scribed 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 spheriod 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 flow in the air. The varying attractions of the sun, moon, and planets on the atmosphere, will occasion tides in it analogous to those of the ocean. or an alteration in the heights of vertical columns of air, winds and currents arising from the resulting inequalities of horizontal pressure; but La Place has proved the action of this cause to be scarcely appreciable. The atmospheric agitations of which we are sensible, both the more violent and gentle, appear to proceed either from a change in the temperature of a portion of the air, or from a change in the quantity of water which it holds in a state of vapour. In both these cases a temporary destruction of the equilibrium subsisting between different parts of the atmosphere is produced, and its particles are set in motion to restore the balance. The effect of heat upon a volume of air is to rarefy and expand, to increase its bulk and diminish its density. When any portion, therefore, of the earth's surface is more heated than the surrounding districts, the air there ascends and flows over the adjoining cooler and denser strata, causing an upper outward current, while the colder and denser fluid rushes towards the spot where the balance has been lost by expansion, and a lower inward current is produced. An easy experiment will illustrate this interchange. In a room warmed by a good fire, if a candle be held at the crevice between the door and the floor, an inward current will be observed from the exterior colder air, but near the ceiling, by the same means, an outward flow will be detected. In the other condition an addition of vapour to the atmosphere gives rise to a wind blowing on all sides away from the district of evaporation, while an abstraction of it by showers creates a partial vacuum, towards which the air rushes from all points of the compass. The diversity of the winds in power is principally owing to the different degrees of vigour with which these causes act.

The currents of the atmosphere display an endless variety in their velocity and force, from the zephyr which scarcely stirs the leaves of the forest, to the gale under which its mightiest branches bend, and the hurricane which tears up its trees by the roots, and destroys the habitations of mankind. It has been observed that in the temperate zones the most violent winds occur, when neither the heat nor the cold common to such localities is at its maximum-that they generally extend over a considerable tract of country — and are accompanied by sudden and great falls in the mercury of the barometer. The latter circumstance attends the storms of the tropics, but they are often confined within narrower limits than the extra-tropical hurricanes. It was noticed by the supperstitious as a coincidence, not without meaning, that at the time of Cromwell's death the enchained winds were liberated, and went forth raving and howling through the land, uprooting the largest trees, and whirling them about like straws, and toppling down chimneys and turrets; but the same tempest, at the self-same hour, dashed the vessels of the Baltic seamen upon the strand, and buried Venetian argosies in the Adriatic, shivered the pines of Norway, and swept before it the cypresses of the Bosphorus - a similar war of the elements attending the termination of the earthly career of Cardinal Wolsey, Buonaparte, and George IV. Sometimes the upper regions of the atmosphere have been remarkably agitated, while the lower stratum of the air has been quite calm. Lunardi, on one occasion, travelled at the rate of seventy miles an hour in his balloon, while at Edinburgh, where he ascended, the air was quite tranquil, and continued so throughout his expedition. To ascertain the velocity and force of winds, a variety of experiments have been conducted with instruments constructed for the purpose. The following table contains some results obtained by Smeaton, inserted in a volume of the Philosophical Transactions:

The currents of the atmosphere far surpass in velocity those of the rivers and the ocean, a gentle pleasant wind blowing at a rate equal to that of the mighty Father of Waters when in flood, but a hurricane will outstrip the swiftest locomotive in its speed. In speaking of the direction of currents of air and water, the indicating terms are employed in an inverse sense, an easterly wind signifying a breeze coming from that quarter, an easterly stream a flow of water towards it. Winds may be divided into three classes or genera, the Permanent, the Periodical, and the Variable; of which, the first excepted, there are many different species. We shall prefer, however, to consider them under their local recognised titles.

1 Trade winds. These are permanent, following the same direction throughout the year. They are met with between the tropics, and a few degrees to the north and south of those limits. The well-known name applied to them is a phrase of doubtful origin, but probably derived from the facilities afforded to trade and commerce by their constant prevalence and generally uniform course, though Hakluyt speaks of the "wind blowing trade," meaning a regular tread 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° 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 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 towards the shore.

In the Pacific Ocean, a similar zone is occupied by permanent 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 easterly 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 maritime 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 afterwards 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, favourable to the success of his expedition, speedily excited the apprehensions of his crew, who found themselves 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 remarkably 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 grey and cloudy.

The theory respecting the origin of the trade winds, adopted by Dr. Dalton, Professor Daniell, and Sir John Herschell, 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 231° 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 towards the poles, where, having been cooled and condensed, they descend, and flow over the surface towards 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 northward and southward to the poles; and if the earth were at rest, there would be a 'constant wind in the lower regions of the atmosphere blowing directly from the poles to the équator, while in equatorial regions. the two streamlets would meet and neutralise each other's influence. But the earth is not at rest! It is incessantly whirling upon its axis, the surface moving at a rate which varies according to the extent of the circumference. The velocity at the equator, where the circumference is the greatest, is about sixteen miles a minute; at 30° of latitude, which is below the most southerly point of Europe, it is about fourteen miles in the same time; and at 45°, or about the centre of France, it is about eleven miles. As the distance from the equator increases, north and south, the rate of the rotation thus becomes less, because the circle of the earth's circumference diminishes in extent. Now a current of air flowing from the north or south polar regions, and setting towards the equator, will encounter as it proceeds an increased rotatory motion eastward, the direction of the earth's axical revolution, and, not acquiring the new velocity at once, it will be left behind, and seem to deflect towards the west just in proportion as it does not keep up with the earth to the east. Hence, what would simply be a north or south wind but for the earth's rotatory motion, becomes a north-east and south-east wind as it approaches those regions where, the velocity of the globe being so much greater than where it