The Science of the Weather 771 



broad features are indicated in the general circulation of the wind 

 and the distribution of pressure and temperature on the earth's 

 surface. Along the Equator there exists a belt of calms, called 

 the "doldrums"; this belt is the mainspring of the great atmos- 

 pheric currents, for here the maximum effect of the sun's heat 

 is felt; the heated air is rising and overflowing in the upper 

 regions. To take the place of this rising air, a further supply 

 flows in from north and from south, and this supply comes from 

 the neighbourhood of the Tropics of Cancer and Capricorn 

 respectively. 



Now these winds are moving over a globe rotating on its 

 axis, and a curious thing happens to the winds on account of 

 this rotation of the earth. Every passenger who has boarded a 

 tramcar as it is turning a street corner has experienced the sensa- 

 tion of not being able to walk straight along the car, but being 

 thrown to the side instead. The reason is that the passenger 

 tends to keep moving in the direction of which he began, but 

 the car is twisting beneath him. So it is with the wind which 

 moves over the earth's surface above or below the Equator the 

 earth's surface is twisting beneath the wind, and thus the wind 

 appears to become deflected from its original direction relative 

 to the surface of the earth. The air which surrounds us is, in a 

 word, being carried along with the revolving earth, just as the 

 passenger is being carried by the car, but as the wind can move 

 independently of the earth (or the passenger independently of 

 the car), a change in the direction of the latter produces an 

 apparent change in the direction of the former. In the Northern 

 Hemisphere the deflection is to the right, and in the Southern 

 Hemisphere to the left. 



The Trade Winds 



Therefore the winds moving southward and northward to- 

 wards the Equator become deflected, and are found blowing from 

 north-easterly and south-easterly directions respectively. These 



