'7 



The National Geographic Magazine 



Winds, Calms, and Sailing Routes, May-October 



Upon emerging from the canal into the 

 Pacific Ocean a vessel will enter an ex- 

 tensive belt of calms and light airs, 

 which render navigation by sail more 

 tedious than in any other frequented 

 portion of the sea. 



In order to understand the feebleness 

 of the winds throughout this region, a 

 word is necessary as to the agency by 

 which winds are produced. Wind is 

 air in motion, the direction of the mo- 

 tion being always from an area through- 

 out which the atmospheric pressure is 

 high toward an area throughout which 

 it is low, the strength of the wind at 

 any point being within certain limits 

 proportional to the barometric gradient 

 or slope at that point. Where these 

 gradients are steep the winds are strong; 

 where they are gentle the winds are 

 light ; where they are absent, or, in 



other words, where the barometric press- 

 ure is uniform, the force giving rise to 

 the winds vanishes and the air is ac- 

 cordingly at rest. 



It is such a uniformity of pressure 

 that exists throughout the region under 

 consideration. Under normal conditions 

 the distribution of atmospheric pressure 

 over the earth's surface consists of two 

 extratropical belts of high pressure, 

 separated by an equatorial belt of low 

 pressure, each of these three belts com- 

 pletely encircling the globe. On the 

 equatorial slope of the high-pressure 

 belts there is a steady movement of the 

 air toward the low-pressure trough 

 which separates them, this movement 

 constituting in the northern hemisphere 

 the northeast trades, in the southern 

 hemisphere the southeast trades respect- 

 ively. Between these two, coinciding 



