66 PHYSICAL GEOGEAPHT OF THE SEA. 



I 

 local injuries, by the constant and inestimable benefits they 

 confer on the whole body of mankind. 



On taking a comprehensive view of their origin, we find 

 that, like the oceanic currents, they are chiefly caused by the 

 unequal influence of solar warmth upon the atmosphere under 

 the line and at the poles. In the torrid zone, the air, rarefied 

 by intense heat, ascends in perpendicular columns high above 

 the surface of the earth, aad there flows off towards the poles, in 

 the same manner as in a vase filled with cold water and placed 

 over the flame of a lamp, the warmed liquid rises from the 

 bottom and spreads over the surface. 



But cold air-currents must naturally come flowing in an 

 opposite direction from the poles .to the equator to fill up the 

 void, as in the example I have cited, colder and consequently 

 heavier water comes streaming down the sides of the vase to 

 replace the liquid which is rising in the centre under the 

 influence of heat. 



Thus the unequal distribution of solar warmth over the 

 surface of the earth evidently generates a constant circulation 

 of air from the equator to the poles, and from the icy regions to 

 the tropics, and by this means the purity of the atmosphere is 

 chiefly maintained. The sun is not only the great fountain of 

 warmth, he is also the universal ventilator ; he not only calls 

 forth animal life, but at the same time, by a simple and admirable 

 mechanism, provides for its health by constantly renewing the air, 

 which is essential to its existence. 



If caloric were the sole agent which influences the direction 

 of the winds, or if the earth were one uniform plain, the opposite 

 air-currents I have mentioned would naturally flow straight to the 

 north and south ; but their course is modified or diverted in the 

 same manner as that of the ocean-currents by the rotation of 

 the globe. Thus, the cold air-current (polar-stream) which 

 comes rushing upon us from the Arctic regions, is felt in our 

 latitude as the biting east or north-east wind, so trying to our 

 nerves and organs of respiration, while we enjoy the warm 

 air-current from the tropics as the mild western or south-western 

 breeze. 



But besides the rotation of the earth, there are many other 

 local influences by which the winds are deflected from their 

 course, or by whose agency partial air-currents are called forth. 



