ABOUT THE WEATHER. 119 



north wind ; in the southern, a south wind. We must 

 bear in mind, however, that this current or wind is only a 

 portion of the atmosphere in motion, and this is bound 

 wholly to the earth and its destiny, revolving with it, as has 

 already been mentioned, round the axis from west to east. 

 Now, this revolution, as a glance at the globe shows, takes 

 place with different velocities in different places. While 

 the air at the poles only turns upon itself, without pro- 

 gressing forward, the air at the equator speeds on through 

 more than 1,000 miles in an hour. Then, if we imagine 

 the air from the pole to be suddenly removed to the equator, 

 some time must elapse before it could acquire the same ve- 

 locity of motion from west to east which is possessed by 

 the air always there ; it would remain behind, therefore, the 

 earth gliding away, as it were, from beneath it, or in other 

 words, it would have the appearance, in going from east 

 to west, of an east wind. Applying this to the Polar 

 currents, we see that the longer they blow, the nearer they 

 approach the equator, the more must they appear as north- 

 east and south-east winds. In reality, there is a region on 

 each side of the region of calms and storms, in which, year 

 after year, there blows in the northern an east-north-east, in 

 the southern an east-south-east wind, which all sailors know 

 by the name of the trade- winds. 



I have now only to mention that the polar air is heavier, 

 colder, and drier ; that, therefore, in the north, north-east, 

 and east winds, (they are all one wind), the barometer 

 rises, the thermometer sinks and the sky becomes clearer : 

 and thus all the essential peculiarities of one principal 

 wind, the Polar current, have been named. 



We must now inquire farther about the fate of the 

 heated air, which forms the constantly ascending stream 

 in the tropics. The higher it rises, the cooler it becomes, 



