The counter-trade of the northern hemisphere ranges at different heights
in different latitudes, in the same latitude at different seasons, and
also upon different days of the same season; and, like the line of
perpetual snow, has its greatest elevation in the tropics, descending
gradually to the surface of the ocean at the poles. At the northern limit
of the N. E. trades, it does not, ordinarily, approach the earth
sufficiently near for decided reciprocal action. Hence, at that point,
storms do not often originate; the winds are lighter and more variable,
and calms are more frequent than at any point, except at the meeting and
elevation of the trades, or in the polar regions. Doubtless this state of
things is increased by the feebler action of north polar magnetism, and
the irregular action of the longitudinal magnetic currents, evinced by the
irregular, and often, feeble action of the trades, near their extreme
limits. They are not unfrequently wholly wanting, near the northern limit,
for several days in succession, and calms and baffling winds are found in
their place--another effect of the irregular action of terrestrial
magnetism, consequent upon the ever-changing transit of central activity
from south to north, and from north to south. Upon the islands, however,
and continents, which have elevated mountain peaks and ridges, especially
if of volcanic origin and activity, which approach more nearly the path of
the counter-trade, a different state of things exists. There, showers and
gusts are frequent. Thus, upon the Sandwich Island, Kauai, the most
northern one, which is within the region of the N. E. trade during ten
months of the year, and upon its volcanic peaks and elevated table-lands,
and north-easterly from them, over the district of Waioli, rain falls in
abundance during the year, while the coastlines upon other portions of the
island can not be cultivated without irrigation. (See Wilkes' Exploring
Expedition, vol. iv. pp. 61 and 71; and American Journal of Science and
Art, for May, 1847).
A like state of things, in degree, may be found upon the Canaries, and the
more elevated of the West India Islands. The Cape de Verdes are an
exception, and the Christian world are quite often called upon for
contributions of provisions, to save the inhabitants of these islands from
starvation. They lie at the northern limit of the equatorial belt, and for
a period of two months only (July and August), are supplied with rain. If,
from any cause, the belt does not move as far north as usual during any
season, unbroken drought and famine are sure to overtake them. The islands
contain some elevated peaks, and are of volcanic origin, but not of
present volcanic activity, and the counter-trades as they issue from the
equatorial belt at their highest elevation, are too far above them for
reciprocal, influential action. If the islands could be placed 10 deg. further
north, we should hear no more of drought or famine from them, and their
quantity of rain and fertility would be not only more permanent, but much
increased. Superadded to this, is the fact, that at that point the belt of
rains precipitates feebly because the S. E. trade originates upon the
southern part of the continent of Africa, and the N. E. mainly, upon the
desert and the Barbary States--and both are sparingly supplied with
moisture.
The same state of things is strikingly obvious upon continents wherever
the mountains are sufficiently elevated, even within the trade-wind
region. Thus, in South America, the Andean ranges are of great elevation,
and spurs and table-lands extend from them a considerable distance to the
eastward. There, the S. E. and N. E. trades of the Atlantic meet in very
considerable volumes, and not only is the equatorial belt much wider than
upon the Atlantic and Pacific, but the counter-trades are met upon the
elevated peaks and mountain-ranges, and showers and storms on their
eastern slopes and summits are frequent during the dry season--down even
to the extra-tropical belt. I have already said that it was probable that
the great elevation of the Andes diverted and turned south a portion of
the N. E. counter-trade which would otherwise pass over the western coast
of Peru.
The report of Lieutenant Herndon, which has come to my notice since that
was written, states facts which strongly corroborate that opinion. It
seems that the trades and counter-trades actually _bank up_, in their
passage to the westward, against those mountains, and the true elevation
of their eastern slopes can not be barometrically ascertained. (See report
of the Exploration of the Amazon, p. 261). Lieutenant Herndon says:
"I was surprised to find the temperature of boiling water at Egas to
be but 208 deg. 2', the same within 2' of a degree that it was at a point
one day's journey below Tingo Maria, which village is several hundred
miles above the last rapids of the Huallaga river; at Santa Cruz, two
days above the mouth of the Huallaga, it was 211 deg. 2'; at Nauta, three
hundred and five miles below this, it was 211 deg. 3'; at Pebas, one
hundred and seventy miles below Nauta, 211 deg. 1'. I was so much
surprised at these results that I had put the apparatus away,
thinking that its indications were valueless; but I was still more
surprised, upon making the experiment at Egas, to find that the
temperature of the boiling water had fallen 3 deg. below what it was at
Santa Cruz, thus giving to Egas an altitude of fifteen hundred feet
above that village, which is situated more than a thousand miles up
stream of it. I continued my observations from Egas downward, and
found a regular increase in the temperature of the boiling water
until our arrival at Para, where it was 211 deg. 5'.
