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air, to occupy the These currents do not flow from due north and due south, because, as the earth rolls every day once round itself, from west to east, air that has acquired slow movement at the poles, finds the globe travelling too fast for it at the equator, and is obliged, therefore, to drop more and more behind.
The current from the north becomes a northeast wind; but that from the south is not due south, but southeast. These winds are constant, where there is no local interference, within about twenty-eight degrees on each side of the equator, being, in fact, the northeast and southeast trade-winds. Why do they not blow all the way from pole to tropic?
There, you have the upper current to consider; the hot air that ascended at the equator has been gradually cooling, and becoming therefore denser-heavier as it ran over the cold current below. The cold air from the pole, too, had been getting warmer, therefore, lighter, on its travel; so that in temperate climates, to which we belong, it becomes a disputable point between the two currents which shall have the upper, which the lower seat. In these regions, therefore, there is no uniform wind; but as the currents from the equator generally succeed in maintaining that it is now their turn to go below, winds from the south prevail outside the trade-winds north of the equator, which are, of course, represented by north winds on the other hemisphere. Southwest and northwest these winds are, because they are fast currents, which started from the earth where it was rapidly revolving, and vote polar regions slow. Winds from the southwest then prevail in Europe; and the southwester now blowing whistles with immoderate exultation at a victory over some polar current with which it has for the last few days been exchanging blows.
Well, you say, there must be a pretty clashing of cymbals when the great trade-winds from the north and south run against one another; and they must do that somewhere near the equator. Yes, the scene of their collision occupies a broad band about six degrees north of the equator, more or less. The trade-winds of the southern hemisphere encroach over the line at all seasons, owing to peculiarities of land and water; but the limits of the trade-winds are not marked out by a fixed straight line. They vary, in extent, with the season, and their outline varies with the nature of the earth or water over which they blow. But, the scene of collision, as we said, forms a broad zone always north of the equator, which is called the zone of the variable winds and calms. Here it is that a great ascending current strikes off the antagonists on either hand; and then if we are in the current, we perceive no wind; and if we hold a lighted candle in the air, its flame ascends unwavering; but a few feet from the ground we can feel nothing of the upward rush which we denominate a calm. With this current rises a vast mass of vapor, and the sun's decline, or a touch from the trade-wind, or the coldness of the upper air, condenses this; and down come sheets of rain, attended with electrical explosions. How the trade-winds, when they come together, twist and twirl, and generally how two winds. cause an eddy, and a veering of the weathercock when they come down upon each other, any man may understand who ever sat by a brook-side. Currents of water coming upon each other, round the stones, from different directions, act upon each other just as the air currents act: carving miniature gales and model whirlwinds from a kindred element.
Within this zone of variable winds and calms, vapor ascends perpetually, and rain falls almost every day; the rainy season being distinguished only by a more determined
drench, just as a doctor, paid by items, pours forth more bottles in the season of an epidemic, though he at all times is unmercifully liberal. That vapor rises from water and from every moist body under the influence of heat, any body knows. The greater the heat, the more the vapor; but even in winter, from the surface of an ice-field, vapor rises. The greater the heat, the greater the expansion of the vapor. It is the nature of material things to expand under heat, and to contract under cold; so water does, except in the act of freezing, when for a beneficent purpose it is constituted an exception to the rule. Vapor rises freely from lakes, rivers, and moist land; but most abundantly, of course, it rises from the sea, and nowhere more abundantly than where the sun is hottest. So it rises in the zone of variable winds and calms, abundant, very much expanded, therefore imperceptible. There comes a breath of colder air on the ascending current; its temperature falls; it had contained as much vapor as it would hold in its warm state; when cooled it will not hold so much; the excess, therefore, must part company, and be condensed again; clouds rapidly form, and as the condensation goes on in this region with immense rapidity, down comes the discarded vapor in the original state of water, out of which it had been raised; down it comes, a hogshead in each drop. Sudden precipitation, and the violent rubbing against each other, of two air-currents unequally warmed, developes electricity; and then in this zone you can hear such thunder, and behold such lightning, as we quiet folks at home are never plagued with. We may stop here to remark that in all climates this is the whole theory of rain. Our present weather is sufficient illustration. There was a noisy wind from the southwest this May morning-a wind from the warm regions which has come over the sea loaded with vapor.
Though violent, it felt warm to the face; but in the sky were scattered clouds, and the wind veered frequently towards the north, with sudden showers, one of which pelted upon Tom. It was a contest between the southwest current, and a current from the north, which now and then forced a way down, and, where it did so, cooled the atmosphere, and obliged it to part with some of its vapor, either. in the form of clouds or rain. The winds are quiet now, and if you look out, you will see that the fight is over, and the southwest beaten after all its crowing; north wins. You see by the smoke that there is a north wind, which, being a cold polar current, cannot hold, in an expanded state, one half of the vapor brought into our atmosphere by the southwest. The north wind, therefore, marks its victory by a general precipitation; the whole sky is uniformly clouded, and a steady rain falls, and will fall, until the balance is restored. When the north wind has turned out of the sky all the vapor that it cannot manage, we shall have fine weather, until a warm wind interferes. The warm wind, then, must bleed some drops before it gains possession, but, having conquered, will possess a sky containing less than its due quantity of vapor; therefore precipitation will not be continued. The southwest wind, however, soon brings moisture with it; and then, if the noon be fine, clouds form at evening, when the temperature falls, and it may rain at night. Every thing contains its regulated quantity of latent heat-a body in the form of air more than a liquid, and a liquid more than a solid. Latent heat is sensible heat mysteriously transformed, used in the processes of nature, swallowed up, become insensible. Water contains more of this, then, in the state of a thin vapor than in the condensed form. When, therefore, clouds form, heat that was used up and made latent is restored and ren
dered sensible; that is one reason why cloudy weather is warm. After a shower, the whole earth is moist, and evaporation takes place on the entire surface. Water, to become vapor, seizes, appropriates, and thrusts into the latent form some of the sensible heat lying in its neighborhood, and, therefore, a coolness or a chill succeeds the rain. But there is chill, also, during the rain-fall, when the condensation is at its greatest; how is that? Doubtless you know that air and water conduct heat but badly. You could not heat a tub of water from the top, and the sea retains through all seasons a remarkable imperturbability as to its temperature. So you, or the sun, cannot heat any amount of air from the top; but the sun's rays that reach the earth warm that, and it retains the warmth, and radiates it back again; and so it is the heat of the sun sent from our own earth which fills the air about us. If we walked on such high stilts as to raise our mouths and noses far above the sod, we should be very glad to have our stilts cut shorter; for the radiant heat lessens as we rise from the earth's surface, in proportion no less rapidly than light lessens as we quit a candle; and at any distance from the earth the atmosphere is very cold. So rain descending from the cold heights brings a chill with it. So clouds that cover over the earth and prevent its heat from radiating into space, but rather reflect it back again, act as a blanket does over a man's warm body when he is in bed, and so we have a second reason why it is warm-close-in cloudy weather.
Since water retains in a remarkable degree an even temperature, and land heats and cools in correspondence with all changes of the sun, it follows that where land and water are commingled, inequalities of temperature will be various and frequent; every inequality being the cause of a