WIND 5 



denser, and thus exercises a greater pressure on the 

 surface, and tends to spread into the areas of lower 

 pressure. The air thus set in motion is wind, and the 

 wind is part of a great and never-ceasing system of 

 circulation of the atmosphere of the globe. 



Broadly speaking, the heating of air between the 

 tropics and the cooling of air within the polar circles 

 are responsible for the whole system of atmospheric 

 circulation, including the movement of surface air 

 from the tropics towards the poles and the return 

 currents of air in the upper atmosphere. The fact 

 that these movements are taking place, not on a 

 motionless plane, but on the surface of a rotating 

 sphere, accounts for the deviation of the winds from 

 a north and south direction. It is convenient to 

 remember that any moving body (whether air, water, 

 or projectile) on the surface of the Earth is caused by 

 the Earth's rotation to deviate towards the right as 

 it moves, no matter in what direction, in the northern 

 hemisphere, and towards the left as it moves, no matter 

 in what direction, in the southern hemisphere, the 

 observer in all cases looking in the direction towards 

 which the body is moving. A pole-seeking wind thus 

 acquires a trend from the west as well as from the 

 Equator, and an Equator-seeking wind acquires a trend 

 from the east as well as from the pole. 



There is an intimate connection between the dis- 

 tribution of atmospheric pressure on the surface and 

 the direction of prevailing winds, the general relation- 

 ship being that wind blows obliquely from areas of 

 high pressure towards areas of low pressure. The dis- 

 tribution of pressure is expressed in isobaric charts, 

 on the same principle as the representation of heights 

 or depths by contour-lines on orographical maps, or on 

 the ordinary sea charts ; and, in order to deduce from 



