PNEUMATICS, 309 



degrees of resistance which the air meets with in 

 passing over the surface, and the different capaci- 

 ties of that surface for receiving and communica^ 

 ting heat. 



The motion of the inferior and superior currents, 

 may be seen exemplified on opening a door, between 

 two apartments of different temperatures. The 

 flame of a candle near the ground, will shew the 

 stream that sets from the colder room to the war- 

 mer ; near the top it will indicate a sream in the op- 

 posite direction. 



As the average quantities of the air carried by these 

 opposite currents are equal, the surface that sepa- 

 rates them is probably not far different from that 

 at which the barometer would stand at 15 inches, 

 or half its medium height at the surface. 



If we suppose the mean temperature is 3, this ele- 

 vation will be found 3010 fathoms, or 18,060 

 feet = 3.425 miles, not so high as the summits of 

 the Cordilleras. 



The upper stream in each hemisphere being directed 

 to one point, the pole of that hemisphere, there must 

 arise a considerable condensation, and acceleration of 

 the air, as the currents approach that point, and 

 hence the causes of irregular winds must be increased 

 on approaching the poles. 



The general direction of the upper current must be 

 to the westward, for the same reason that that of 

 the lower was toward the east. 



419. The 