"From an after-investigation, I am led to believe that the cause of
this phenomenon arises from the fact that the trade-winds are dammed
up by the Andes, and that the atmosphere in those parts is, from this
cause, compressed, and, consequently, heavier than it is further from
the mountains, though over a less elevated portion of the earth. The
discovery of this fact has led me to place little reliance in the
indications of the barometer for elevation, at the eastern foot of
the Andes. It is reasonable, however, to suppose that this cause
would no longer operate at Egas, nearly one thousand miles below the
mouth of the Huallaga."
The report of Lieutenant Gibbon, is also exceedingly instructive.
Separating from Lieutenant Herndon at Tarma, upon the Andes, he pursued a
southern course, along the eastern slopes of the chain from 11 deg. 30'
south, almost to 18 deg. south, at Ohuro, making a journey of about 7 deg. 30' of
latitude.
A considerable portion of this journey was over eastern and less elevated
portions of the Andes; but little below, however, the line of perpetual
snow. Here, during the dry season, he met with frequent showers and fogs
from the eastward, but left them as he descended into the plains upon the
table-land. There he found the dry season more distinctly marked; but
occasional irregularities were found upon the table-lands, as every where
upon corresponding elevations. The S. E. trades, however, were there
obvious, during the dry season, notwithstanding the irregularities. The
rainy season, from December to May, he spent at Cochabamba, and at its
close he traveled north down the Madeira and its tributaries, to the
Amazon. Although scarcely consistent with my prescribed limits, I can not
forbear making a few extracts. Thus, when on the mountains, east of
Huanvelica, in the N. E. counter-trade, he says:
"Our course is to the eastward. The snow-capped mountains are in
sight to the west. Temperature of a spring 48 deg.; air 44 deg.. Lightning
flashes all around us; as the wind whirls from _north-east_ to
south-west, rain and snow-flakes become hail, half the size of peas.
Thunder roars and echoes through the mountains; the mules hang their
heads, and travel slowly; the thinly-clad aboriginal walks shivering
as he drives the train ahead; the dark cumulus cloud seems to wrap
itself around us."
Again, at the Bombam Post-house, in the focus of change from cirrus to
cumulus, and stratus, and storm:
"The winds are very gentle, and curl the cirrus or hairy clouds in
most graceful shapes about the hoary-headed Andes, in rich and
delicate clusters; when the peak is concealed, all but the blue tinge
below the snow, we see a natural bridal vail. An _easterly wind_
lifts and turns them to dark, cumulus clouds, settled on the frosty
crown, like an old man's winter cap; the physiognomical expression is
that of anger. The change is accompanied by thunder, and seems to
command all around to clothe themselves for storms. The cold rain
comes down in _fine drops_ upon us; the day grows darker, and the
_clouds press close upon the earth_."
During an excursion east of Cuzco--
"Turning from the river, we ascend a steep ridge of mountains--the
eastern range at last. A heavy mist _wafts upward as the winds drive
it against the side of the Andes_, so that our view is shortened to a
few hundred yards. We hope the curtain will rise that we may view the
productions of the tropical valley below; but the mist thickens, and
the day gets dark with heavy, heaped-up black clouds; a rain-storm
follows. The grasses are thrifty, and the top of the ridge covered
with a thick sod. By barometer, we stand eleven thousand one hundred
feet above the level of the sea."
In May following, having spent the rainy season in Cochabamba, he travels
north--
"Our route from Tarma to Oruro was south. We traveled ahead of the
sun. In December, when we arrived in Cochabamba, the sun had just
passed us. As soon as he did so, the rains descended heavily on this
side of the ridge; it was impossible to proceed. The roads were
flooded, the ravines impassable, and the arrieros put off their
journey until the dry season had commenced. After the sun passed the
zenith of Cochabamba, and had fairly moved the rain belt after him
toward the north, then we came out from under shelter, and are now
walking behind the rain belt in dry weather, while the inhabitants
are actively employed in tending their crops."
