66 PHYSICAL GEOGRAPHY OF THE SEA. 
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, and 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 
Sottom 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-stveam) 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 enrth, 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. 
