318 THE CHEMISTRY OF CREATION. ' 



chimney, and rising into the air in an almost 

 perpendicular column ; the leaves on the tall 

 poplars beyond are still, and the few fleecy 

 clouds which rest on the blue sky have no 

 sensible motion. The atmosphere, then, is 

 calm and motionless. The apparent wind is 

 the result of the resistance a swiftly moving 

 body experiences in passing through the air. 

 The current is, in fact, the difference between 

 the motion of the carriage and that of the air 

 through which it passes. In like manner the 

 mariner placed on the watery surface of our 

 swiftly rotating globe is whirled along insen- 

 sible to himself at a greater velocity than the 

 bed of air which lies above him, and the sen- 

 sible effect is that he perceives a strong and 

 equable wind in the opposite direction. 



Having thus hastily traced the lower current 

 to the equator, let us now follow the ascending 

 column. Upon rising to a certain altitude, it is 

 there to some extent cooled, by parting with its 

 heat into space by radiation, and its upward 

 progress is necessarily arrested ; the current is 

 then deflected, and flows towards the poles in a 

 grand stream. As it proceeds it still loses heat, 

 and at about the 30th degree of latitude, it is 

 so cold as to descend and change places with 

 the lower current from the poles. Proceeding 

 onwards still it receives heat from its contact 



