THE TRADE WINDS.

61

upon it, and this would produce an apparent or relative motion in the air from east to west, that is, an easterly wind. Thus the cold current moving towards the equator is influenced by two sources of motion; the first caused by the heat of the torrid zone producing a partial vacuum, to fill up which the cold air from the temperate zones rushes in towards the equator and at right angles to it; the second source of motion is that which has been communicated to it in a direction due east by the rotation of the earth in the temperate latitudes it has left. The resultant of these two motions is the south-east trade-wind in south latitude, and the north-east trade-wind on the northern side of the equator.

When the slower moving air of the temperate zone first arrives at the quick-moving or tropical belt of the earth, the difference of their velocities is great compared with the other motion of the cooler air towards the equator, and consequently the wind blows at the extreme edges of the trades nearly from the east point. This gradual change in the direction of the trade-winds is thus accounted for :-as the difference in velocity between each parallel and the next becomes less and less in approaching the equator, so the lagging behind of the air, which constitutes its westward tendency (or easting as it is called by sailors), becomes less and less. The 30th parallel, which moves at the rate of 903 miles an hour, has a less velocity by 77 miles than the 20th parallel, which moves at the rate of 980 miles an hour, while the 20th is only 46 miles an hour slower than the 10th parallel. Thus, the air in travelling from the 30th to the 20th parallel must be more retarded, or have more easting than that which travels from the 20th to the 10th parallel. As the air from the cooler regions draws near the equator, its velocity

is checked by becoming heated, which gives it a tendency to rise rather than to flow along the surface; and it is further checked by the meeting of the two opposite currents, one from the north and the other from the south.

As every current towards the equator must be rendered easterly, so every current from the equator must become a westerly wind, because it proceeds from a quick moving into a slower moving latitude, and must, therefore, rotate quicker than the part of the earth on which it arrives. And as this cause operates least powerfully near the equator, and becomes more powerful in receding therefrom, this would cause the upper tropical current to become more and more westerly as it advances towards the temperate zones; thus describing the same apparent curve as the trade-winds below it, and moving everywhere in a direction exactly opposite to them,-a fact which has been established by observation. When this upper current first precipitates itself on the earth's surface, about the 30th parallel of latitude, it has lost but little of its equatorial velocity, because the only friction to which it has been subjected is that of the lower current; and hence the furious westerly gales which are so prevalent beyond the limits of the trade-winds, or about latitude 30° in each hemisphere.

sun.

38. The equatorial limit of the two trades, or the region of calms and light variable winds, is subject to continual change, depending on the position of the As the circle of greatest heat does not coincide with the equator, except at the time of the equinoxes, but with a parallel to the north or south of it, it will readily be seen how the whole system of tropical winds must to a certain extent follow the sun; that is, the limits of all these winds must advance northward in

WINDS OF EACH HEMISPHERE.

63

our summer, and recede southward in our winter. Hence the regions of calms and light variables is bisected by the equator only at the equinoxes. In July and August this region is wholly north of the equator, and extends as far as 12° north latitude, while in January, it is, on the contrary, almost entirely in the southern hemisphere. Thus, in our summer, the southern trade sometimes crosses the equator, and advances a degree or two to the north of it; while, in our winter, the northern trade advances nearly up to the equator, but never crosses it. The reason for this difference is the greater average amount of heat in the whole northern compared with the southern hemisphere. Indeed the line of greatest mean yearly heat is situated entirely within the northern hemisphere, nowhere approaching the equator nearer than three degrees. The phenomena of the winds are, therefore, symmetrical on each side of the line of greatest mean yearly heat rather than on that of the real equator.

39. The outer or temperate limits of each trade-wind also partake of this northward and southward shifting, because, the warmer either hemisphere may be, the farther will the upper current travel into the temperate zone before it becomes sufficiently cooled to sink and usurp the place of the lower current, which constitutes the trade-wind. Hence there are places situated about the 28th parallel in each hemisphere which have a constant east trade-wind during their hottest months only, and the above-mentioned westerly gales during the cooler part of the year.

40. The following figure, Fig. 17, will serve to show the prevailing direction of the winds or lower atmospheric currents at different parts of the earth's surface, apart from the effects of local peculiarities, such as sea and land breezes. The facts intended to be

impressed on the mind by this diagram are, 1st, the

the general lower temperature of the southern hemisphere than the northern. 2ndly. The constancy of the trades from the 10th to the 28th degree of latitude, their directions being polar where they approach nearest the equator, and gradually more and more easterly in receding therefrom, until outside the tropics they become east. 3rdly. The conflict and alternate preponderance of these east winds and of west gales about the 30th parallels. 4thly. The prevalence of easterly and equatorial westerly winds from the 30th to the 60th parallels. 5thly. The prevalence of polar gales about the arctic and antarctic circles.

It is of course impossible, in Fig. 17, to show the upper currents*, which should theoretically blow at every spot, nearly contrary to the prevailing lower ones. Free from all the irregular disturbances of the earth's surface, these upper currents are supposed to

* These will be found represented in Fig. 16.

PERIODICAL WINDS.

65

flow with a slow but unruffled and undeviating constancy, as seen in the motions of lofty clouds, and confirmed by the experience of aeronauts. Mr. Green states, that in this country, whatever may be the direction of the wind below, within 10,000 feet above the surface of the earth its direction is invariably from some point between the north and west.

41. We must now notice certain modifications in the trade-winds, produced by the presence of large masses of land. If the earth were a uniform mass of water or of land, these winds would blow with the utmost regularity; but, as the surface of the earth is broken very irregularly by masses of land and water, the law of the trade-winds is greatly disturbed. This is especially remarkable in Mexico and India. That part of the Pacific which extends from the Isthmus of Panama to the peninsula of California lies between 8° and 22° N. latitude. The sun's rays striking directly on the great territory of Mexico heat the land strongly, thereby causing the air over it to rise; the vacuum is filled up not only from the northward, but also by the comparatively cool air of the equatorial regions in the neighbourhood. The air coming from that part of the globe which revolves quickest to a part which moves more slowly, produces not an easterly wind, but westerly and south-westerly winds; so that the navigator is often very much embarrassed who expects east or north-east winds, according to the usual theory of the trade-wind.

The monsoons of the Indian Ocean are also modifications of the trade-winds due to the presence of vast masses of land. These winds are called periodical, to distinguish them from the trades which are constant. They blow for nearly six months of the year in one direction, and for the other six in an opposite direction. The Malays call them mooseen, which signi