Although the motion of the air depends on a great number of causes, there are nevertheless principal ones, of which it is difficult to estimate the effects, because of the modifications from secondary causes. The most powerful cause is the heat of the sun, which produces successively a considerable rarefaction in different parts of the atmosphere, and gives rise to an east wind that constantly blows between the tropics, where rarefaction is the greatest.
The force of the sun's attraction, and even that of the moon on the atmosphere, are inconsiderable in comparison with that just mentioned; it is true, this force produces in the air a motion similar to that of the flux and reflux in the sea, yet it must not be supposed that the air, because it has a spring, and is 800 times lighter than water, receives, by the action of the moon, a more considerable motion than that of the waters of the sea; for the distance of the moon being the same, a sea of any fluid matter will have nearly the same motion, because the force which produces it penetrates the matter, and is proportional to its quantity; thus a sea of water, air, or quicksilver, would elevate itself nearly to the same height, by the action of the sun and moon; hence we see that the the influence of the planets in the atmosphere is not considerable, and although it must cause a slight motion of the air from east to west, this motion is insensible in comparison with that produced by the heat of the sun; but as the rarefaction will be always greatest when the sun is at the zenith, the current of air must follow the sun, and form a constant wind from east to west. This wind blows continually over the sea in the torrid zone, and in most parts of the land between the tropics; it is this wind we feel at the sun's rising; and in general the east winds are more frequent and impetuous than the west; this general wind from east to west extends even beyond the tropics, and blows so constantly in the Pacific Sea, that vessels which sail from Acapulco to the Philippines, perform their voyage, which is more than 2700 leagues, without any risque, and almost without any need of directing their course. In the Atlantic, between Africa and Brazil, this wind is also constant: it is felt also between the Philippines and Africa, but not in so constant a manner, by reason of the islands, and different obstacles that are met with in that sea; for during the months of January, February, March, and April, it blows between the Mozambique coast and India, but during the other months, it gives place to different winds: and although this east wind is less felt on the coasts than in the open sea, and still less in the middle of continents than on the coasts; nevertheless there are places where it blows almost continually, as on the east coasts of Brazil, on the coasts of Loango, in Africa, &c.
This east wind continually blowing under the line, is the cause, that sailing from Europe to America, the course of the vessel is directed from the north to the south, along the coasts of Spain and Africa, to within 20 degrees of the equator, where this east wind is met with, which carries them directly to the coasts of America. The voyage from Acapulco to the Philippine islands, is made in two months by the favour of this east wind: but the return from them to Acapulco is longer and more difficult. At 28 or 30 degrees on this coast from the line, the western wind is nearly as constant, which is the reason that vessels returning from the East Indies to Europe, do not follow the same track as in going; those from New Spain sail north along the coasts till they arrive at the Havannah, and from thence they continue northward, until they meet with the westerly winds which carry them to the Azores and afterwards to Spain. So likewise in the South Sea, those which return from the Philippines, or China, to Peru, or Mexico, sail north as far as Japan, and navigate under that parallel to a certain distance from California, from whence, coasting along New Spain, they arrive at Acapulco. These winds do not always blow from one point, but in general from the south-east from April to November, and from the north-east from November to April.
The east wind, by its action, increases the general motion of the sea from east to west; it also produces currents which are constant, some flowing from east to west, others from west to east; and from the east to the south-west or north-west, following the direction of the eminencies and chains of mountains at the bottom of the sea, the vallies that divide them serving as channels to these currents. The alternative winds which blow sometimes from the east, and sometimes from the south, produce also currents which change their direction at the same time with these winds.
The winds which blow continually for some months, are generally followed by contrary winds, and therefore mariners are obliged to wait for that which is favourable to their voyage. When these winds change, a calm or dangerous tempest generally ensues, and which continues for several days, sometimes a month, and has been known for more than two.
These general winds caused by the rarefaction of the atmosphere, are differently combined and modified by different causes in different climates. In that part of the Atlantic, under the temperate zone, the north wind blows almost constantly during the months of October, November, December, and January, which makes those months the most favourable to embark from Europe to India, in order to pass the line by the aid of these winds; and it is known by experience, that ships which quit Europe in the month of March frequently do not arrive sooner at Brazil than those which sail in the October following. The north wind almost continually reigns during winter in Nova Zembla, and other northern coasts. The south wind blows during the month of July at Cape de Verd, when the rainy season, or winter of these climates sets in. At the Cape of Good Hope the north-west wind blows during the month of September. At Patna, in the East Indies, the north-west wind blows during the months of November, December, and January, and produces heavy rains; but the east wind blows during the other nine months. In the Indian ocean, between Africa and India, as far as the Malacca islands, the monsoons reign from east to west from January to the beginning of June, the west winds begin in the months of August or September; during the interval of June and July, there are dreadful tempests generally from the north winds; but on the coasts these winds vary much more than in the open sea.
In the kingdom of Guzarat, and on the coasts of the neighbouring sea, the north winds blow from March till September, and during the other months south winds almost always reign. The Dutch, to return from Java, generally set sail in the month of January or February, when they have the assistance of an easterly wind which is felt as far as the 18th degree of South latitude; afterwards they meet with the south winds which carry them to St. Helena[E].
[E] See Varen. Geography, gener. cap. 20.
There are regular winds produced by the melting of snows, which the ancient Greeks have noticed. During summer a north-east wind, and in winter one from the south-east, were noticed in Greece, Thrace, Macedonia, the Egean sea, and as far as Egypt and Africa; the same kind of winds have been remarked at Congo, at Guzarat, and at the extremity of Africa, which are all produced by the melting of the snows. The flux and reflux of the sea also produce regular winds which remain only a few hours, and in many places winds are observed to blow from the land during night, and from the sea during the day, as on the coasts of New Spain, Congo, the Havannah, &c.
The north winds are pretty regular in the polar circles; but the nearer we approach the equator, the weaker they become: a circumstance equally common to the two poles.