Lessen xvn.] WINDS. 93 



from the neighbouring parts : it will be willingly 

 allowed that the heat is at all times greatest in the 

 torrid zone, and decreases gradually in proceeding; 

 northward or southward ; also that the poles may 

 at all times be considered as the centres of cold. 

 Hence it manifestly results, that, abstracting from 

 accidental circumstances, there will be a constant 

 ascent of air over the torrid zone, which air will 

 afterwards fall northward and southward, whilst 

 the colder air below is determined by a continual 

 impulse towards the equator. 



When the effects of the earth's rotation are takea 

 into consideration, our reasoning must be as fol- 

 lows :- The air over any part of the earth's surface, 

 when apparently at rest or calm, will have the 

 same rotatory Telocity as that part ; but if a quan- 

 tity of air in the northern hemisphere receive an 

 impulse in the direction of the meridian, either 

 northward or southward, its rotatory velocity will 

 be greater in the former case, and less in the latter, 

 than that of the air into which it moves ; con- 

 sequently, if it move northward, it will have a 

 greater velocity eastward than the air, or surface 

 of the earth over which it moves, and will there- 

 fore become a south-west wind, or a wind be- 

 tween the south and west.' And vice versa, if it 

 move southward, it becomes a north-east wind. 

 From similar considerations it will appear, that in 

 the southward hemisphere the winds will be north- 

 west and south-east respectively. 



The Trade-winds may therefore be explained 

 thns : The two general masses of air proceeding 



from 



