Thus, in the Atlantic and Pacific Ocean we find the trade-winds perpetually blowing from the east, the north-east trade-wind between 9° and 27° N. lat., and the south-east trade-wind between 3° N. lat. and 25° S. lat. It was by their assistance that Columbus was enabled to discover America, and that the wretched barks of Magellan traversed the wide deserts of the Pacific from end to end.
Between these two regions of the trade-winds lies the dreaded zone or girdle of the equatorial calms (doldrums), where long calms alternate with dreadful storms, and the sultry air weighs heavily upon the spirits.
"Down dropt the breeze, the sails dropt down,
'Twas sad as sad could be;
And we did speak, only to break
The silence of the sea.
"Day after day, day after day,
We stuck, nor breath, nor motion,
As idle as a painted ship
Upon a painted ocean."
On their polar limits, the trade-wind zones are again girdled with calm belts, the horse latitudes, whose mean breadth is from ten to twelve degrees. The boundaries of these alternating regions of winds and calms are not invariably the same, on the contrary, they are perpetually moving to the north or south, according to the position of the sun.
From 40° N. lat. to the pole, westerly winds begin to be 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 conducted 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. &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 direction 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 importance 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.