CHAPTER V.

One, and the principal end attained by the power of the agent, is the gathering of a volume of atmosphere from, or near, the surface of the land and sea, so as to ensure its possession of all the moisture of evaporation which rises from the locality, and the highest degree of temperature, and from a space ranging from one to two thousand miles in width, in one hemisphere, and to carry it over into the other. Not over the top, or upon the top, of the whole mass of atmosphere situated in the opposite hemisphere--out of reach of all influences from the earth--but through it, and curving gradually down near to, and within influential distance of the surface of the earth, soon after it passes the outward limit of its fellow trade; and to continue the current onward, leaving portions of it and its heat and moisture on the way, but taking a considerable volume up and around the magnetic poles--it being impossible for the entire volume to be thus carried around the poles in consequence of the diminished circumference of the earth. To this end it is obvious it must possess polarity.

Another end to be attained is to combine the moisture of evaporation with the air, so that the cold atmosphere through which, or the earth over which it passes, may not be continually condensing its moisture, and thereby enveloping the earth in a perpetual mist; but so that it may part with it at intervals, making cloudy and clear days; and part with it in portions, so that a regular and necessary supply may be furnished to the entire hemisphere, even up to the geographical poles. Is there such an agent? There is, precisely and perfectly adapted to the ends to be attained, ever there and ever active, and that agent is magnetism.

The earth is a magnet. It has its magnetic poles, and they are distinct from its geographical ones; and there are two in each hemisphere. They are situated from 17 deg. to 19 deg. distant from the geographical poles; and ours is not far from longitude 97 deg. W. from Greenwich, and 71 deg. north latitude. Navigators have gone north and north-west of it, and found its situation by the declination of the needle. From these poles, lines of magnetic intensity extend to the opposite and corresponding pole of the other hemisphere, and upon or near those lines the needle points north without variation; and toward these lines of no variation the needle every where, on either side declines. The foregoing diagram shows the situation of our magnetic pole and line of no variation, the dip of the needle by the arrows, and the magnetic equator.

Recent discoveries have shown that the magnetic force is exerted in lines and currents; that such currents, as physical lines of force, surround magnets, and currents of electricity. Doubtless such lines of force exist around the earth and the magnetic poles. There are also longitudinal lines of force existing and active, between the poles, and extending from one side of the center to the other, occupying nearly one third of the magnet. If you take a large needle thoroughly magnetized, place it upon paper and drop filings of iron upon it, they will become arranged about it in circular and perpendicular, and also in longitudinal lines, conforming to the currents.

This experiment is illustrated in all our books on natural philosophy.

The foregoing diagram, copied from Olmstead's Philosophy, does not show as accurately as Faraday's projection of the lines upon a globe-magnet the comparative distance from the poles of the needle, at which the longitudinal currents commence and terminate, and where the filings will not adhere to any considerable extent. The lines shown upon the needle should bear the same proportion to its length as the trade-winds bear to that of the earth, measured from pole to pole, and if the needle had a globular form they would so appear.

These lines are made by currents arising from one side of the magnetic equator, and passing over to the other. Doubtless, just such currents rise, and pass over upon the earth.

Magnetic and electric currents carry the air with them. This is well settled by experiment. Oxygen, too, is magnetic, and capable both of receiving and retaining polarity and of combining with, or attracting and retaining vapor, and of course the moisture of evaporation. Here then we have a power existing, capable of producing the result--precisely, and with evident wisdom adapted to its production--ever present and active; and no other known agent can.

Is it not then the agent?

Let us look a little further. This result is affected by the action of the sun: the trades with the central belts of rains travel north and south after it; so does the sun affect the magnetic currents every where, even the magnetic needle is daily affected by its action, as it increases the intensity of the terrestrial magnetic currents, and hence its well established diurnal oscillations.

Again, along the eastern lines of the continents which skirt the great oceans on the west, run the northerly and southerly lines of no variation, and of greatest magnetic intensity. Here are the trade currents gathered into a volume, which curve and carry unusual fertility to South-eastern Asia, and North America, and in those great aerial gulf streams we find the intense electric action which produces the typhoons of the former, and the hurricanes of the latter. It may still be said that these conditions and phenomena of the trade-wind region, are not produced by magnetism or magneto-electricity, but the objector can point to no other adequate power. That it must be heat, electricity, or magnetism, must be admitted. There is no other power known. Heat demonstrably can not produce them. Magnetism or electricity therefore must, and they are doubtless states or phases of the same power, producing in their different states or phases the different results. And even heat--atmospheric temperature, is often, if not always the result of their action. In the present state of science, it is enough for me that the magnetic longitudinal currents are there; that they are lines of force and adequate; that oxygen is magnetic, and therefore the atmosphere must be affected by them--that so far as we can reason from analogy, they ought to produce the effect upon the atmosphere which we find produced, and until further light is thrown upon the subject I shall presume that they do. Every step we take hereafter in this investigation will confirm the presumption.

There is one peculiarity to be more particularly noticed before we leave the trade-wind region, and we are now prepared to notice it.

The belt of rains, formed by the currents of the two trades, threading their way through each other--how are they produced? Why should the place where the currents thus pass through each other be a place of almost daily precipitation? There is, in fact, no ascension, except that which the currents have in their line of ascent to attain the elevation which the magnetic law of the current requires.

