CHANGED BY EARTH'S TTJBNING. 217 



since the first voyage of Columbus. Their long 

 continuance caused much fear to the Spanish 

 sailors, who believed that this everlasting east- 

 wind would render their return to Europe very 

 difficult. The illustrious astronomer Halley, as 

 early as the year 1685, gave a satisfactory 

 explanation of them, by proving, that the 

 currents of air, as far as regards their main direc- 

 tions, from north to south, and from south to 

 north, could only blow, on an earth at rest, in 

 lines parallel to the meridians, but that these 

 directions must suffer a very considerable change 

 from the tui'ning of the earth round its axis. The 

 speed with which any point on the surface of the 

 earth moves round, is greater or less according to 

 the size of the circle of latitude in which it lies. 

 It increases from the poles, where it is nothing, 

 to the ecpiator, where it is greatest, amounting 

 there to fourteen hundred and twenty-seven feet in 

 a second. 



In time of calm the air has no other motion 

 than that which it shares with other earthly 

 bodies ; it is moving round with the speed of the 

 place at which it lies. If, however, the wind 

 blows, — that is, if the air have another motion be- 

 sides that which belongs to it as a part of the earth, 

 — it can keep its direction unchanged in spite of 

 the rotation of the earth only in one case. And 

 this is when the wind is exactly due east or due 



