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_.
[Illustration: Fig. 12.]
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.
[Illustration: Fig. 13.]
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.[2]
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.