

RELATIVE POSITION OF THE TWO WINDS. 251 



rates it, and when it finally enters the temperate zone, it falls to 

 the surface of the earth with clouds and rains, and strives for su- 

 premacy with the polar current. The difference of specific cold 

 between tl^p opposed masses of air must be very small, since by turns 

 each gains the advantage. Often the current coming from the 

 torrid zone, recognizable from below by its trains of cirri, cannot 

 reach the surface of the ground, and maintains itself as far as the 

 pole in the upper strata of the atmosphere, whilst the wind that 

 blows from the frigid zone forms a continuous current over the earth, 

 from the pole to the equator. Still, we must consider the south-west 

 wind as the prevailing wind of the northern temperate zone, for it 

 makes itself felt there much more frequently than the contrary cur- 

 rent, the proportion of the former being nearly double between the 50th 

 and 55th degree of latitude.* "We know that sailing vessels formerly 

 required 46 days on an average for the voyage from Europe to the 

 United States, whilst the return, facilitated likewise by the Gulf- 

 stream, f was accomplished in 23 days. The winds from the south- 

 west and west, which are nothing else than the counter-current of 

 the trade- winds, blow with such regularity in these parts, that one 

 might give the names of " ascending voyages " to the passages from 

 Europe to America, and " descending voyages " to those in the op- 

 posite direction. Corresponding phenomena occur in the southern 

 hemisphere ; there it is the north-west winds which blow most fre- 

 quently beyond the southern limits of the trade-winds. 



Thus the two permanent winds which are drawn towards the 

 equator by the expansion of the warm air have each their proper 

 domain, limited in one direction by the calms of the equinoctial line, 

 in the other by the irregular winds of the temperate zone. Still these 

 limits oscillate incessantly from month to month and from season to 

 season, and one cannot indicate them in a precise manner. On a 

 general map of the trade winds, it is sufficient therefore to trace the 

 extreme frontiers of these currents for winter and summer.^ On an 

 average, the space in the Atlantic over which the north-east wind 

 blows, embraces from 18 to 20 degrees of latitude, or from 1245 to 

 1275 miles ; in the South Pacific the domain of the south-east trade 

 wind would not be less than 30 degrees, § or 2045 miles. 



Between the atmospheric and oceanic currents the analogy is 

 evident. The maritime river which is founded at the junction of 



* Maury, Filot Charts. t See above, p. 76. 



X See above, Fig. 106. 

 h Kerhallet, Considerations GeneraUs sur V Ocean Facijique. 



