over the surface of the globe, phenomena like these occur in nature, on a widely extended scale; for if two neighboring countries are unequally heated, the air above them partakes of their respective temperatures, and there arises at the surface of the earth, a wind blowing from the colder to the warmer region, while at the same time, a directly contrary current prevails in the upper strata of the atmosphere.

79. VELOCITY. Every gradation exists in the speed of winds, from the mildest zephyr, that scarcely bends the flower, to the most violent hurricane, which prostrates the giant oak, and hurls to the ground the proudest works of man. They have been classed as follows, by Smeaton, according to their rapidity and force.

Velocity of the wind, Perpendicular force on one square
miles per hour.
foot in lbs. avoirdupois.

1

Common appellation of
such winds.

80. The velocity of the upper currents of the atmosphere, is as variable as that of the winds which sweep over the surface of the globe; for the aëronaut, Green, who ascended from Liverpool, in 1839, to the height of 14,000 feet, encountered a current that bore him along at the rate of five miles per hour, but upon descending to the altitude of 12,000 feet, he met with a contrary wind, blowing with a velocity of eighty miles per hour.

How does it explain the origin of winds?

What is said of the velocity of winds?

Give the common appellations of winds, with their velocity and force. What is said of the speed of the upper currents?

Give examples.

On one occasion, his balloon was carried over the space of ninety-seven miles in fifty-eight minutes.

81. ANEMOMETER. The velocity of the wind is estimated by the anemometer, an instrument so called from the Greek words, anemos, wind, and metron, measure. One of the best is Woltmann's. It consists of nothing more than a small windmill, to which is attached an index, in order to mark the number of revolutions per minute; the number of course increasing with the speed of the wind. Now if the atmosphere is still, and the anemometer is carried against it at the rate, for instance, of ten miles per hour, the number of its revolutions will be exactly the same as if the instrument was stationary, and the vanes revolved by the force of a breeze possessing the same velocity.

82. If then, upon a calm day, the anemometer is taken upon a railroad car, moving, for example, at the speed of twenty miles an hour, and the number of revoutions for half an hour accurately noted, we can obtain, (by dividing this result by 30,) the number of revolutions per minute, corresponding to those of a wind having a velocity of twenty miles per hour. In this manner, a table adapted to the instrument can be constructed for all winds, moving with a greater or less rapidity.

The velocity of the higher aërial currents is ascertained by the speed with which the shadow of a cloud passes over the surface of the earth.

83. FORCE. The force of the wind is obtained, by observing the amount of pressure it exerts upon a given, plane surface, perpendicular to its own directions. If the pressure-plate acts freely upon spiral springs, the power of the wind is denoted by the extent of their compression, and that weight will be a measure of its force, which produces the same effect upon the springs. This instrument, which is also termed an anemometer,

What is an anemometer? Describe Woltmann's, and the mode of computing by it the velocity of the wind.

How do we judge of the speed of the upper currents?
In what manner is the force of the wind estimated?

is constructed in exactly the same manner as a letter weigher, where a weight of half an ance 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) lbs.

lbs.

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-east 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 pene trating 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.
What is said of the breadth of the trades?

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.