WINDS. 



367 



an upper-current of air proceeding from the warmer to the colder 

 region, and an under-current in an opposite direction. Now, as this 

 state of matters exists on the surface of the globe, where the mean 

 difference of temperature between the regions of the Equator and 

 those of the Poles amounts to upwards of 82°, and as the inequality 

 is nearly permanent, its effect is, to produce a constant interchange 

 between the polar and equatorial regions." 



" Thus a cold and dense under-current from the Poles replaces the 

 rarefied air of the Equator, which, ascending and forming an upper- 

 current, flows towards the Poles, north and south, in order to restore 

 the equilibrium of the atmosphere. 



" It is known that the earth revolves on its axis, in a direction 

 from west to east, increasing from the Poles to the Equator, where it 

 attains a velocity of 1000 miles per hour, — so that the air, in passing 

 from the highest latitudes towards the Equator, progressively arrives 

 at regions of increased rotary velocity ; and, as they cannot keep pace 

 with this increase of motion, they necessarily hang back, and form 

 currents flowing in a direction opposite to that of the rotation of the 

 earth, or from east to west, and thus by these combined efforts, the 

 northern and southern currents of air are deflected and modified, so 

 as to become the permanent northeasterly and southeasterly currents, 

 forming the magnificent phenomena of the Trade Winds. 



" The southwesterly winds, so prevalent in the Atlantic Ocean, 

 between the latitudes of 30° and GO , are produced by the upper- 

 current being drawn down to supply the superficial current which 

 goes towards the Equator ; and as it has a greater rotatory motion 

 than the earth in these latitudes, it produces a southwesterly wind. 

 The same is the case in the southern hemisphere, where northwesterly 

 winds prevail. 



" The theory is summed up, that whenever the air has greater 

 velocity of rotation than the surface of the earth, a wind more or less 

 westerly is produced, and when it has less velocity, a wind having an 

 easterly tendency results. 



" Thus there is a perpetual exchange between the different masses 

 of the atmosphere, — the warm air tempering the cold of the higher 

 latitudes, and the cold mitigating the heat of the lower. 



" The difference of temperature puts the air in motion, and the 

 direction of the resulting wind, at every place, depends upon the 

 difference between the rotary motion of the wind and the rotary 



