IT has been mentioned as a part of the planetary constitution of our globe, that a gaseous envelop environs its mass, the atmosphere, which requires the attention of the astronomer, on account of its influence in displacing the celestial bodies, and contributing to their visibility, by refracting and reflecting the rays of light. This elastic fluid is the scene of interesting phenomena, and performs important functions in the economy of nature. Besides being essential to the life of man, and the animal races, whose exist ence would terminate in a few minutes without the respiration of it; the exhalation of moisture from the surface of the earth is mainly owing to the common air we breathe, which receives and sustains the vapors formed into clouds, distributes them over different regions by its incessant motions, and tempers by its currents those extremes of heat and cold to which various localities are subject. It is in these last-named offices that the atmosphere demands the notice of the physical geographer. The consideration of its actual constitution does not belong to his province, but a general view of the fluid may be appropriate before we proceed to those agitations and changes which are in constant action, and upon which the welfare of organized beings so materially depends.

The atmosphere is, then, an integral portion of the earth, a body of air revolving with the solid mass upon its axis, the higher strata, of course, increasing in velocity with the distance from the axis of revolution. From hence a conclusion may be drawn respecting its height, for an absolute limit is put to its elevation by this feature of its physical condition. There is a point where the centrifugal force, or the tendency to fly off from the centre, will counterbalance the centripetal, or the gravitation toward the centre, and beyond that point the latter will be vanquished. It is obvious that no portion of the atmosphere can extend beyond the point where the two influences balance, or are in equilibrium, and the

projectile force becomes greater than that of gravitation, or its projection into space would follow. At the distance of 6.6 radii from the centre of the earth, or at an elevation of 22,200 miles, about the eleventh part the distance of the moon, this point is fixed, beyond which it is impossible for the atmosphere in the smallest quantity to extend. This consideration is only of importance to show that physical laws rigidly restrict it within finite bounds, for any portion of air at that distance must have a tenuity which is utterly inconceivable. The indications of the height of the atmosphere drawn from its weight, as shown by the barometer, reduce its elevation within a vastly circumscribed limit. A column of the whole circumambient air is nearly equal in weight to a similar column of mercury of thirty inches, or of water of thirty-four feet, which would give it an elevation of but 27,000 feet, or rather under five miles, if its density were uniform. But the elasticity of the air causes it to expand with the diminution of its own pressure, which becomes less at every step from the surface of the earth; and owing to this expansion we must place the limit to its height at a far greater distance than that suggested by the simple barometrical measurement of its weight. A pretty common opinion prevails that its extreme boundary does not exceed forty or fifty miles, and we have sensible evidence on the high lands of the globe, that for all the purposes serviceable to vegetable and animal life, the atmospheric zone is of very contracted elevation. It is a well-known property of the air that the temperature diminishes with its height, a circumstance referable to the general physical law, that as the density of gases decreases they acquire an increased capacity for heat. The higher, therefore, a body ascends in the atmosphere, the greater is the quantity of heat abstracted from it, the surrounding fluid becoming more rare. Hence the perpetual snow, and the piles of glaciers, that crown the summits of mountains, at whose base the orange and the citron bloom, and man pants in the fierce sultriness of a torrid climate.

But while the atmosphere may be considered generally as an aerial zone of the earth, the companion of the massy spheroid in its annual revolution round the sun, and rotating with it upon its axis, it has independent movements which present very complex phenomena, however clear the causes which put them in operation. The particles of air are constantly suffering displacement, and it is easy to conceive of various circumstances disturbing the dilatable and elastic fluid in which we live. A body in movement will communicate its motion to the adjoining particles, which may be sensibly propagated by them to a considerable distance; but this cause operates so slightly in the production of atmospheric currents that it might be entirely overlooked. It will be sufficient to state that some of the vast oceanic streams are supposed to produce a corresponding

or track. The parallels of 28° north and south latitude mark the medium external limits of the trade winds, between which, with some variations, their direction is from the north-east, north of the equator, and from the south-east, on the other side of the line, hence called the north-east and south-east trades. They are separated from each other by the region of calms, in which a thick foggy air prevails, with frequent sudden and transient rains attended by thunder and lightning. This region, in the Atlantic, extends across the whole ocean from the coasts of Africa to those of America, but its position shifts, being sometimes entirely north of the equator, and but rarely reaching one or two degrees south; and hence it may be considered as belonging to the northern hemisphere. The region also varies in breadth from two and a half to ten degrees, but usually occupies a width of four or five. These variations are dependent upon the position of the sun, which has an influence likewise upon the strength, direction, and situation of the trade winds themselves. When the sun has a northern declination, and approaches the tropic of Cancer, the boundary line of the north-east trade wind extends to 32° north latitude, and the wind has a more easterly direction, but the parallel of 25° degrees is its northern boundary, and the wind inclines more north when the sun is south of the equator, and approaches the tropic of

