68 PNEUMATICS. 



Phenomena of the Winds. 



The phenomena of the winds, however various, 

 are supposed to result from two general movements ; 

 that of the cold air from the pole towards the torrid 

 zone, and of an upper and contrary current from the 

 equator towards the pole. The line of division be- 

 tween the upper and lower currents should be where 

 the mercury stands at 15 inches, which is at the height 

 of 83 miles. This ought to be the boundary of the 

 region of the clouds, which indeed are sometimes 

 seen to approach this elevation. 



In Great Britain, on an average of ten years, 

 westerly winds exceed the easterly in the proportion 

 of 225 to 140 days. 



Effects of the Winds. 



The different ingredients of the atmosphere are 

 mixed together by high winds, which, when they are 

 strong, move at the rate of from 60 to 100 miles per 

 hour. In our winter the south-west winds convey 

 the air that has been purified by the vast forests and 

 savannahs of South America, and which, passing over 

 the ocean, arrives uncontaminated. 





 Atmospheric Pressure. 



The ordinary pressure of the atmosphere, at the 

 surface of the earth, is equal to 1 5 pounds on the 

 square inch of surface. Ten atmospheres therefore 

 are 150 pounds, twenty, 300 pounds, and so on. 



Variation of the Atmospheric Pressure. 



The annual range of the barometer, which indicates 

 the variation of the pressure, does not exceed ^th to 

 | inch in the torrid zone ; is about 2 inches at Liver- 

 pool and St. Petersburgh, and 1 -^ inches at Melville 

 Island. The extreme variation) which no where ex- 



