INDS; 129 



from it, precisely similar to the movement of the air, heated by 

 contact with the land, and perhaps the sloping sides of moun- 

 tains. The most remarkable aerial currents, and of the greatest 

 importance in navigation, are the trade winds, which, within the 

 tropics, blow continually from the east, though with variable force, 

 and declining north and south, according to the latitude, and sea- 

 son of the year. The primary cause of the trade winds is anala- 

 gfrus to that of the lan,d and sea breezes, though much more ex- 

 tensive, and maintaining a constant direction, and likewise united 

 with the influence of another cause, viz : the rotation of the 

 earth. We have already had occasion to remark that within the 

 two tropics lies a belt, over some part of which the sun is verti- 

 cal at noon, at all seasons of the year, hence this equatorial belt, 

 in the torrid zone, is continually heated by the sun, and a large 

 body of air warmed by contact with the heated ground, rises con- 

 stantly to the upper regions. Its place is supplied by colder air 

 moving along the surface of the earth, from the colder northern 

 and southern climates. The effect of this would be to produce 

 a north wind north of the equator, and a south wind south of the 

 equator. The portion of air thus transferred from the higher lati- 

 tudes to the equator has a slower diurnal motion than at the equa- 

 torial regions. Perhaps a diagram will make this more plain. 



Let P be ane of the poles of the earth, A B a parallel of latitude; 

 and E E, the equator. Now it is eviden^that the diurnal move- 

 ment of a body attached to the parallel A B, is slower than one 

 at the equator E E, since the equatorial belt or circle, is of much 

 greater circumference than the circle of latitude, and both are 



