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presses upon the surface of the earth, with a determinable, but variable force, that we can measure by means of the barometer. Elastic in direct proportion to the force that acts upon it, it decreases in density from the surface of the earth to an elevated region, at which, by the late researches of Wollaston, it appears that its expansive power terminates; varying in its capacity for specific heat, in a ratio inversely as a function of its density, it is warmest near the surface of the earth, and its heat decreases as we ascend into it; deriving heat from the earth, as well as communicating it, the temperature of the air changes, at a given place, with the alternations of day and night, and the vicissitudes of the seasons: this varying temperature we measure by means of the thermometer.
Were the earth a sphere of uniform temperature, and at rest in space; its atmosphere a perfectly dry and incondensible elastic fluid ; the height of this atmosphere would be equal over every point of the earth's surface, and its density and elasticity, at equal elevations, every where the same. The barometer would therefore stand at equal altitudes on every part of the surface of the sphere; and, at equal heights above this surface, would subside to the same level in all climates. The atmosphere would be absolutely at rest; did not its relations to heat vary with its density, the latter would decrease in a regular geometric proportion, while the elevation from the surface of the earth increased in arithmetical progression. But as this relation to specific heat is variable, it would become necessary to associate indications of temperature with those of pressure, in attempting an exact measure of the progressive density. Should a change of temperature occur, affecting every part of the sphere, the elasticity of the atmosphere, and its consequent height, would be increased; this increase of altitude would however take place without producing any motion in the lateral direction, or any change in the pressure on the surface; but the pressure will be changed at all other altitudes.
Let us next suppose, that the temperature of the sphere, around which the atmosphere is diffused, should, instead of being equal in all its parts, increase regularly from the poles to the equator. In this case, the pressure at every point on the surface would still continue the same; but the altitude of the atmospheric column would be greatest at the equator, and least at the poles ; while the density at the surface would follow an opposite law. The heavier fluid at the poles would press mechanically upon the rarer, and a current be established in the lower part of the atmosphere, from the poles towards the equator. This, however, could not continue long to flow, as it would soon cause an accumulation in the equatorial regions, that would compensate the difference of density, were it not for a counter current, that takes place in
the higher regions of the atmosphere. This counter current is thus caused :-the difference in the density of the polar and equatorial columns becomes less as we ascend in the atmosphere, while the elasticity which is constant at the surface, lessens with the height, and the barometer stands higher, at equal heights, in the equatorial, than in the polar column; it will hence happen, that at some definite height in every latitude, the unequal density of the lower strata will be compensated, and the current from the poles towards the equator cease to flow; at a higher elevation, the pressure will be reversed, and the current will flow back towards the poles, thus restoring to the polar columns the air that had passed in the lower current to the equator. The state of equilibrium will therefore continue, and the pressure remain constant at the surface, notwithstanding the constant flow of the currents. The temperature, however, is not thus regularly distributed over the surface of our earth. In consequence of the varying length of the natural day, and the continual change in the inclination of the solar rays, the distribution of heat becomes unequal and variable, with the change of season. Thus it will happen, that the surface of a high latitude may be as much heated in the summer season, as that of parallels nearer to the equator. If we assume any given parallel to be heated to a higher degree than it would be upon the hypothesis of an equable decrease from the equator to the pole, -the current, for some degrees nearer to the pole, would be increased in velocity, while that for an equal number towards the equator, would have its direction changed ; a corresponding change of velocity and direction ensues, and a compensation of pressure again takes place. The state of equilibrium, however, would not be permanent under such variations of temperature; for the air itself would be affected, and the first access of heat would diminish the pressure, while the continuance of the current, for a time after the cause had ceased, would cause a rise in the barometer. The variation in the pressure, is not confined to the parallel, where the cause of the secondary currents exists ; but extends as far as the currents themselves. The greater the variation of the temperature, from the mean appropriate to the latitude, the greater the force of the currents, and the greater the oscillations of the barometer. Thus in equatorial regions, and low latitudes, the polar currents flow almost uninterruptedly; in higher latitudes, the variations are more and more frequent, and the violence of the winds becomes greater and greater.
A variation of the heat of the meridians, will cause changes also in the principal currents, by inducing an influx of air on both sides, towards the heated meridian, in the lower regions, and produce corresponding counter currents in the upper. But, in this case, the polar currents are not destroyed, but merely de
flected towards or from the heated meridian. The simple alternations of day and night, do not produce a sufficient variation of temperature to cause any important effect; but there are many local causes that influence the course of the winds in a very remarkable degree.
We have hitherto considered the sphere as at rest; let us now ascribe to it a diurnal motion upon its axis. By this, an apparent modification will be caused in the direction of the currents. Those which flow from the pole towards the equator, will have impressed upon them another motion, whose velocity is the same in magnitude and direction, with that of the points of the parallel of latitude whence they proceed. They will, therefore, in passing over points of the surface, whose velocity of rotation is greater, appear to move in a direction, not only from the pole towards the equator, but also in one contrary to that of the rotation of the earth, or from east to west. The diagonal of a parallelogram, one of whose sides is in the direction of the meridian, and represents in magnitude the true velocity of the currentthe other in a direction from east to west, and representing in magnitude the difference of the velocity of rotation of the two parallels, will exhibit the apparent velocity and direction of the wind. The same cause will affect currents proceeding from the equator towards the poles, in the opposite manner; and cause them to appear to deviate to the eastward of their real direction, or to blow from a point between the direction of the meridian, and that of the western point of the horizon.
