«H PHYSICAL GEOGBAPHY OF THE SEA. 



prevalent, and in the Atlantic Ocean their proportion to the 

 easterly winds is as two to one. 



In the Northern Indian Ocean and in the Chinese Sea we also 

 find the trade-wind, which is there called the north-east monsoon; 

 here, however, it only blows from October to April, as during 

 the summer terrestrial influences prevail which completely divert 

 it from its course. 



From the wide plains of central Asia, glowing with the 

 rays of a perpetually unclouded sun, the rarefied air rises 

 into the higher regions. Other columns of air rush from the 

 equator to fill up the void, and cause the trade-wind to vary 

 its course, and change into the south-western monsoons of the 

 Indian Ocean, which blow from May to September. The 

 regularly alternating monsoons materially contributed to the 

 early development of navigation in the Indian seas, and con- 

 ducted the Greeks and Eomans as far as Ceylon, Malacca, and 

 the Gulf of Siam. Similar monsoons, or deflections from the 

 ordinary course of the trade-winds, occur also in the Mexican 

 Gulf, in the Gulf of Guinea, and in that part of the Pacific 

 which borders on Central America, through the influence of 

 the heated plains of Africa, Utah, Texas, and New Mexico. 



The passage from one monsoon to the other is of course only 

 gradual, since the land also is only gradually heated and cooled. 

 Thus at the change of the monsoon, an atmospheric war oi 

 several weeks' continuance occurs, during which the trade-wind 

 and the monsoon measure their strength, and calms alternate 

 with dreadful storms (typhoons, cyclones, tornadoes). 



According to the researches and observations of Franklin, 

 Cooper, Eedfield, Eeid, &c. &c, these storms are great rotatory 

 winds, that move along a curved line in increasing circles. In 

 the northern hemisphere, the rotatory movement follows a direc- 

 tion contrary to that of the hands of a clock ; while the opposite 

 takes place in the southern hemisphere. The knowledge of the 

 laws which regulate the movements of storms is of great impor- 

 tance to the mariner, since it points out to him the direction he 

 has to give his ship to gain the external limits of the tornado, 

 and thus to remove it from danger. 



Water-spouts are formed by two winds blowing in opposite 

 directions, and raising or sucking up the water in their vortex 

 They generally form a double cone : the superior part with its 



