is constructed in exactly the same manner as a letter weigher, where a weight of half an once compresses the spiral, bringing down the index to a certain division of the scale. 84. If, however, the velocities of the different winds are already known, and the force of one obtained, those of the rest can be found by the following rule, viz. that their forces are as the squares of their velocities. For instance, if the power of a gale, possessing the speed of twenty miles an hour, is known to be 1,968 pounds on a square foot, that of a storm with a velocity of fifty miles can thus be ascertained by a simple proportion. (20×20) (50×50) 400 is to 2500 as 1,968 is to the answer 12,30. Should the forces be known, it is obvious that the velocities can be computed by reversing this process. Winds may be divided into three classes, CONSTANT, PERIODICAL, and VARIABLE. CONSTANT WINDS. TRADE WINDS. 85. The most remarkable instance of the first class, is that vast current, which, in the torrid zone, is ever sweeping around the globe, in a westerly direction; and, from its advantage to commerce, in always affording a steady gale to the bark of the adventurous mariner, is denominated the trade wind. 86. So uniform is its motion, that on the voyage from the Canaries to Cumana, on the northern coast of South America, it is scarcely necessary to touch a sail; and with equal facility, the richly laden Spanish galleons were accustomed to cross the Pacific from Acapulco to the Philippine Isles. 87. ORIGIN. The cause of this wind has been thus explained by Halley, an English philosopher. From the vertical position of the sun, the regions near the equator If the velocities are known and one force, what else can be obtained? Give the rule and the example. If the forces are known, what can be computed? Into how many classes are winds divided? Name them. What is the trade wind? How does it originate? are intensely heated, while those more remotely situated are less so; the temperature gradually diminishing towards either pole. (Art. 49.) In accordance with the principles just unfolded, (Art. 78,) an upper current will flow from the equator towards the poles, and a cold current at the surface of the earth, from the poles and the higher latitudes, towards the equator. Here the air, becoming rarefied by the heat, rises, and mingling with the upper wind flows back again to the polar climes; thus establishing a perpetual circuit. If then the atmosphere was subject to no other influences, a north wind would prevail in the torrid zone, in the northern hemisphere, and a south in the southern; but these directions are modified by the rotation of the earth, in the following manner. 88. Every thing upon the surface of the globe at the equator, is carried towards the east, at the rate of about sixty-nine miles in four minutes; but as we recede to the north or south of this line, the eastern velocity is so diminished, that at the latitude of 60° it is reduced to one-half, and at 83° to less than one-eighth of its original amount. A wind, therefore, blowing from the high latitudes towards the equinoctial clime, is constantly passing into regions where all terrestrial objects have a greater easterly velocity than itself. They will consequently move against it, and as they are apparently stationary, it will thus acquire a relative westerly motion. Just as when a traveler, outstripping the wind that blows at his back, feels a breeze directly in his face. 89. Thus, the polar wind in the northern hemisphere is influenced by two forces at the same time, one of which carries it to the south, and the other to the west; and the course it assumes by their combined action must be according to the laws of compound motion, (C. 249,) some intermediate direction, tending from the northeast to the south-west; and such is the fact, according to all observations. What two forces influence the polar wind in the northern hemisphere? What is the direction of the trade wind in this hemisphere ? In a similar manner, the lower current in the southern hemisphere, acquires a direction from the south-cast to the north-west. The passage of a vessel across a river is an illustration in point. If the vessel is steered before the wind, from east to west, while the stream is flowing from north to south, she will be seen by a spectator on shore sailing from north-east to south-west. 90. In the Atlantic and Pacific, the breadth of the trades increases as they flow towards the western shores of these vast oceans, the wind gradually changing to the east, by the mutual action of the two currents. 