Capricorn. At that season, the southern boundary of the south-east trade wind extends to 30° S. lat., and the whole ocean is swept by it between that line and about 1° N. lat. The general width of the south-east trade is about 9° greater than that of the north-east, the region of calms, as before stated, being almost wholly in the northern hemisphere. In the basin of the Atlantic, the zone of the trade winds becomes broader, and their direction more easterly, as the coast of America is approached, the breezes blowing to the very shore. This is not the case on the African side of the Atlantic, where, through a tract of sea extending from fifty to eighty miles off shore, these winds are are not found at all, but contrary westerly breezes prevail. The irregularity is easily explained. Owing to the rarefaction which the air undergoes over the great hot desert of the Sahara, the colder air from the contiguous sea rushes in to supply the partial vacuum created, and keep up the equilibrium of the atmosphere, producing winds blowing toward the shore. In the Pacific Ocean, a similar zone is occupied by permament north and south-easterly breezes, or trade winds, though subject to a variety of interruptions. An instance of irregularity occurs along the coasts of Peru and Chili, where the general direction of the wind is south, and a steady south-easterly wind is only experienced at the distance of five or

six hundred miles from the shore. The numerous shoals and islands which are found in the Pacific, prevent uniformity in the tropical movements of the atmosphere. That intelligent hydrographer Captain Horsburgh has observed, that where shoal coral banks shoot up out of the deep water in many places between the tropics, a decrease of the prevailing wind is frequently experienced; for when a steady wind is blowing over the surface of the deep water, no sooner does a ship get upon the verge of a shoal coral bank, than a sudden decrease of the wind is often perceived. This he supposes to be occasioned by the atmosphere over these banks being less rarefied by the increased evaporation than that over the deep water, and consequently not requiring so great a supply of air to restore the equilibrium as the circumjacent parts, which are more rarefied and heated. It would undoubtedly be the case, if the earth were entirely covered with a mantle of water of uniform depth, that the trade-winds would everywhere prevail, throughout a zone, bounded by the parallels of from 25° to 32° on each side of the equator. But the large masses of land, of uneven surface, which occur between the tropics, and the consequent inequalities of temperature, check the tendency of the intertropical atmosphere to a regular course, introduce derangement in its movements, so that it is only in the great open seas that the trade-winds are experienced. Still, it has been observed that, in some countries under and near the equator, constant eastterly winds are found, which are no doubt identical in their cause with those that distinguish the equatorial regions of the ocean. They are met with on lands which exhibit extensive level plains, where nothing occurs to obstruct their passage and alter their direction. Thus, along the immense low tract drained by the Amazon an easterly wind prevails, by the assistance of which, the voyager is enabled to ascend rapidly against the strong current of the river. This wind blows from the estuary of the Amazon, where it is moderate, to its sources at the foot of the Andes, where it has gathered such strength, that Humboldt found it difficult to make head against it. The plain traversed by the lower course of the Orinoco has a similar easterly breeze,

but of less force.

We owe the discovery of the trade winds to Columbus, and this would have been prominently connected with his name, had it not been supplanted by the glory of a greater achievement, the revelation of a new world to the knowledge of mankind. The ancients were entirely unacquainted with these permanent breezes, and though maritine adventure had been largely prosecuted by the Portuguese at the instigation of Prince Henry, they had not penetrated into the region of the trades. Proceeding cautiously along the shores of Barbary, they had explored the coasts of Africa to Cape de Verde, rescued the Azore Islands from the "oblivious empire of the ocean, and afterward, under Vasco di Gama, doubled the Cape of Good Hope; but these voyages carried them clear of the district of the north and south-east trade winds. But soon after leaving the Canaries in the

Santa Maria, Columbus fell in with the former, which in the summer extend to the latitude of those islands, and-for the first time-a sail from the Old World swelled before the steady breath of the northern tropic. This circumstance, favorable to the success of his expedition, speedily excited the apprehensions of his crew, who found themselve borne, day after day, by a permanent breeze, farther from their native shores, and inferred the impossibility of returning, as they observed no change in its direction. Fortunately for his fame, and for the world, the great navigator firmly held on his course, reached the bounds of the before-supposed illimitable ocean, and re-crossed it in the region of the variables, to the north of the northern trade wind. Now, in passing from the Canaries to Cumana, on the north coast of South America, it is scarcely ever necessary to touch the sails of a ship; and with equal facility the passage is made across the Pacific, from Acapulco, on the west coast of Mexico, to the Philippine Islands. If a channel were cut through the Isthmus of Panama, the voyage to China would be remarkaby facilitated by the trade winds of the Atlantic and Pacific Oceans; be more speedy, agreeable, and safe than the usual route by the Cape, the chief interruption to its uniformity occurring in the Caribbean Sea and the Gulf of Mexico, where the trade wind blows impetuously, the sea is stormy, and the sky gray and cloudy

The theory respecting the origin of the trade winds, adopted by Doctor Dalton, Professor Daniell, and Sir John Herschel, was first proposed by George Hadley, the brother of the inventor of the quadrant, and embodies features of the previous theories of Halley and Galileo, who both grappled with this great geographical phenomenon. It is founded upon the rarefaction of the atmosphere of the torrid zone by the powerful heat to which that region is subject, in connection with the different velocities of the earth's surface, in different degrees of latitude, in the diurnal rotation. Heat rarefies and expands a volume of air in a ratio equivalent to an addition of about seventy feet to the ordinary height of the atmosphere for every degree of thermometrical measurement. As the sun is always vertical at some place within the tropics, the average temperature of the earth's surface in that region, bounded by the parallels of 234° on each side of the equator, is much higher than in latitudes to the north and south; and the incumbent air acquiring this higher temperature, is thereby rarefied and expanded. The consequence is, that in obedience to hydrostatical laws, masses of air are continually buoyed up from the surface, or swelled round the torrid zone in the form of a protuberant belt, the upper strata flowing over, and running off in streams north and south toward the poles, wherehaving been cooled and condensed-they descend, and flow over the surface toward the equator, pouring in a perpetual current of air to supply the place of that buoyed up by the heat of the tropics. Thus, there is a constant current in the higher regions of the atmosphere, proceeding from the equator north. ward and southward to the poles; and, if the earth