So on the north of the equatorial belt, along the whole line of the Andes,
up to the northern boundary of the desert valley of the Gila, rain falls
on the high mountain-ranges, owing to the contiguity of the counter-trade
and the diversion of showers to the north, along their eastern sides.
During the survey of the boundary line between Mexico and California,
etc., by the commission under Mr. Bartlett, it became necessary to find
some spot where water and grass were abundant, for the head quarters of
the commission. This was found, and _could only be found_, upon the
Mimbres Mountains, at an old abandoned Spanish copper mine, 7,000 or 8,000
feet above the level of the sea, surrounded with peaks of still greater
height. These elevated ranges were within influential distance of the
counter-trade, and here snow fell in the winter, from the extra-tropical
belt, and rain, in showers, in summer, at the period of the most northerly
extension of the tropical belt; when fifteen miles off, in the valley, it
was unbroken drought. Mr. Bartlett thus describes it in his Personal
Narrative:
"We reached this district on the 2d of May. Vegetation was then
forward, though there had been no rain. But it must be remembered
that during the winter there is snow, and hence a good deal of
moisture in the earth when the spring opens. The months of May and
June were moderately warm. On the third of July the first rain fell.
It then came in torrents, accompanied by hail, and lasted three or
four hours. Many of our adobe houses were deluged with water, and the
mountain-sides exhibited cataracts in every direction. The Arroyo,
which passes through the village, and which furnishes barely water
enough for our party and the animals, became so much swollen as to
render it difficult to cross; and, by the time it had received the
numerous mountain torrents, which fall into it within a mile from our
camp, it became impassable for wagons, or even mules. The dry gullies
became rapid streams, five or six feet deep, and sometimes fifty feet
or more across. On this day, a party, in coming to the copper mines,
from the plain below, _where there had been no rain_, found
themselves suddenly in a region overflowing with water, so that
their progress was arrested, and they were obliged to wait until the
flood had subsided. After this we had occasional showers, during the
months of July and August."
The location of this mountain station is near the thirty-third degree of
north latitude, while the northern limit of the equatorial belt, nowhere,
except upon the mountain ranges and table-lands of Mexico, extends above
25 deg..
There, for the reason we have been considering, it does extend further
north during July and August, in occasional showers, and in the vicinity
of Mount Picacho, Mr. Bartlett met one of its mountain thunder-storms on
the 13th of July, on his return south through Mexico, in latitude 32 deg., in
the following year. (Personal Narrative, vol. ii. p. 285). These showers
originated in strata of counter-trade, which had followed up along the
eastern side of the mountains and not from strata which had crossed them
and curved to the eastward, as is shown by the course of progression of
the showers.
Let us look, in this connection, at a fact or two of great interest,
though not directly connected with the point in hand. The southern limit
of the extra-tropical belt in winter, on the Pacific coast of North
America, is in the vicinity of San Diego, at about 32 deg.. In summer, that
limit is carried up above Astoria, which is in latitude 46 deg. 11'--about
14 deg.--yet New Mexico receives little if any rain in winter in the vicinity
of Albuquerque, but does receive a limited supply of about seven inches in
summer and autumn, five and a half inches of which falls in June, July,
and August. Albuquerque is in latitude 35 deg. 13', below the southern summer
limit of the extra-tropical belt, and north of the northern limit of the
equatorial belt. This anomaly is explained by the extension west over
northern New Mexico, of the extreme western edge of our concentrated
counter-trade, by reason of its issuing further west from the equatorial
belt in its northern extension in the summer months. This western edge, in
curving to the east, north-east of New Mexico, covers the north-western
States, Iowa, Minnesota, Wisconsin, etc., and furnishes them that great
excess of summer precipitation which is a peculiarity of their climate;
and its absence further east in winter, and the very great elevation of
the Rocky Mountains and other ranges over which their ordinary
counter-trade of that season curves, account for the absence of much
precipitation and snow there, or over the valley of the Rio Grande in New
Mexico, in winter.
