EXCURSIONS OF AN EVOLUTIONIST 



We have supposed the lowering of the aver- 

 age winter temperature of England, for exam- 

 ple, due to the great aphelion distance of the 

 sun, to be 35 F. There is one way in which 

 this effect might be partially modified, and that 

 is by the equalizing influence of the Gulf 

 Stream. But in the case we have supposed, this 

 influence would almost certainly be cut off. The 

 direction of the main ocean-currents is deter- 

 mined by the trade-winds, and the trade-winds 

 are caused by the difference of temperature be- 

 tween the poles and the equator. As the heated 

 air at the equator rises, the cooler air from 

 north and south flows in to take its place, and 

 these atmospheric currents flowing from the 

 north and south poles toward the equator are 

 what are called the trade-winds. The strength 

 of the trade-winds depends entirely upon the 

 difference in temperature between the equator 

 and the pole ; the greater the difference, the 

 stronger the wind. Now, at the present time, 

 the south pole is much colder than the north 

 pole, and the southern trades are consequently 

 mucli stronger than the northern, so that the 

 neutral zone in which they meet lies some five 

 degrees north of the equator. The trade-winds, 

 pushing stupendous masses of surface ocean- 

 water, produce the main ocean-currents ; and 

 accordingly these currents now tend chiefly from 

 south to north, so that most of the heated water 

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