The trades have passed over an evaporating surface and are charged with moisture. This moisture they hold in magneto-electric combination. Evaporation does not depend upon temperature. Ice and snow evaporate at all temperatures (Howard, vol. 1, p. 86). So the cold N. W. wind, full of positive electricity, will lap up, as it were, the pools from the earth, with astonishing quickness; and when this electricity is deranging the action of the machinery and material of the manufacturer, he allays it by a supply of moisture, with which the electricity can combine. Nor does the air lose its moisture when below the freezing point. In all parts of the atmosphere, as at the surface of the earth in winter, moisture is held in large quantities in the coldest and severest weather; and it is not till it moderates, and a perceptible electric change takes place, that it is precipitated as rain or snow. Doubtless there is an exposure of considerable surfaces, of opposite currents, charged with opposite polarity, and a constant depolarization where their surfaces meet. May there not be a consequent dissolution of the electro-magnetic combination between the air and moisture, or the excitation of that electric action which attends or produces like rains every where? and hence the constant precipitation. This is rendered probable, by the fact that precipitation, at the meeting of the trades, takes place in level countries in the day-time, between 10 A. M. and sunset, in showers, with thunder and lightning, as with us in summer, although among the mountains the rain sometimes falls in the night also. The precipitation in the heat of the day is obviously induced by the action of the sun, although it is by no means certain that the friction of the opposing surfaces does not assist in the operation.

I am well aware that the lines of magnetic force curve upward and carry the trades with them, and that, therefore, precipitation by condensation from the mere cold of the upper stratum of the atmosphere is possible. But, there are three reasons why I do not believe such to be the fact.

1st. Precipitation takes place in the day time mainly, and in sudden, isolated, heavy showers and not in steady continuous rain. Nor is there condensation or continual mist at other hours of the day.

2d. They occur at a time of day when the sun is affecting the magnetic currents most powerfully, viz., between ten o'clock A. M. and sunset, and mainly at the time of greatest heat.

3d. The counter-trades do not precipitate after they leave the rainy belt, although at a great elevation, until they reach the outward limits of the trades; and they do precipitate again, although they gradually descend nearer the earth, as soon as they become subject to the action of the currents of an opposite magnetism. Their precipitation is partial too, even then, and they carry a portion of their moisture through an atmosphere of the coldest temperature up to the geographical poles.

A similar result attends the action of the sun in the extra-tropical regions. Cumuli commence forming in the counter-trade, or at the line between that and the surface current, at the same time of day that the diurnal motion of the magnetic needle commences, or the rain clouds form in the tropics; they continue to enlarge here as there, till about the same hour of the day that the needle obtains its maximum diurnal variations; and when the influence of the sun upon the needle ceases, and it returns to its original status, the cumuli disappear. Hail storms too, it is said, always, or generally occur in the day time.

In like manner the sea-breezes and other fair-weather surface winds, rise in the forenoon with the influence of the sun upon the magnetic currents and the needle, and die away at nightfall when the influence ceases.

There are other electro-magnetic, or to speak more correctly, magneto-electric, effects of the sun's action equally illustrative, which tend to show that the precipitation at the passing of the trades, is the result of their action upon each other, aided by the sun, to which we shall allude when we come to speak of the causes and character of the surface winds of the extra-tropical regions.

As, however, this takes place only, or mainly, where the threading surfaces meet, it is but partial, and the body of the respective polarized currents pursue their way unaffected, toward the opposite magnetic pole--and there for the present we leave them.

Storms sometimes originate in these currents, when concentrated, as in the West Indies, the China Sea, the Bay of Bengal, and Indian Ocean, while passing through the rainy belt, and move with the current to the north-west if issuing on the north side of it, and to the south-west if issuing on the south side of it, until they respectively get beyond the extreme limits of the trades, and then they curve to the eastward, imbedded in and following their current. The peculiar extension of the land to the east on the northern portions of South America, prevents the gathering of an aerial gulf similar to the one which we have described to the north-west, entering upon our division of the continent over the Gulf of Mexico. It is otherwise in the Indian Ocean, and there the storms are found issuing from the rainy belt on the southern side, sweeping over the Mauritius and other islands of that ocean, and often simultaneously with storms issuing on the north over the Bay of Bengal. Colonel Reid mentions instances and gives a diagram.

These storms in milder forms issue from the rain belt at other points, and may issue any where, but will always be found most extensive and most violent, that is to say, as hurricanes and typhoons, in the concentrated volumes of counter-trade on the western side of the great oceans, within a few hundred miles of the lines of magnetic intensity and no variation, and when they form in the rainy belt they are highly electric. Most frequently, however, as we shall see, they form in these currents after they have issued from the rainy belt, and after they have passed the extreme limits of the trades and become subject to the circular and perpendicular magnetic currents which exist north and south of the longitudinal ones, and which when seen upon the magnetic needle, attract the filings and cause them to adhere--although but slight attraction or adhesion takes place where the longitudinal currents exist.

Such, then, are the atmospheric arrangements and phenomena of the trade-wind region, and the cause that produces them; such is the character and cause of the enlarged volume of counter-trade, which spreads out and blows over our country as permanently as the S. E. trades blow on the South Atlantic and South America, returning to us the rivers which had run from us to the sea.