THE WINDS. 283 



tion, that the position of these trade-wind zones is variable ; that 

 is, that the equatorial edge of the southeast trade-wind zone is far- 

 ther to the north in our summer, when the northeast trades are 

 most feeble, than it is in winter, when they are strongest. 



818. We have here, then, at work upon these trade-wind zones, 

 a force now weak, now strong, which, of course, would cause these 

 zones to vibrate up and down the ocean, and within certain lim- 

 its, according to the season of the year. These limits are given 

 on Plate YIII. for spring and autumn. During the latter season 

 these zones reach their extreme northern declination, and in our 

 spring their utmost limits toward the south. 



819. Changing of the Monsoons. — Lt. Jansen, in his appendix 

 to the Dutch edition of this work, thus describes this phenomenon : 



" We have seen (§ 262) that the calms which precede the sea- 

 breeze generally continue longer, and are accompanied with an 

 upward motion of the air ; that, on the contrary, those which pre- 

 cede the land-breeze are, in the Java Sea, generally of shorter 

 duration, accompanied by a heavy atmosphere, and that there is 

 also an evident difference between the conversion of the land- 

 breeze into the sea-breeze, and of the latter into the former. 



820. " Even as the calms vary, so there appears to be a marked 

 difference between the changing of the monsoons in the spring and 

 in the autumn in the Java Sea. As soon as the sun has crossed 

 the equator, and its vertical rays begin to play more and more 

 perpendicularly upon the northern hemisphere, the inland plains 

 of Asia, North Africa, and of North America are so heated as to 

 give birth to the southwest monsoons in the China Sea, in the 

 North Indian Ocean, in the North Atlantic, and upon the west 

 coast of Central America: then the northwest monsoon disap- 

 pears from the East Indian Archipelago, and gives place to the 

 southeast trade-wind, which is known as the east monsoon, just 

 as the northwest wind, which prevails during the southern sum- 

 mer, is called the west monsoon. 



821. "This is the only monsoon which is found in the south- 

 ern hemisphere, while in the northern hemisphere the northeast 

 trade-wind blows in the China Sea and in the Indian Ocean ; in 

 the East Indian Archipelago the west monsoon prevails ; and here. 



