2^2 THE ATMOSPHEEE AND METEOEOLOGY. 



sometimes even neutralize them for a time. In tlie neighbourhood of 

 the coast, the extremes of heat and cold which succeed each other on 

 the continents cause the winds to deviate in their course,* and 

 consequently it is only in the open sea, at a great distance from the 

 coast, that the sails of ships are swollen by a breeze blowing almost 

 constantly from the same point of the horizon; but even then 

 the wind is stronger in the morning and evening than during the 

 heat of the day. In the Atlantic, bordered on each side by continents 

 tolerably regular in form, the trade- winds have the most uniform speed. 

 In the Pacific, the multitude of islands scattered over the surface of 

 the waters modify greatly the normal condition of the winds, and 

 over a very great extent of their natural domain the trade- winds are 

 transformed into monsoons. To the north of the equator the north- 

 east winds only blow in a constant manner, between the Revillagigedo, 

 and the Marianne Islands. As to the southern trade-winds, they are 

 still more restricted ; they commence with the group of the Gallapa- 

 gos at 1G20 miles from the coast of America, and towards the west, 

 they do not pass the Archipelago of Noukahiva and the Low Islands. 



In rushing one against the other, the two opposite winds hold 

 each other in check, and consequently their force is neutralized; 

 it is thus that a circular zone of calms, variable winds, and sud- 

 den aerial eddies is formed all round the earth, which, according 

 to the seasons, occupy a breadth of from 155 to 620 miles on the 

 surface of the sea. ^Nevertheless, we must not suppose that, in this 

 zone of so-called calms, the air is generally tranquil ; but the at- 

 mosphere is more often in a state of equilibrium there than in any 

 other part of the surface of the globe. According to the Pilot Charts 

 of Maury, the mean duration of the calms of the Atlantic between the 

 5th and 18th degrees of north latitude is to that of the winds in the 

 proportion of 98 to 802, or of 1 to 8. During the period when the 

 calms are most frequently produced, that is to say, in the month 

 of November, and in the space comprised between the 12th and 13th 

 degrees of north latitude, they prevail on an average half as much as 

 winds coming from any point whatever of the horizon. 



We can understand that this zone which separates the two trade- 

 winds of the north and south must necessarily be altered according 

 to the seasons by the position of the sun, since it occupies on the 

 circumference of the globe precisely those latitudes where the atmo- 

 spheric strata are most strongly heated by the solar rays, and where the 

 vertical movement of the expanded air is produced. When the sun, 

 * See below, Fig. 107. 



