WIND 143 



one of the dishes with colored water and the other with kerosene or 

 some light oil. 



Although the two liquid columns are similar, yet the pressure at 

 the bottom of the dish of water will be greater than that at the bottom 

 of the dish of oil since the water is heavier than the oil. These are 

 the conditions that exist on the surface of the earth at two places one 

 of which has a high and the other a low barometer. Release the 

 Hoffman's screw upon the top tube and then the one at the bottom. 

 Notice carefully what happens as the lower tube is allowed to open. 

 The dishes are not now filled with oil and water respectively. In the 

 transfer of the liquids, through which tube did each pass ? (If part 

 of each rubber tube is replaced by a glass tube, the action in the 

 experiment can be seen to better advantage.) 



Experiment 79. Fill a convection apparatus with water, putting in 

 a little sawdust and mixing it well with the water. Heat one side of 

 the tube and observe the convection currents set up. 



In Experiment 78 the interflow from one dish 

 to the other is due to the fact that the water is 

 heavier than the oil and 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 qne 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 movement 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 spoken of as due to a difference in barometric 

 pressure or to the barometric 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 



