Now, in the case of these constant winds, the inertia is very nearly overcome, as they start from a circle in which the velocity to the eastward is about 750 miles per hour. If the inertia were fully overcome, there would be no perceptible wind; as the velocity is actually but fifteen to eighteen miles per hour, it appears that the friction encountered actually destroys from thirteen-fourteenths to fifteen-sixteenths of the inertia. Hence, we find these constant air currents toward the west are, in reality, the result of the earth carrying any object on its surface a little more rapidly than the atmosphere moves; so that these winds are precisely the same in principle as the well-known fact that when you run rapidly in still air (so-called), it seems that the wind is blowing directly in your face.
In like manner, it appears that a wind from west to east is merely an air-current moving a little more rapidly than the earth revolves at that point. The relative difference between the velocity of air-currents must vary greatly; for a violent easterly or westerly wind very near the poles may equal or even exceed the speed of the rotation of that point; while the most violent tropical storms average between one-twentieth and one-eighth of the local rotation. The latter is not often exceeded. But whatever the relation of the respective velocities, it is clear that the velocity of the wind in general must depend largely on the amount of air abnormally heated, and upon the rapidity with which it is heated. So men have come to recognize that a period of unusually oppressive heat forebodes a storm of some sort. But few regard the unusual warmth as a reason of the storm. They are linked, in the popular mind, as antecedent and consequent, rather than as cause and effect.
These constant winds near the equator have been named trade-winds, because of their importance to commerce. Unknown before the first voyage of Columbus, they filled the minds of his crew with fear that they could never return home, if the wind blew always in one direction. The same gentle wind bore Magellan in his voyage around the world, and caused him to give the name of “Pacific,” or “peaceful,” to the great ocean on our west; and the same steady breezes made the fortune of many a noble galleon in the days when Peru was an Ophir, Mexico an El Dorado, and the Philippine Isles a Tarshish where they took shipping for the distant land of gold.
Owing to the fact that the continents intercept the regular trades by reason of their elevation and irregular conformation, and also because of their much greater specific heat, whereby they set in motion many other local currents, the trades are found to begin only a considerable distance to the west of the continents. Yet the influence of the trades is sufficient to make easterly winds the prevailing ones on the great inland plains: as in the Sahara, Arabia, Southern Siberia, and portions of North and South America.
It is clear that other nearly constant currents must exist to supply the vacuum that would be otherwise caused by the trades. These are found to the south and north of the trade belts, and, as might be expected, blow nearly in the opposite direction, being descending currents; while the trades, as before stated, are ascending. The column of hot air from the equator starts toward the poles above the trades, while a polar current sets in toward the equator; but as the amount of air displaced at the equator is by far the greatest, much of it can, of course, never reach the poles. On meeting the polar current, the two partially mingle and descend, forming what is called the return trade. This blows, most of the year, to the southeast, the equatorial current prevailing and coming from a region whose easterly rotation is more rapid. At certain seasons of the year, however, the polar current prevails to some extent, though not sufficiently to overcome the eastward trend; so the wind in this belt blows alternately to the southeast and the northeast.
Between the region of trades and alternating winds is a belt, on either side of the equator, of calms and variable winds, which shift northward or southward, parallel to the belt of calms between the trades. These two zones, however, are much less clearly defined than the great central one, and are not liable to such extraordinary disturbances.
Such is the great constant wind, with its dependents. So long as the sun has warmed the earth, it has hurried on its course, subject to unceasing law, and destined to cease only when the heavens and the earth shall pass away, and chaos or annihilation shall end the things that be. A Wandering Jew of the atmosphere, it flies ever onward, bearing the merchant to his port, and the rain-cloud to the land; ever and anon desolating the isles with its bursts of fury; then resuming its restless course, like the remorseful Salathiel.
CHAPTER III.
PERIODIC WINDS.
“Earth has each year her resurrection hours
When the spring stirs within her, and the powers
Of life revive; the sleeping zephyrs rouse,
The blushing orchards clothe their naked boughs,
The swallow skims above the lakelet’s verge.
Swift summer speeds with fire in every vein,
And autumn’s glories crimson hill and plain.
Then warmth and life from Nature take their flight,
And winter robes her in a shroud of white,
While mournful Boreas chants her funeral dirge.”