We may now see, too, why the western coast and the Pacific region of the
continent, below 45 deg., are so deficient in moisture. The S. E. trades,
which arise from the western portion of the south Atlantic and the
continent of South America, which, if it were not for the Andes chain, in
their natural course, after passing the equatorial belt, would continue on
to the north-west until they passed the limits of the N. E. trades, and
curve in upon the western portion of our continent below 45 deg., and supply
it bountifully with rain, are, in part, perhaps, diverted along the
eastern side of those mountains to swell the volume of our counter-trade,
and in part pass them, almost exhausted of their supply of moisture by
their contiguous reciprocal action. Hence, too, the deficiency of
precipitation at the base of the Andes, on the western side, and the
peculiar and irregular character of the winds under the western lee of the
Andean range. Baffling airs and bands of calms prevail on this portion of
the Pacific, except where the mountains fall off, and then there is a
westerly or south-westerly monsoon under the equatorial belt. Says
Lieutenant Maury in his Charts, sixth edition, p. 731:
"The passage, under canvass, from Panama to California, as at present
made, is the most tedious, uncertain, and vexatious that is known to
navigators.
"My investigations have been carried far enough to show that at
certain seasons of the year a vessel bound from Panama to California,
must cross at least three, at some seasons four, such meetings of
winds or bands of calms, before she can enter the region of the N. E.
trades. Hence the tedious passage."
Such will ever be the state of things on this continent and upon the
eastern Pacific, so long as the S. E. counter-trades are compelled to pass
over the mountain chain of South and Central America.
Again, if we examine carefully the belt or zone of extra-tropical rains,
we shall find that the focus of greatest precipitation is considerably
north of its southern limit, and that, other things being equal, this
focus travels north in summer, and gives to higher latitudes their needed
summer rains. This is very apparent upon the north-western portion of our
continent, as the following table will show:
+-----------------------------------------------------------
| | Lat. |Jan.|Feb.|Mar.|Apr.|May.|June.|
| |-------|----|----|----|----|----|-----|
|San Diego, Cal. |32 deg. 41'| 0.3| 1.7| 1.1| 0.9| 0.5| 0.0 |
|San Francisco. |37 deg. 48'| 1.7| 0.5| 4.4| 2.1| 0.4| 0.0 |
|Cant., Far W., Cal.|39 deg. 02'| 3.3| 0.6| 6.4| 2.2| 0.9| 0.0 |
|Astoria, Oregon. |46 deg. 11'|27.0|10.9| 6.1| 4.4| 5.9| 2.6 |
|Puget's S'd, Ore. |47 deg. 07'|11.8| 3.9| 4.7| 4.1| 0.8| 0.6 |
|Sitka, Russ. Am. |57 deg. 3'| 2.5| 9.6| 3.5| 3.3| 1.9| 5.9 |
+-----------------------------------------------------------
---------------------------------------+
|July.|Aug.|Sept.|Oct.| Nov.|Dec.|Year.|
|-----|----|-----|----|-----|----|-----|
| 0.0 | 0.2| 0.0 | 0.1| 1.5 | 3.4| 9.6|
| 0.0 | 0.0| 0.4 | 0.6| 3.0 | 5.5| 18.8|
| 0.0 | 0.0| 0.3 | 0.1| 3.5 | 4.6| 21.9|
| 0.0 | 2.3| 1.9 | 6.7|13.2 | 6.2| 87.2|
| 0.5 | 1.3| 1.6 | 3.6| 5.9 | 6.1| 44.8|
| 3.7 |10.1|14.8 |12.7| 7.4 | 4.2| 79.5|
---------------------------------------+
The figures are for inches and tenths of an inch of rain.
Thus, it will be seen that in January, when the southern line is at San
Diego, at the south line of California, the focus of precipitation is over
Oregon; and that in August and September when the southern line is carried
up and over Oregon, the focus has traveled north to Sitka, and that it is
always at least 10 deg. north of the southern line of the belt upon that
coast. The increased quantities of rain which fall at the focus of
precipitation there, from Oregon up, are doubtless much enhanced by the
equatorial oceanic current which flows over opposite that part of the
continent. A like effect, precisely, is produced in Europe. The quantity
of rain which falls at Bergen, in Norway, being 87-61/100 inches per year,
more than three times the average for that continent.
