Of Winds, &c.

Winds.

The atmosphere is, like the ocean, subject to Cause of movements, which, considered generally, depend on the same cause, the invariable tendency of all fluids to preserve an equilibrium: hence a change in the temperature of a column of air, the condensation of a part of the atmospheric gas into rain, congelation, or whatever else causes a void, a contraction, or dilatation, and which consequently breaks the equilibrium of the different parts of the atmosphere, necessarily produces a deplacement of a mass of air or a wind.

The observations made on the velocity of wind Velocity. afford the following results:*

A light breeze travels ten feet in one second of time,

A moderate breeze

A fresh breeze

A strong breeze

A strong gale

A storm

A hurricane

Winds

may properly be divided into constant or Division general, periodical, and variable. There are two constant and general movements of the atmosphere, similar to the constant currents of the ocean;

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* A Professor of the University of Petersburgh has invented an instrument, which he has named Anemometer, for ascertaining exactly the velocity of the wind.

Trade winds.

ocean; the first is the general equatorial movement, which carries the air relatively to the earth to the west, and the other that which draws the air from the poles towards the equator.

The general equatorial winds* seem to be produced principally by the solar heat, which rarifies and elevates the air between the tropics, and the air thus rarified must either be dissipated in the heavens, or must move towards the poles in the upper region of the atmosphere; at the same time that a cold air must arrive from the poles through the inferior region to fill up the void. But the velocity which each molecule of air receives from the rotation of the earth must be less in proportion to its nearness to the poles, whence it results that the polar air advancing towards the equator, and for a time preserving its primitive movement, will move with less velocity than the corresponding parts of the earth, the objects on the surface of which must therefore strike against the atmospheric fluid with a force corresponding to the excess of their velocity,t and thereby experience a resistance to their movement of rotation. Thus to the observer, who thinks himself at rest, the air appears to move in a direction directly opposite to the rotation of the globe, or from east

to west.

The

* Called trade winds by the English probably because those of the Atlantic conveyed the trade from Europe to America.-Vents alizés of the French.

} + The simple effect of riding or running fast in a calm clearly explains this theory, the person moving faster than the atmosphere always feels a wind in his face.

lantic.

The currents of polar air in approaching the tropics encountering the equatorial movement, a combined movement is the consequence, producing a N. E. wind in the northern tropic, and a S. E. in the southern. These trade winds extend on the eastern side of the Atlantic between the pa- In the At rallels of 28° N. and 23° S. increasing their limits as they go westward, being met some degrees farther north and south near the coast of America. Their limits also vary with the season, extending farther beyond the tropics in the respective summers of the two hemispheres. The N. E. trade is usually lost between 12° and 2° N. and never crosses the equator, while the S. E. particularly in the northern summer season, is often met with in 5° N. This extension of the S. E. trade to the north is probably dependant on the superior cold of the southern hemisphere, whence a more considerable current of dense air rushes towards the equator. The strength of the respective trades is greatest in the summer of the opposite hemisphere, which is the necessary consequence of the greater rarefaction of the air in this latter; it is also observed that the strength increases in going to the west, which seems to be partly the effect of the increased velocity of the equatorial current of the ocean, the movement of which must be considered as affecting the current of air over it, and partly caused by the proximity of the heated land, which draws towards it with accelerated movement the denser air of the sea. The directions of the trades also deviate towards the north and south with the sea

Deviations on the coast of

Africa.

son, blowing more directly towards the region of greatest rarefaction; thus, while the sun is in the southern tropie, the N. E. trade inclines more northerly, even to the N. N. E.; and when in the northern, the S. E. trade inclines in like manner towards the south, even to the S. S. E.

It is from the knowledge of the trade winds that ships, bound from Europe to the West Indies, first steer to the south till they meet them; but in proportion as they are favourable to those bound to the western hemisphere, they are against those returning from it: hence it is the object of these last to get as soon as possible to the north of their limit into variable winds.

We have seen currents of the ocean, contrary to its general movement, setting towards the coast of Africa; a similar deviation is observed with respect to the winds near this coast: thus, north and N. W. winds blow in the vicinity of Cape Verd, and between it and Sierra Leone; and south and S. W. winds prevail on the coast of Guinea throughout the year, particularly between Cape Palmas and Cape Lopez. These winds are doubtless the combined effects of the currents and of the heated atmosphere over the burning deserts of Africa. It is in consequence of these south and S. W. winds that ships, bound from Europe into the Gulf of Guinea, usually prefer what is called the great or outer passage, as more certain than the inner one. In the former the course is the same as if bound to the East Indies until they have reached the twenty-eighth or thirtieth degree of latitude,

when

when they steer for the coast of Africa; in the inner passage they keep close along the coast from Cape Verd, to take advantage of the land and sea breezes and southerly currents,

T

The space of the North Atlantic included be- Sea of rains. tween the parallels of 10' and 4° north, and between the meridians of Cape Verd and the eastern of the Cape Verd Islands, has been called the sea of rains. It is generally described as subject to very long calms, accompanied by a suffocating heat, with transient squalls of thunder, lightning and violent rains. Though these effects seem to have been considerably exaggerated, they nevertheless exist to a certain degree; and are probably produced by the two trade winds having spent their force when they reach this limit, and long calms are the consequence, while the vapours conveyed by both winds, ceasing to be impelled onwards, condense into heavy rains.

in the Pacific.

In the Pacific Ocean the general trade winds Trade winds are similar to those of the Atlantic, blowing constantly between the north and east in the northern tropic, and between the south and east in the southern. Near the west coast of America their limits are strictly confined to the tropics, or even within them, but they widen as they move onwards, extending on the coast of Asia to the latitude 52° N. and on that of New Holland, or Terra Australis, to about 30° S. With the N. E. trade, and the equatorial current, the Spanish galleons from Acapulco to the Philippines crossed the grand ocean in an undeviating track between the parallels

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