THE UPPER CURRENTS. 
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per currents—the return trades, or antitrades, as they 
are often called—reach the horse latitudes the greater 
part of the air settles down to the earth and thus be¬ 
comes the feeder of the trade-winds. The rest of it 
continues east, slightly northeast, spirally around the 
earth toward the poles. This circular motion around 
the poles creates a valley or rare area there. This 
idea may be illustrated by suspending a pail of water 
from the ceiling and twisting the rope. When the 
pail is left free to whirl, the water heaps up at the 
edge of the pail, leaving a depression in the center. 
The air moving toward the poles would, of course, 
soon pile up there if it did not in some way flow back 
to the equator. Where and how it does that is not 
so well defined. Some of it gets back as a surface 
wind, and part of it as upper currents ; sometimes it 
seems to glide south between the upper and lower 
currents of the earth. All of us have experienced 
