136 The National Geographic Magazine 



fined regions such as the tropics, cannot 

 but impress us with the idea that it is in 

 a general way cyclical — that is, that the 

 same water, after the lapse of time, re- 

 traverses approximately the same path. 



The source of the energy required to 

 set and keep this vast mass in motion 

 has been productive of endless discus- 

 sion. The attractive force of the moon, 

 the vis inertia or lag of the water itself, 

 the difference in temperature and specific 

 gravity of the equatorial and polar re- 

 gions, the unequal distribution of at- 

 mospheric pressure — each in its turn 

 has been proposed and strenuously ad- 

 vocated as the true and only cause of 

 the ocean currents. To the seaman, 

 however, the cause of the ocean currents 

 has always been the winds, the motion 

 of the waters of the sea taking its origin 

 in the region where the latter attain 

 their maximum constancy, viz. , in the 

 region of the trades. 



The trade winds cover a belt on the 

 earth's surface extending roughly over 

 50 degrees of latitude, from 30 N. to 

 20 S., including within this belt a 

 greater water area than could be in- 

 cluded in any other position. Through- 

 out this wide zone the wind blows for 

 90 per cent of the time from some point 

 in the eastern semicircle. In the south- 

 ern hemisphere the trades are somewhat 

 stronger and more constant than in the 

 northern, owing probably to the freedom 

 from interrupting land areas. Over the 

 eastern half of the ocean they extend 

 far higher in latitude than over the 

 western. This is true of both hemi- 

 spheres, the northern and the southern, 

 the northeast trades in the Atlantic 

 during the northern summer often ex- 

 tending far up on the coast of Spain, 

 the southeast trades during the southern 

 summer often extending beyond the 

 Cape of Good Hope. Similar conditions 

 hold for the Pacific. The southeast 

 trades, too, blow well across the equator 

 into the northern hemisphere. 



The trade winds, however, are not 



continuous throughout the entire belt 

 from north to south. Just north of the 

 equator and confined entirely to the 

 northern hemisphere, extending east 

 and west, is an elongated triangular 

 area, the base of the triangle, in length 

 some 1 5 of latitude, resting in the case 

 of the Atlantic Ocean on the coast of 

 Africa, in the case of the Pacific on the 

 coast of Central America and Mexico, 

 throughout which the trades are absent, 

 their place being taken during a large 

 portion of the year by light, variable 

 winds and calms, during the remainder 

 of the year by winds whose prevailing 

 direction is southwest — the so-called 

 southwest monsoon of the African and 

 American coast, most apparent during 

 July, August, and September. 



THE CHARACTER OF THE TRADE 

 WINDS 



Among those who have not sailed in 

 them the impression is general that the 

 trades blow day after da3* steadily in one 

 direction and with a constant force. 

 This is distinctly not the case. The 

 trade winds are quite as susceptible to 

 variation, and fortunately so, as the 

 winds of higher latitudes. The one 

 thing about them is that, not being sub- 

 ject to the large variations of barometric 

 pressure which characterize higher lati- 

 tudes, the wind rarely goes around the 

 compass and, indeed, rarely gets out of 

 the eastern semicircle. As an example 

 of their constancy, let us consider the 

 percentage of winds coming from each 

 compass point for a certain region, for 

 instance, the square bounded by the 

 parallels 20°-25° N. and the meridians 

 50°-55 ° W. , in the heart, therefore, of 

 the northeast trades in the north Atlan- 

 tic. The figures are for the month of 

 June, and may be regarded as giving 

 the number of hours in each hundred, 

 or, approximately, in 4 days, that the 

 wind may be expected to blow from the 

 given point: 