The great perennial and periodical winds that are found prevailing in certain regions of the globe, and the more frequent of the variable winds that affect the climate of the temperate and frigid zones, grow out of the great general causes we have just exhibited.
In the Atlantic and Pacific Oceans, and extending to the distance of about thirty degrees on each side of the equator, the constant experience of navigators shows perennial winds to prevail; which, from their convenience to those who pursue the commerce of the Indies, have been called the Trade Winds. These are neither more nor less than the great under-current caused by the higher temperature of the equatorial regions, affected in the manner we have described, by an apparent change of direction, arising from the rotation of the earth on its axis. On the north side of the equator, therefore, they blow from points lying between north and east; and on its south side, from those between south and east. It has generally been stated, in books on Natural Philosophy, that near their northern limit, they blow from a point nearest to the north, and gradually change their direction, as the latitude lessens, to due east; and it has been assigned as a cause, that the velocity from north ta VOL. ITI.-No. 5.
south is lessened by the action of the current pressing towards the same zone, from the opposite pole. A similar direction has been predicated for the south-east trade wind. On the maps representing these winds, their course is therefore delineated as forming a graceful curve, from the outer limits to the dividing zone. A more strict course of reasoning would show, that this cannot be the case. The eastern part of the direction, arises from the difference of the velocity of rotation of the parts over which the polar current flows; and when the last ceases, the first must cease also. Whatever, then, be the velocity of the wind, the deviation from the direction of a meridian, will depend upon the comparative velocity of the points on the earth's surface, over which it passes. As this velocity of rotation increases more rapidly in higher than in low latitudes, the deviation of the wind towards the east, will be greatest near its northern and southern limit, and least near the zone that divides the north-east from the south-east trade. Instead of meeting, and forming by their union a due east wind, both should cease in a low latitude ; and there should be an intervening space, rendered neutral by the counteracting focus. This is found to be consistent with actual experience. A navigator sailing from the United States for India, instead of making his course in a direct line for the Cape of Good Hope, crosses the Atlantic, to the longitude of the Canary Islands; a few degrees south of these, he meets the trade winds; not as books on Natural Philosophy would indicate, blowing from NE., or even NNE., but more nearly due E.; as he proceeds south, the wind hauls more to the north, until, passing NE., it becomes NNE., or even north, at the southern verge of the north-east trade. This last direction is at right angles to that usually assigned it. The southern verge of the north-east trade winds, varies in position with the season of the year; sometimes reaching as far to the north as twelve degrees-at others, approaching to the equator, within three or four; but it never crosses that great circle. The northern limit also varies from 27° or 28° N. to 31°, or even 32° in some places. This change arises from the variation in the declination of the sun, by which the heat of the two hemispheres is constantly changing. The change, however, is not nearly as great in amount, as the change in the sun's position, in consequence of the surface heated being water, in which a circulation will arise, under variations of temperature, that will tend to equalize the heat of the mass. To the south of the verge of the north-east trade, the navigator meets a zone, of inconstant breadth, called the Variables, in which calms are frequent, the direction of the winds uncertain, and rains, so rare in the regular trades, frequent and excessively violent. This zone varies in breadth, from 21° 10 9°, according to the time of the year. On passing across this zone, the SE. trade is again met, blowing from almost due S., and the navigator is forced over to the west, and frequently, until he make the coast of Brazil. It was this circumstance, in truth, that led to the discovery of that part of the American continent, by accident, but a few years after Columbus, by following the dictates of sound reason, had been led to the search of this quarter of the globe. Proceeding to the south, the navigator finds the direction of the wind gradually changing to SE., SSE., and, finally, nearly to E., at the southern verge of the south-east trade; he thus is enabled to change his course, to one more in the direction of his contemplated voyage. That the received theory of the trade winds, should be so contrary to their real phenomena, is an instance of the looseness with which this subject has hitherto been treated. An application of the very first problem in Mechanics, ought to have been sufficient to show its fallacy; it is therefore clear, that the several authors who have treated on the subject, have been content to copy each other, without calling in the aid of calculation, whose results would have been consistent with the phenomena. This repetition of error is the more remarkable, as an early navigator, (Dampier,) whose work is in every body's hands, has described most minutely and accurately, all the different phenomena of the trade winds. There is an admirable paper on this subject, by Captain Basil Hall, in the work before us. We have stated, that the northern limit of the Variables never passed, at any season, to the south of the equator, while at times it was found as far north as 12°; this arises from the greater heat of the northern hemisphere, caused by the longer continuance of the sun in the northern half of his apparent path.
In the South Atlantic, the North and South Pacific, and in the Indian Oceans, westerly winds are found to blow constantly between the latitudes of 30° and 40°; a westerly wind is also the prevailing, although not the only wind, in these latitudes, in the North Atlantic. It is usual to explain this, by stating, that, beyond the limits of the trade winds, their counter current descends to the earth, to enter into the lower current, and keep up the circulation; coming from a latitude in which the velocity of rotation is greater, it seems to move in the same direction with the earth, or from W. to E. Our author appears to have adopted this theory of his predecessors, without close examination. We conceive, however, that his own principles afford a better explanation, and furnish, besides, a reason for the definite limit of the trade winds; which ought otherwise to extend, in the summer of each hemisphere respectively, to much higher latitudes.
Within the tropics, and to a short distance beyond them, the variations of temperature, particularly in the ocean and the atmosphere above it, are so small, that they differ insensibly from the