BOOK XVI. Kinds of winds. Origin of the con The direction of winds is not designated, like that of marine currents, by the point of the compass to which they tend, but by the point from which they come; thus, a northerly wind is directly contrary to a northerly current. In regard to duration, winds are distinguished as constant and variable; in regard to extent, as general and partial. Two general and constant movements exist in the atmosphere; the one prevails in the torrid zone, and carries the air westward in respect of the earth, in a direction conformable to that of the general movement of the ocean; the other, which is principally felt in the temperate zones, carries the polar air towards the equator, and by this means produces two polar currents or effluxes, similar to those already pointed out in the ocean. We shall first consider the equatorial movement of the atmosphere; that which occasions the trade-winds, or the erly winds. constant east wind which blows in the torrid zone. The primary cause of those winds seems to be the expansion. which the air undergoes from the action of heat; for the sun (which we suppose to be situated in the plane of the equator) must evidently, by its heat, rarefy the columns. of air beneath it, and raise them above their level, from which they must of necessity either be dissipated in the celestial space, or sinking again by their weight, they flow towards the poles, along the higher parts of the atmosphere. At the same instant, however, a fresh quantity of air must arrive; proceeding from the polar regions, it comes to replace the air which has been rarefied at the equator. Thus two opposite currents of air will be formed; one in the higher, another in the lower strata of the atmosphere. But the real velocity impressed on each particle of air, being caused by the earth's rotation, must be smaller the nearer that particle is to the poles; and hence the circumpolar air, as it advances towards the equator, and preserves for a time its primitive velocity, must revolve more slowly than the corresponding parts of the earth; bodies situated at the earth's surface will therefore strike against it with this excess of XVI. velocity, suffering, from its re-action, a resistance opposed BOOK to their rotatory motion; and to the observer, who imagines himself to be at rest, the air will appear to move in a direction precisely contrary to that of the rotation of the globe, in other words, from east to west. tions of the constant winds. The different positions of the sun, at different seasons of Modificathe year, will produce certain modifications in those atmospheric movements. We shall indicate their general cha- easterly racter. When the sun passes to the north, in April, May, and June, the air of our hemisphere is proportionably dilated, from the equator to the pole. It will rise therefore, and the void produced by its rarefication, in the lower strata, will be filled by a polar current. At this season, accordingly, we shall have a north wind; but in a certain latitude, suppose 30°, our north wind will encounter the general east wind; if they meet at right angles, a compound movement will result, and we shall have a north-east wind. On arriving at the summer solstice, the sun will warm every part of the northern hemisphere, more or less considerably ; the heat will continue for a certain period of time; and hence in July and August we shall have fewer north-winds. But our luminary retires; the polar air is cooled and condensed anew; the equatorial atmosphere becomes more and more dilated. The general north wind will, therefore, reappear in September and October. It will go on increasing as the sun recedes from us, and approaches the winter solstice. There will be a limit, at which the condensation of the air north of the equator, and the expansion of the air to the south of it, will cease; an equilibrium more or less perfect will then reign throughout all the atmosphere. The same phenomena, in a contrary order, must happen in the southern hemisphere. Over all the globe, therefore, we should find equatorial and polar winds of regular occur. rence, if those general movements were not resisted and deflected by an infinite number of causes. Similar variations, we have seen, produce an influence on the currents of the ocean; and the atmosphere being incalculably more subtle, XVI. BOOK and also less homogeneous than water, we cannot wonder that it should be more sensible to the smallest impulse, and more liable to unexpected changes. Origin of variable winds. Sea and es. The inequalities of the earth's surface, and the diversity of its soils, have, no doubt, a powerful influence on the constitution of the atmosphere. At one place, mountains covered with eternal snow arise, and prevent the air from undergoing the same expansion as in valleys; at another, burning sands, forests, marshes, savannahs, are spread out, and exhale various inflammable gases; at a third, we observe large basins of water surrounded and irregularly indented by land. The air must, therefore, suffer relative and partial condensations and expansions: hence the sea-breeze, the land-breeze, the mountain-breeze. Those changes too will occur differently in summer and in winter, during the day and during the night. Hence the morning and the evening breezes,— those aurae matinales, those zephyrs, whose refreshing