These winds are somewhat changed by the seasons of the year; for when the sun is far northward, the Brasil S. E. wind gets to the south, and the N. E. wind to the east; and when the sun is far south, the S. E. wind gets to the east, and the N. E. winds on this side of the equator veer more to the north.
8th. Along the coast of Guinea, from Sierra Leone to the island of St. Thomas, (under the equator) which is above 500 leagues, the southerly and south-west winds blow perpetually: for the S. E. trade-wind having passed the equator, and approaching the Guinea coast within 80 or 100 leagues, inclines towards the shore, and becomes south, then S. E. and by degrees, as it approaches the land, it veers about to south, S. S. W. and when very near the land it is S. W. and sometimes W. S. W. This tract is troubled with frequent calms, violent sudden gusts of wind, called tornadoes, blowing from all points of the horizon.
The reason of the wind setting in west on the coast of Guinea, is in all probability owing to the nature of the coast, which being greatly heated by the sun, rarefies the air exceedingly, and consequently the cool air from off the sea will keep rushing in to restore the equilibrium.
9th. Between the 4th and 10th degrees of north latitude, and between the longitude of Cape Verd, and the eastermost of the Cape Verd isles, there is a track of sea which seems to be condemned to perpetual calms, attended with terrible thunder and lightnings, and such frequent rains, that this part of the sea is called the rains. In sailing through these six degrees, ships are said to have been sometimes detained whole months.
The cause of this is apparently, that the westerly winds setting in on this coast, and meeting the general easterly wind in this track, balance each other, and so produce the calms; and the vapours carried thither by each wind meeting and condensing, occasion the almost constant rains.
The last three observations shew the reason of two things which mariners experience in sailing from Europe to India, and in the Guinea trade.
And first. The difficulty which ships in going to the southward, especially in the months of July and August, find in passing between the coast of Guinea and Brasil, notwithstanding the width of this sea is more than 500 leagues. This happens, because the S. E. winds at that time of the year commonly extend some degrees beyond the ordinary limits of 4° N. latitude; and besides coming so much southerly, as to be sometimes south, sometimes a point or two to the west; it then only remains to ply to windward: And if, on the one side, they steer W. S. W. they get a wind more and more easterly; but then there is danger of falling in with the Brasilian coast, or shoals: and if they steer E. S. E. they fall into the neighbourhood of the coast of Guinea, from whence they cannot depart without running easterly as far as the island of St. Thomas; and this is the constant practice of all the Guinea ships.
Secondly. All ships departing from Guinea for Europe, their direct course is northward; but on this course they cannot proceed, because the coast bending nearly east and west, the land is to the northward. Therefore, as the winds on this coast are generally between the S. and W. S. W. they are obliged to steer S. S. E. or south, and with these courses they run off the shore; but in so doing they always find the winds more and more contrary; so that when near the shore, they can lie south; but at a greater distance they can make no better than S. E. and afterwards E. S. E.; with which courses they commonly fetch the island of St. Thomas and Cape Lopez, where finding the winds to the eastward of the south, they sail westerly with it, till coming to the latitude of four degrees south, where they find the S. E. wind blowing perpetually.
On account of these general winds, all those that use the West India trade, and even those bound to Virginia, reckon it their best course to get as soon as they can to the southward, that so they may be certain of a fair and fresh gale to run before it to the westward: And for the same reason those homeward-bound from America endeavour to gain the latitude of 30 degrees, where they first find the winds begin to be variable; though the most ordinary winds in the north Atlantic ocean come from between the south and west.
10th. Between the southern latitudes of 10 and 30 degrees in the Indian ocean, the general trade-wind about the S. E. by S. is found to blow all the year long in the same manner as in the like latitudes in the Ethiopic ocean: and during the six months from May to December, these winds reach to within two degrees of the equator; but during the other six months, from November to June, a N. W. wind blows in the tract lying between the 3d and 10th degrees of southern latitude, in the meridian of the north-end of Madagascar; and between the 2d and 12th degree of south latitude, near the longitude of Sumatra and Java.