76 SCIENCE OF COMMON THINGS. 



"Wind always blows. Effect of mountains on winds. 



movements of the atmosphere we apply the designation 

 of wind. 



471 Does the wind always blow f 



Yes ; there is always some motion in the air ; but the 

 violence of the motion is perpetually varying. 



473 Does the rotation of the earth upon its axis affect the motion of 

 the air f 



1 Yes, in two ways : 1. As the earth moves round its 

 axis, the thin movable air is left somewhat behind, and 

 therefore seems (to a stationary object) to be blowing 

 in the opposite direction to the earth's motion ; and 



2. As the earth revolves, different portions of its sur- 

 face are continually passing under the vertical rays of 

 the sun. 



473 When are the rays of the sun called vertical rays ? 



When the sun is in a direct line above any place, his 

 rays are said to be "vertical " to that place. 



474 When the sun is vertical, or nearly over head at any place, what 

 time of day is it at that place ? 



Noon. 



475 How does a change in the heat of air produce wind f 



The air always seeks to preserve an equilibrium so 

 cold air rushes into the void made by the upward cur- 

 rent of warm air. 



470 Why does not the wind always blow one way, following the direc- 

 tion of the sun f 



Because the direction of the wind is subject to per- 

 petual interruption from kills and valleys, deserts, 

 seas, &c. 



477 How can hills or mountains affect or change the direction and course 

 of the wind ? 



If a current of air, blowing from a particular direc- 

 tion, strike against the side of a mountain, it will neces- 

 sarily be deflected from a straight line, and must either 

 ascend the mountain, turn back, or assume a lateral 

 direction. 



478 Why are those winds which llow over large continents or tracts 

 of land generally dry ? 



