EXPLORING THE UPPER AIR 



of the rim of the amphitheatre it came under the 

 control of the air currents characteristic of the 

 district and might pass out of sight behind the rock 

 walls. Fortunately the amphitheatre was open 

 at the west and the balloon in most cases started 

 off in that direction, to reverse direction later when 

 it was at a higher level and so more easily seen 

 from the amphitheatre. From this first season 

 our pilot balloons were followed upon the average 

 to an altitude of about 7,000 meters or about 4 

 miles above the surface of the earth. Many were 

 followed to 10,000 meters, and one to 14,000 me- 

 ters, or about eight miles. Rather generally they 

 traveled westward toward the coast in the lower 

 levels to reverse direction and disappear toward 

 the ice-cap to the eastward, thus showing that for 

 much of the time the winds above our observing 

 station were controlled by ice-cap conditions. 



The pilot balloon reveals the characteristics of 

 the wind currents only, not the pressure, tempera- 

 ture, or humidity of the air. These important 

 elements are supplied by the larger register- 

 ing balloons, which carry instruments to record 

 automatically the three elements at each level 

 passed through by the balloon. Such balloons are 

 also followed throughout their course by the theodo- 

 lite in the same manner as the pilot balloons. The 



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