The trade-winds are met with on both aides of the equator. The mean boundary-line of the region in which they blow, and beyond which variable winds prevail, is about 28° lat. in the eastern parts of the ocean, but in the western parts this line is generally two or three degrees farther north and south. To the north of the equator they blow in the eastern parts of the ocean from the north-east, seldom front the eastward of east-north-east, or from the northward of north north-eaet. In proceeding farther west they become more easterly, and often they blow from due east, and sometimes from the south of east, but generally they are one or two points north of east. To the south of the equator the trade-winds in the eastern parts of the ocean blow from south-east, and usually between south-east and east, but they also decline more to due cast in reaching the western portion of the ocean. They do not occur in the vicinity of the continents, but are chiefly separated from them by a tract of sea in which either periodical or variable winds prevail. The trade-winds therefore are only experienced when we are well out from the land in the open sea. The wind blows with less force and steadiness in the eastern than in the western portion of the ocean. It is also stronger and more con stant in the hemisphere where the sun is not, than in that which is exposed to its perpendicular rays ; in the latter, however, it is more easterly than in the former. The region in which the trade-winds occur is distinguished by an almost continual serenity and fair weather. Though the trade-winds of the northern and southern hemisphere blow in an oblique direction towards one another, they do not meet in general, hut arc divided by a tract of sea in which calms frequently prevail, and also variable light winds, mostly from the west, are met aid,. This region of the calms is distinguished by a thick foggy air, and frequent rains of short duration attended by thunder and light ning. The region of calms which separates the north-east trade-winds from the south-eastern, and which usually occupies a width of four or five degrees of latitude, is not always found at the same part of the ocean, but advances farther north when the sun has a northern decli nation, and farther south when it is in the southern hemisphere. [Camas.] The same is observed respecting the winds themselves. Though tho maul boundary-line of the trade-winds is 28° of lat. in the eastern parts of the ocean, it extends two, three, and even four degrees farther north when the sun approaches the northern tropic, and about the same distance farther southward when the sun is near its greatest southern declination. It sotnetimes happens that a north eastern wind occurs as far north as 40° in the Atlantic, along the southern coasts of Spain and Portugal, but as this is seldom the case, it is supposed that ouch a wind cannot be considered as a trade-wind, but only as one of the variable winds which prevail to the north and south of the trade-winds. There are also a few instances on record in which the north-east and south-east trade-winds have not been found separated by a region of calms, hut in which a vessel, with the inter vention of a calm of short duration, has passed from one trade-wind into the other.
The true explanation of these "magnificent phenomena," as they have justly been called, according to Dove and Herschel, was first delivered in the Philosophical Transactions' for 1735, by George Hadley. It has often been erroneously attributed to Dr. Edmund Halley, in part, doubtless, from the similarity of his name, and partly on account of a theory of these winds, now long proved to be fallacious, having been actually proposed by him, as already noticed. The true
theory was also divined by the sagacity of John Dalton, who, not knowing what Hadley had done, printed it in his Meteorological Obser vations and Essays' in the year 1793, but discovered Hadley's priority in time to acknowledge it in his preface.
The astronomical point of view from which the objects of meteoro logy have always been viewed by Sir John F. W. Herschel, has given to his treatment of atmospheric phenomena, a breadth and perspicuity scarcely to be found in the writings of other meteorologists. His view of the theory of the trade-winds presents a remarkable example of this, and, as a whole, is, we think, unequalled. In what follows we give an epitome of it, condensed from two works—his Outlines of Astronomy,' and Treatise on Meteorology,' with some adaptations of our own.
It is a matter of observed fact that the sun is constantly vertical over some one or other part of the earth between the tropics, and that the whole of the zone or belt so included between the tropics, and equally divided by the equator, is, in consequence of the great altitude attained by the sun in its diurnal course, maintained at a much higher temperature than those regions to the north and south which lie nearer the poles. The heat thus acquired by the earth's surface, agreeably to the principles explained above, is communicated to the incumbent air, and becomes the universal primary cause of the phenomena of the winds, in conjunction with the earth's rotation. The colder and heavier air glides in on both sides, along the surface, from•the regions beyond the tropics ; while the displaced air, thus raised above its due level, and unsustained by any lateral pressure, flows over, as it were, and forms an upper current in the contrary direction, or towards the poles ; which being cooled in its course, and also pressed down by the mass of the atmosphere above to supply the deficiency in the extra tropical regions, thus keeps up a continual circulation. "That this is a real cause (sera cause) is placed in complete evidence by the general fact that the atmospheric pressure at the surface of the sea diminishes regularly from either tropic to the equator, where the barometer stands habitually about 0.2 in. lower than in the temperate zones." The principle whose action was made known and applied by Hadley now comes into play. The equatorial portion of the earth's surface has the greatest velocity of .rotation, and all other parts less in the proportion of the radii of the circles of latitude to which they corre spond. But as the air, when relatively and apparently at rest on any part of the earth's surface, is only so, because in reality it participates in the motion of rotation proper to that part, as indicated at the beginning of this article, it follows that when a mass of air near the poles is transferred to the region near the equator by any impulse urging it directly towards that circle, in every point of its progress towards its new situation it must be found deficient in rotatory velocity, and therefore unable to keep up with the speed of the new surface over which it is brought. Hence, the currents of air which set in towards the equator from the north and south, must, as they glide along the surface, at the same timo lag, or hang back, and drag upon it in the direction opposite to the earth's rotation, that is, from east to west. Thus these currents, which but for the rotation would be simply northerly and southerly winds, acquire, from this cause, a relative direction towards the west, and assume the character of permanent north-easterly and south-easterly winds.