63 PHYSICAL GEOGRAPHY 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 alsu 
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 Romans 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 of 
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, Redfield, Reid, &c. &e., 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 