91. The land is heated by the sun far more intensely than the ocean. This is owing to the fact that the solar rays warm only the surface of the earth, scarcely penetrating an inch in the course of a day, while during the same time they pierce the water to the depth of many fathoms. It has been computed that the beams of the sun communicate daily a hundred times more heat to a given extent of ground than to an equal surface of water. On this account, the proximity of highly heated continents produces local variations in the direction of these winds; for the air, being more rarefied over the land, ascends, and to supply its place, the cooler air of the trades sets in from the sea towards these localities. 92. Thus, on the African coast, between Cape Bajador and Cape Verde, a north-west wind prevails within the limits of the north-east trade; and off the coast, from Sierra Leone to the Isle of St. Stephen, the trade wind gradually changes to the south and south-west, veering to the west as it approaches the shore. From the same cause, the south-east trade becomes a south wind along the coasts of Chili and Peru. 93. LIMITS OF THE TRADE WINDS. In the Pacific, the north-east trade wind prevails between the 25th What in the southern hemisphere? Illustrate the subject. Why is the land more intensely heated than the ocean? How does this difference cause a local variation in the direction of the trades? Give instances of such changes. State the limit of the trade winds. and 2d degree of north latitude. The extent of the south-east trade is not precisely ascertained, but it probably ranges from the 10th to the 21st degree of south latitude. In the Atlantic, the former is comprised between the 30th and 8th degrees of north latitude, and the latter within the limits of the 3d degree of north and the 28th degree of south latitude. 94. The limits, however, are not stationary, but are dependent upon the season-advancing towards the north during the summer of the northern hemisphere, and receding to the south as the sun withdraws to the southern tropic. Thus, on the west coast of Europe, the north-east trade has been found to extend as far as Madeira, and even to Mafra, in Portugal. 95. CALMS. In the vicinity of the Cape Verde isles, between the 8th and 3d degree of north latitude, is a tract denominated by mariners the rainy sea. This region is doomed to continual calms, broken up only by terrific storms of thunder and lightning, accompanied by torrents of rain. A suffocating heat prevails, and the torpid atmosphere is disturbed, at intervals, by short and sudden gusts, of little extent and power, which blow from every quarter of the heavens, in the space of an hour-each dying away ere it is succeeded by another. In these latitudes, vessels have sometimes been detained for weeks. In the Pacific, the region of calms is comprised within the 2d degree of north and south latitude, near Cape Francis and the Galapagos islands-a narrow belt of ocean separating the two trades. Here, likewise, dreadful tempests prevail. 96. According to Humboldt, a similar state of the atmosphere exists, during the months of February and March, on the western coast of Mexico, between the 13th and 15th degrees of north latitude, and 103d and 106th degrees of west longitude. A ship, richly laden with cocoa, was here becalmed for the space of twenty Are the limits stationary? Upon what do they depend? eight days, when the water failing, the crew were compelled by their sufferings to abandon the vessel and seek the shore, eighty leagues distant, in an open boat. 97. The calms are supposed to be caused in the following manner. The adjacent continents to the east of these stagnant regions being far more intensely heated than the sea, the air over the latter would rush easterly towards the land, were it not arrested by a contrary impulse in the direction of the trade wind. If these opposing forces are at any time equally strong, the atmosphere is motionless, and a dead calm ensues— just as a vessel, in ascending a stream, continues stationary when the power of the wind is exactly balanced by that of the current. When, however, the relative strength of these forces rapidly changes, those short and sudden gusts which have been noticed will arise, as one or the other of these impulses prevails. 98. The presence of a highly heated region is strikingly marked in the case of the rainy sea. To the east lies the great African desert, from whose burning surface a vast volume of hot and rarefied air is perpetually rising. Another cause must not be forgotten, which applies, more particularly, to the calms near Cape Francis. This tract is directly under the equator, and from its peculiar situation, the upward current of rarefied air is probably here so strong as to neutralize the action of the trade winds. The limits of the calms vary also with the seasons. Thus, in the Atlantic, the range in August is between 3° 15' and 13° N. Lat., but in February, extends from 1° 15' to 6° N. Lat. WINDS OF THE HIGHER LATITUDES. 99. The upper equatorial currents, flowing off towards either pole, descend, on their passage, to the earth, and since they carry with them an excess of easterly velocity, will become, upon the principles already How do the calms originate? What are their limits? |