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POPULAR SCIENCE MONTHLY. 



blows from each side, and a descending current of air near parallel 

 32 of each hemisphere, from which the air flows north and south. 



Friction between the surface of the earth and the atmosphere 

 tends to carry the air with the earth in its rotation. As the velocity 

 of the earth's surface is nothing at the poles and increases toward the 



Fig. 3. — Showing the Origins of the Highs and their Paths. 



equator, those winds which blow toward the equator will lag behind 

 and have a westerly direction, and those that blow toward the poles 

 will retain their greater velocity of the lower latitudes and travel 

 faster than the more northern parallels, resulting in an eastward 

 direction over the earth's surface. 



In the temperate zone of the northern hemisphere there is a 

 general northeasterly drift of the surface atmosphere at a fairly 

 regular rate of motion, and this causes our storms almost without 

 exception to travel from west to east. This eastward tendency of all 

 atmospheric disturbances is the basis of all predictions. 



In the great ocean of air, as in the ocean of water, there are con- 

 stantly occurring waves and hollows, or areas where the air is piled 

 to an unusual height, showing increased pressure on the barometer, 

 and areas where the height and pressure are less than the normal. 

 These high and low areas, or "highs" and "lows," as they are 

 technically known, travel in a general northeasterly direction. In 

 Fig. 2 is shown an actual case of a low between two highs in the 

 United States, and it is extremely interesting to notice the laws of the 

 winds around them. The finer, oval lines are used to connect all 

 points having equal barometric pressure. They are known as " iso- 



