120 ABOUT THE WEATHER. 



and consequently the heavier, so that it begins to sink; 

 but since the heavy, cold Polar current forms, as it were, 

 a firm floor underneath, it flows away over this layer 

 of air towards the poles, and thus forms the second great 

 wind prevailing over the earth, which, from its origin, 

 is called the Equatorial current. To us, it comes as a 

 south wind ; in the southern hemisphere it is, of course, 

 the north wind. But just as the Polar current, in its 

 progress towards the equator, becomes gradually changed 

 into an east wind, from the same cause, the stream of air 

 flowing from the equator to the poles, diverted in the 

 opposite direction, becomes gradually a west wind. The 

 Equatorial currents naturally possess directly opposite pecu- 

 liarities to the Polar currents, being lighter, warmer 

 and moister; they cause the fall of the barometer, the 

 ascent of the thermometer, and give rise to the formation 

 of clouds, rain and snow. By these two streams in con- 

 junction, a continual circulation of the whole atmosphere 

 is maintained, rendering it impossible that any local 

 influence should anywhere cause a complete consumption 

 of those substances, in the atmosphere, necessary to life 

 oxygen and nitrogen ; or that the noxious one, carbonic 

 acid, should accumulate in excess. Thus is the existence 

 of all living nature dependent on this circulation. 



At the first glance, the simple and grand features of 

 the fundamental laws of atmospheric changes, as I have 

 endeavoured to sketch them, do not seem to agree at 

 all with what appears to us the capricious variation of 

 the weather, which in virtue of that very character, is 

 taken as the type of changeableness and inconstancy. The 

 following may, perhaps, explain this apparent contradiction. 

 The surface of the earth may be divided into two unequal 

 portions, according to their meteorological phenomena : 



