Contrary currents are often observed in the air; some clouds move in one direction, while others, which are higher or lower, move in a directly opposite one; but this contrariety of motion does not remain, being commonly produced by the resistance of some large clouds that force the wind into another course, but which returns again as soon as the obstacle is dissipated.
The winds are more violent in mountainous places than in plains, and increase until we reach the common height of the clouds, that is to say, to about one quarter, or one third of a league perpendicular height; beyond that height the sky is generally serene, at least during the summer, and the wind gradually diminishing. It is even asserted to be quite insensible at the summit of the highest mountains; but these summits being covered with snow and ice, it is natural to suppose that this region of air is agitated by the wind when the snow falls, and only during summer that the winds are not to be perceived. In summer the light vapours which are raised above the summit of these mountains fall in the form of dew, whereas in winter they condense and fall in snow or ice, which in winter may raise considerable winds, even at that height.
A current of air increases in velocity where the space of its passage is straitened: the same wind which was moderate in an open plain becomes violent in passing through a narrow passage in a mountain, or between two lofty buildings; and its most violent action is at the top of these structures or mountains, for air being compressed by these obstacles, its density and mass becomes increased, and as the velocity remains, the force or momentum of the wind naturally becomes much stronger. This is the cause that near a church or castle the winds seem to be stronger than at a distance from them. I have often remarked, that the wind reflected from a lone building is more violent than the direct wind which produced it. This can only be occasioned by the impelled air being compressed against the building, and by that means adds to its force.
The density of the air being greatest at the surface of the earth, we might be led to imagine that the greatest action of the wind should be there also; and I indeed think this is really the case when the sky is serene; but when it is covered with clouds, the most violent action of the wind is at the height of these clouds, which generally fall in rain or snow. The strength of the wind, therefore, must be estimated, not only by the velocity, but also by the density of the air; for it will often happen that one wind, which shall have no more velocity than another, will, nevertheless, root up trees and overturn buildings, only from the air impelled by this wind being denser; and this evinces the imperfection of the machines invented to measure the velocity of the wind.
Particular winds, whether direct or reflected, are more violent than general ones. The interrupted action of land-winds depends on the compression of the air, which renders each blast much more violent than if the wind blowed uniformly. A strong continued wind never occasions such disasters as the rage of those produce which blow, as it were, by fits; but we shall give examples thereof in the following article.
We may consider the winds, and their directions, under general points of view, from which possibly we may derive useful instructions; for example, we might divide the winds into zones. The east wind, which extends to about 25 or 30 degrees on each side the equator, exerts its action round the globe in the torrid zone; the north wind almost always as constantly in the frigid zones. Therefore it may be said that the east wind occupies the torrid zone, the north wind the frigid zones, and with respect to the temperate zone, the winds which reign there are merely currents of air produced by these two winds, whose direction tends to the eastern points. With respect to the westerly winds, which often reign in the temperate zones, both in the Pacific and Atlantic Oceans, they may be regarded as winds reflected by the lands of Asia and America, deriving their origin from the east and north winds.
Although we have said that, generally speaking, the east winds reign around the globe to about 25 or 30 degrees on each side the equator, it is nevertheless certain, that in some parts they do not extend so far, and their direction is not always from east to west, for on this side the equator it is east-north-east, and beyond the equator it is east-south-east, and the further we remove from the equator the more the direction is oblique. The equator is the line under which the direction of the wind from east to west is the most exact; for example, in the Indian ocean, the general wind from east to west scarcely extends beyond 15 degrees. Sailing from Goa to the Cape of Good Hope this wind is not felt till we have past the equator; but after arriving at the 12th degree south latitude, it continues to the 28th degree. In the sea which divides Africa from America there is an interval from the 4th degree north latitude to the 10th or 11th degree south, where this general wind is not perceivable; but beyond the 10th or 11th degree it reigns as far as the 30th.
There is also much exception with regard to the trade winds, whose motion is alternative. Some remain a longer or a shorter time, others extend to greater or less distances; others are more or less regular, and more or less violent. Varenius speaks thus of a principal phenomena of these winds. "In the ocean between Africa and India, as far as the Malaccas, the east winds begin to reign in January and last to the beginning of June; in August or September the west winds begin and continue during three or four months. In the interval of these monsoons, that is from the end of June to the beginning of August, there is no wind on that sea, but they have violent storms which come from the north.
"These winds are subject to the greatest variations near the land, for ships cannot depart from the Malabar coast, nor other western ports on the coasts of the peninsula of India, to sail to Africa, Arabia, or Persia, but from January to April or May; for from the end of May, and during the months of June, July, and August, there are such violent tempests from the north or north-east that ships are not able to keep the sea. On the other side of this peninsula, in the sea which bathes the Coromandel coast, these tempests are not known.
"To sail from Java, Ceylon, and many other places, to the Malacca islands, the month of September is the most proper time, because the west wind begins to blow in these parts; nevertheless, at 15 degrees south of the equator, we lose this west wind and meet with the general winds, which blow south-east. To sail from Cochin to Malacca they depart in March, because the west winds begin to blow at that time; therefore these westerly winds blow at different times in different parts of the Indian sea; and it is necessary to sail at different periods in going from Java to the Malaccas, from Cochin to Malacca, from Malacca to China, and from China to Japan.