The difference shown in the foregoing table, between Astoria and Puget's
Sound, is owing to the fact that the latter lies in the interior and
within the coast range of mountains, while Astoria is situated at the
mouth of the Columbia River, with an open view of the ocean.
A like comparative increase of precipitation in northern latitudes, in
summer, is found every where varying according to the local influences
which operate in the particular case. Thus,
------------------------------------------------------------------------
There falls in |Winter.|Spring.|Summer.|Aut'mn.| Year.
---------------------------------|-------|-------|-------|-------|------
Burlington, Vt., lat. 44 deg. 20' | 5.7 | 7.3 | 11.4 | 9.8 | 33.9
Albany, N. Y., lat. 42 deg. 39' | 8.3 | 9.8 | 12.3 | 10.3 | 40.7
Minnesota, Iowa, lat. 41 deg. 28' | 7.3 | 12.3 | 17.4 | 11.7 | 48.8
St. Peters'g, Russ., lat. 59 deg. 56'| 3.89 | 3.20 | 5.70 | 4.71 | 17.51
Pekin, China, lat. 40 deg. | .54 | 3.35 | 18.80 | 2.29 | 25.68
------------------------------------------------------------------------
Pekin lies in the northern part of China, and would have a much larger
fall of rain from a concentrated counter-trade, but for the numerous
mountain-ranges which intersect its path in winter, but over which it
passes at a greater elevation during the summer--a peculiarity from which
the eastern section of this country is most remarkably and happily free.
Thus, it is obvious that the focus of precipitation in the zone of extra
tropical rains, is some 8 deg. to 12 deg. north of its southern line, and travels
with the whole machinery in its annual transit north and south.
It is a question of some difficulty, perhaps, whether this focus is
increased by the increase of magnetic action at this point, for both the
line of descent of the counter-trade, and the focus of magnetic action,
are carried up in a like manner, and for a like cause, and, in all
probability, both concur in the result.
There is exceeding wisdom in this provision for the gradual subsidence of
the counter-trade, and gradual increase of magnetic intensity, and
consequent gradual precipitation. On the European continent, and over
western Asia, there are 50 deg. of latitude to be supplied with moisture by
this polar belt of rains. If the focus of precipitation was at its
southern border, the counter-trade would be deprived of its moisture at
that point, and little would reach the more northern portions of the globe
which are to be supplied by it. But the movement of the whole machinery
carries up the southern line from the south boundaries of the Barbary
States on to the Mediterranean and portions of southern Europe, and the
focus of precipitation and of near approach of the counter-trade to the
earth, being situated far north of the southern line, is carried up
correspondingly, while the combination of the moisture with the atmosphere
by south polar magnetism and electricity, and the gradual descent of the
counter-trade, enable it to resist, to some extent, the influence of north
polar magnetism and cold, and thus retain portions of its moisture for
distribution in the polar regions.
_The elevation of the counter-trade above the earth varies in the same
latitude with the variations in the phenomena of the weather._ An
attentive observation of the clouds of our climate will soon satisfy any
one of this, after he has become familiar with them, so as to distinguish
with certainty the clouds of the trade. Its range, in this country, is
from 3,000 feet, or less, to 12,000 feet above the earth, and its depth
with us probably, from 6,000 to 8,000 feet. Gay-Lussac, in his scientific
experimental balloon ascension, the first of _that character_ ever made,
except an imperfect one just previous, by himself and Biot, found it at
about 12,000 feet over Paris, and about 4,000 feet in depth. It is
detected by the thermometer when much elevated.
The atmosphere grows cool as it is ascended on mountains, or by balloons.
The rate of cooling is ordinarily about 1 deg. of Fahrenheit for every 300
feet. If it were not for the equatorial current, this progressive decrease
of temperature would doubtless be perfectly uniform. Of Gay-Lussac's
ascension, on this point it was said:
"At forty minutes after 9 o'clock, on the morning of the 15th
September, 1804, the scientific voyager ascended, as before, from the
garden of the repository of models. The barometer then stood at 30.66
English inches, the thermometer at 82 deg. Fahrenheit, and the hygrometer
at 57-1/2 deg.. The sky was unclouded, but misty.
