14 ANCESTRAL MAN. 



is due to difference of temperature ; and this augmented force 

 would be most operative in the trade winds. 



The North East Trades, then, being stronger than the South 

 East Trades, would blow across the equator ; and the line separa- 

 ting them would therefore be at some distance to Jhe south of the 

 equator. 



As the currents of the ocean are due to the prevailing winds, 

 the equatorial waters which are driven from east to west, would 

 also, in the case supposed, be impelled more to the southern than 

 to the northern hemisphere. 



What happens at present, is as folloivs. The equatorial current 

 of the Atlantic, on approaching the American coast, strikes against 

 the wedge-like projection of Cape St. Roque and there bifurcates, 

 one portion of the current being deflected to the south and the 

 other to the north. It is this northern portion which becomes 

 what is called the Gulf Stream. The tepid current of the Gulf 

 Stream moving northwards, ultimately gains admission to the 

 arctic ocean ; but in its course it retains the rotational velocity of 

 the equator and is thereby pressed upon the western shores of 

 Europe and Spitzbergen, greatly modifying the winter climate of 

 those countries. Indeed, the total quantity of heat conveyed by 

 the Gulf Stream is nearly equal to one half of the sun-heat 

 received by the arctic regions. Further, the entrance of the Gulf 

 Stream into the arctic circle necessarily produces, on hydro- 

 dynamic principles, an equal and opposite counter current. This 

 cold stream, known as the Labrador Current, moving down from 

 the pole, is caught, in consequence of the earth's rotation, by the 

 east coast of North America, which receives its chilling influence 

 as far as Cape Cod. 



Quite other things would befall in the case we are supposing, for 

 the equatorial current of the Atlantic would reach the American 

 continent south of the projection of Cape St. Roque, and striking 

 obliquely against the Brazilian coast would be altogether deflected 

 into the southern ocean. The Gulf Stream, therefore, would not 

 exist ; and the waters which constitute it would carry all their 

 heat into the already warm southern regions, whose tropics would 

 receive nearly double their present amount of warm water entirely 

 at the expense of the northern hemisphere, while the temperature 

 of the English winter would become colder by more than 25 F. 

 And the Labrador Current would not exist ; and its cold waters 

 would remain, to be entirely frozen within the Arctic Circle and 

 to aggravate that difference of temperature already considered. 



These, then, are the chief reasons for believing that in the 

 varying eccentricity of the earth's orbit, combined with the 

 equinoctial precession, lies the cause of recurrent glacial periods. 