breath re-animates us in the warm season. These alternate breezes are felt at very high latitudes, as, land breez- for example, at Bergen in Norway. The South Sea Islands, notwithstanding their small circumference, in this manner, during day-time, attract the general east wind, which is thus made to embrace them, as it were, on every side, and to blow from all points of the compass towards the central summit of the island. When night arrives, the air flows back again from the summit, towards the sea in every direction. This observation of Forster's throws great light on the theory of winds. In the last place, chains of mountains may arrest winds in the lower regions of the atmosphere, or turn them from their direct path, and sometimes give them more impetuosity, as marine currents acquire greater force in the neighbourhood of straits and promontories. Such violent movements of the air, when arrested by an obstacle, have given particular notoriety to Cape Horn, the Cape of Good Hope, and the South Cape of Van Diemen's Land, to the Strait of Bab-elMandel in Arabia, and the Dragon's Mouth in America, not to mention others. XVI. Exhalations from the soil impart their own peculiar qua- BOOK lities to winds. Thus, in Arabia, the Simoom carries much nitrous gas along with it; the Harmattan of Guinea much oxygen; the Chamsin of Egypt much azote. Winds of a peculiar quality. Moon's in The positions of the moon may exercise an influence on winds, by producing a sort of ebb and flow in the atmo- fluence. sphere; but we are by no means of opinion, that the immediate agency of the lunar attraction can excite such oscillations. This immediate agency, in so subtle a fluid, must be altogether transient, and, consequently, its effect almost imperceptible. The phenomenon appears rather to be caused by a re-action of the ocean on the atmosphere. Indeed, the movements of the sea must generally have an important influence on those of the air. It is chiefly from the bosom of the sea that the constituent principles of our atmosphere are unfolded; and its parts must, therefore, have a velocity proportionate to that of the aqueous particles from which they have just been disengaged. Clouds, by intercepting or condensing the sun's rays; common rain, by its cooling effect; vegetation, by absorbing great quantities of air; the decomposition of animal and vegetable substances, may all contribute to the formation of local winds. Hurricanes appear to have an electric origin. At the Hurrimoment when the electric spark unites a quantity of oxygen canes. gas with a quantity of hydrogen, to produce a rain-storm, in all probability a considerable mass of hydrogen is consumed, and thus occasions a sudden fall of rain or hail; by which means a very large vacuum is formed, and the ambient air rushes into it, with astonishing rapidity, and sometimes in the most opposite directions. The Antilles, the Isle of France, and the Isle of Reunion, the kingdoms of Siam and China, are the countries in which hurricanes most frequently exercise their ravages. The hurricanes of Europe are not, in any way, to be compared with those of countries farther to the south; generally speaking, the former are nothing more than whirlwinds occasioned by the meeting of two opposite currents. But BOOK Waterspouts, or syphons. in a real hurricane, all the elements seem to have combined and armed themselves for the destruction of nature. The lightnings cross each other; the thunder roars without interval; rain falls down in torrents. The velocity of the wind far exceeds that of a cannon ball, or of the powder which impels it; growing corn, vines, sugar canes, forests and houses, every thing is swept away; one might imagine the ground it had passed over was cleared and levelled. It begins in various ways; sometimes we have one little black cloud appearing on the summit of a mountain; at the instant when it seems to settle on the peak, it rushes down the declivity, unrolls itself, dilates and covers the whole horizon; at other times, the tempest advances in the shape of a fire-coloured cloud, shewing itself suddenly in a calm and serene sky.* The water spout or syphon, is a no less dangerous phenomenon. Syphons are distinguished as terrestrial and marine; it were better to divide them into air syphons and water syphons. The latter usually occur in the manner we are now to describe. Underneath a dense cloud, the sea becomes agitated with violent commotions; the waves dart rapidly towards the centre of the agitated mass of water; on arriving at which, they are dispersed into aqueous vapours, and rise, whirling round in a spiral direction towards the cloud. This conical ascending column is met by another descending column, which leans towards the water, and joins with it. In many cases, the marine column is from 50 to 80 toises in diameter near its base; both columns, however, diminish towards the middle where they unite; so that here they do not extend more than three or four feet in diameter. The entire column presents itself in the shape of a hollow cylinder, or tube of glass empty within. It glides over the sea without any wind being felt; indeed several have been seen at once following different directions. When the cloud and the marine base * Voigt, Magasin de Physique, vii. 36-40, (in Germ.) Comp. Enclyclopédie Méthodique Marine, tome iii. partie 2d, p. 813, et sqq. |