WIND 



217 



runs under it and pushes it up so that the oil overflows 

 into the dish that the water has left. The same thing 

 happens in the atmosphere when from any cause the column 

 of air above one place becomes heavier than that above 

 another place. There will be under 

 these conditions a transfer of air, 

 along the surface, from the place 

 where the pressure is greater to that 

 where it is less great, and this move- 

 ment of the air we call wind. 



The wind on the surface of the 

 earth is not usually in the same 

 direction as that high up. The 

 strength of the wind depends upon 

 differences in air pressures. As the 

 air pressure is measured by the 

 barometer, the wind is commonly . 



/ In this common appliance 



spoken of as due to a difference in the heat of the stove 

 barometric pressure or to the baro- 

 metric gradient. Winds are named 

 from the direction from which they 

 come. A west wind is a wind that 

 blows from the west. 



If there were no other forces that 

 affected the movement of the air, 

 except the high and low pressures, 

 the transfer would be in a straight line from one place to 

 the other, and it could always be told in what direction the 

 high and low pressures were, by direction of the wind. 

 But obstacles like mountains and hills deflect the air 

 currents. Chief among other causes which influence the 

 direction of air movements is the rotation of the earth. 



HOT WATER TANK 



causes the water to circu- 

 late in a way similar to 

 that in which the air is 

 caused to circulate by 

 the heated surface of the 

 earth. The hot water 

 rises to the top of the 

 tank from where the 

 pressure 01 the cold 

 water in the supply 

 cistern will cause it to 

 flow. 