"During the whole of this gradual ascent, he noticed, at short
intervals, the state of the barometer, the thermometer, and the
hygrometer. Of these observations, amounting in all to twenty-one, he
has given a tabular view. We regret, however, that he has neglected
to mark the times at which they were made, since the results appear
to have been very materially modified by the progress of the day. It
would likewise have been desirable to have compared them with a
register, noted every half hour, at the Observatory. From the surface
of the earth to the height of 12,125 feet, the temperature of the
atmosphere decreased regularly, from 82 deg. to 47 deg. 3' by Fahrenheit's
scale; _but afterward it increased again, and reached to 53 deg. 6' at
the altitude of 14,000 feet_; evidently owing to the influence of the
warm currents of air which, as the day advanced, rose continually
from the heated ground. From that point the temperature diminished,
with only slight deviations from a perfect regularity. At the height
of 18,636 feet the thermometer subsided to 32 deg. 9', on the verge of
congelation; but it sunk to 14 deg. 9' at the enormous altitude of 22,912
feet above Paris, or 23,040 feet above the level of the sea, the
utmost limit of the balloon's ascent."
The high range of the barometer indicated a very considerable elevation
of the trade at the time Gay-Lussac made his ascension. I am not aware
that it has since been found at so great an elevation, in so high a
latitude, though it is undoubtedly elevated by the interposition of a
large volume of N. W. air, upon some occasions, to nearly the same
altitude with us.
In the extract in relation to the ascension of Gay-Lussac, we have another
of the thousand hastily-adopted and absurd hypotheses connected with the
caloric theory. It is obviously and utterly _impossible_ that in addition
to the ordinary accumulation of heat at the surface of the earth "_as the
day advanced_"--that is, _during the forenoon_, warm currents should
ascend, unobserved by Gay-Lussac during an ascent of 12,000 feet--not
_affecting in the least_ so large an intervening body of the atmosphere or
his thermometer, and in such immense volumes as to increase the warmth of
a stratum of 4,000 feet in depth, an average of 3 deg. of Fahrenheit, and to
the extent of 6 deg. at the center.
Very few balloon ascensions have been made with a view to scientific and
accurate observation. But other aeronauts have met the counter-trade at
different altitudes, and in both clear and stormy weather.
Recently, in 1852, four ascensions were made in England, under the
direction of the Kew Observatory Committee, of the British Association. I
copy from the August number of the "London, Edinburg, and Dublin
Magazine," for 1853, the following condensed amount of the result:
"The ascents took place on August 17th, August 26th, October 21st,
and November 10th, 1852, from the Vauxhall Gardens, with Mr. C.
Green's large balloon.
"The principal results of the observations may be briefly stated as
follows:
"Each of the four series of observations shows that the progress of
the temperature is not regular at all heights, but that at a certain
height (_varying on different days_) the regular diminution becomes
arrested, and for the space of about 2,000 feet the temperature
remains constant, or even increases by a small amount. It afterward
resumes its downward course, continuing, for the most part, to
diminish regularly throughout the remainder of the height observed.
There is thus, in the curves representing the progression of
temperature with height, an appearance of _dislocation_, always in
the same direction, but varying in amount from 7 deg. to 12 deg..
"In the first two series, viz.: August 17th and 26th, this peculiar
interruption of the progress of temperature is strikingly coincident
with a _large_ and _rapid fall_ in the temperature of the
_dew-point_. The same is exhibited in a less marked manner on
November 10th. On October 21st a dense cloud existed at a height of
about 3,000 feet; the temperature decreased uniformly from the earth
up to the _lower_ surface of the cloud. When a slight rise commenced,
the rise continuing through the cloud, and to about 600 feet above
its upper surface, when the regular descending progression was
resumed. At a short distance above the cloud, the dew-point fell
considerably, but the rate of diminution of temperature does not
appear to have been affected in this instance in the same manner as
in the other series; the phenomenon so strikingly shown in the other
three cases being perhaps modified by the existence of moisture in a
_condensed_ or vesicular form.
"It would appear, on the whole, that about the principal plane of
condensation heat is developed in the atmosphere, which has the
effect of raising the temperature of the higher air above what it
would have been had the rate of decrease continued uniformly from the
earth upward."
