Geology 



In common with all other gases, air when heated 

 expands, and therefore becomes lighter, as there is only 

 the same amount of matter in a larger space. This 

 is an everyday experience, as it is responsible for the 

 rising of hot air and smoke from our chimneys, and 

 for the accumulation of hot air near the ceilings of our 

 rooms. The action can also be seen in the case of 

 the paper fire-balloon used in connection with galas 

 and firework displays. 



It is a matter of general knowledge, that certain 

 parts of the Earth's surface receive a much more generous 

 supply of heat from the Sun than do others, and this 

 fact is responsible for the movements of the air which 

 we call winds. 



First, let us consider the state of affairs at one of 

 the two periods of the year when the Sun is immediately 

 overhead at the equator, known as the equinoxes. 



At these periods the regions in the vicinity of the 

 equator will be much more strongly heated by the 

 Sun's rays than regions more remote, and consequently 

 the air in the equatorial belt will, by reason of its 

 expansion, become lighter than that in the regions to 

 the North and South. The heavier air in the cooler 

 belts will, therefore, flow in towards the equator, 

 displacing and floating upward the air of the equatorial 

 belt. 



Near the surface of the ground, therefore, we shall 

 find air currents or winds, flowing from northerly and 

 southerly directions towards the equator. The warm 

 equatorial air pushed upwards by them overflows the 

 colder currents, and thus comes to move outwards from 

 the equator towards the poles. 



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