306 OUTLINES OP NATURAL PHILOSOPHY. 



If the surface of the earth were wholly covered with 

 water, so that there were no part of it more dispo- 

 sed than another to obstruct the motion of the air, 

 or no part which had a greater capacity than ano- 

 ther, of acquiring or communicating heat, the air 

 would probably circulate continually from the Poles 

 to the Equator, and back again, without any irregu- 

 larity whatsoever. 



416. In consequence of the rotation of the earth 

 on its axis, another motion is combined with that 

 of the currents just described. The air, which is 

 constantly moving from points where the earth's 

 motion on its axis is slower, to those where it is 

 quicker, cannot have precisely the same motion 

 eastward with the part of the surface over which 

 it is passing, and therefore must, relatively to that 

 surface, describe a curve, having its convexity turn- 

 ed to the east. The two currents, therefore, from 

 the opposite hemispheres, when they meet toward 

 the middle of the earth, having each acquired an 

 apparent motion westward ; and as their opposite 

 motions from south and north must destroy one 

 another, nothing will remain but this motion west- 

 ward, by which they will go on together, and form 

 a wind blowing directly from the east. 



This is the cause of the Trade Wind, which (with 

 certain exceptions) blows continually between the 



Tropicsy 



