun is always raising currents of 

 warm air and vapour. The uninterrupted con- 

 tinuance of this action gives rise to streams of air 