These gentlemen do not adopt the absurd explanation of the French
philosophers; they account for the phenomenon by supposing heat to be
_developed_ at that particular part of the atmosphere; but they are
equally wide of the mark. They found the excess of heat there to the
extent of 7 deg. to 12 deg., and on days when there was no condensation, or other
assignable cause for its _development_.
The temperature of the counter-trade partakes, doubtless, of the
temperature of the adjoining strata at its upper and lower portion, and
has never been found much, if any, higher than 60 deg. at the center. Nor
could it be expected. The trade, in its upward curving course, within the
tropics, attains a considerable altitude where the atmosphere is
comparatively cold, and necessarily loses a portion of its heat there, and
during its northern flow. Probably its central summer range, in the
latitude of Paris, is not far from 55 deg., and with us 60 deg..
The contrast between the trade and the surrounding atmosphere, in winter,
is much more striking, and this has been observed particularly upon the
Brocken of the Alps, and in the polar regions.
"In all seasons the temperature is higher on the Brocken, on a serene,
than on a cloudy day, and, in the month of January, _the serene days were
warmer than at Berlin_." (Kaemtz's Meteorology, by Walker, p. 217.--Note.)
As the portion of the counter-trade, which does not become depolarized--in
diminished volume--progresses toward the polar regions, it settles nearer
the earth, and within the Arctic circle is found but little way above it.
Thus, in December, 1821, Parry, at Winter Island, in latitude 66 deg. 11',
flew a kite, with a thermometer attached, to the height of 379 feet, and
found that the temperature, instead of falling 1-1/4 deg., the usual ratio of
decrease, rose 3/4 of a degree.
The same thing was observed at Spitzbergen, in latitude 77 deg. 30' north, and
at Bosekop, latitude 69 deg. 58', by a scientific commission, and by means of
kites, confined balloons, and the ascent of elevations.
"In winter the temperature goes on increasing with the height, up to
a certain limit, which is variable, according to the different
atmospheric circumstances, the influence of which is not yet very
exactly known. The hour of the day appears to be indifferent, since
there exists no thermometric diurnal variation in the strata of the
surface. The mean of thirty-six experiments, made with kites, or with
captive balloons, at Bosekop, latitude 69 deg. 58' north, has given a
mean rate of increase of 1 deg. 6' for the first hundred meters.[6]
Beyond this limit, and even beyond the first 60 or 80 meters, the
temperature again becomes decreasing, at first very slowly, but
afterward the decrease is accelerated. The observations that have
been made on the flanks, or on the summits, of mountains, during the
same expeditions, entirely confirm these results. The cooling
influence of a soil, that radiates its own heat for several weeks,
without receiving any thing on the part of the sun, in compensation
of its losses, the influence of _counter-currents from above_, coming
from the west and the south-west, with a high temperature, account
for this anomaly, which, in winter, represents the normal state of
the most northern parts of the European continent." (Walker's Kaemtz,
p. 515.--Note.)
Mr. Walker is the only author, so far as I know, who has suspected the
true cause of the phenomenon, viz.: "currents from above coming from the
west and south-west, with a high temperature;" but the caloric theory
"sticks like a burr," and he adheres also to the idea that a snow-clad
surface, in the absence of the sun, can aid, by radiation, in warming the
atmosphere for a distance of several hundred yards above it, increasing
the warmth as the distance from the earth increases!
This contrast between the counter-trade and the adjacent atmosphere, in
winter, in latitudes as low as that of the Brocken, is probably heightened
by the increased warmth of the former, at that season. The S. E. trades
then form under a vertical sun, and the difference of temperature can not
be less than from 6 deg. to 8 deg.. Not unfrequently in winter and spring the rain
will fall with a temperature of 50 deg. to 55 deg., when the atmosphere near the
earth is 10 deg. or 20 deg. or more, below those points; and it is frozen to every
object upon which it falls. The trade stratum, from which it descends, is
not warmed by "radiation" or by ascending currents from a snow-clad
surface, and during a cloudy day; nor by a "development of heat" at that
particular altitude, but it has brought its heat from the South Atlantic,
and imparts it to the rain which forms within it. There is every reason to
believe that the counter-trade flows north in a regular descending plane,
not materially differing from that of the line of perpetual snow. The
descent of the latter is well ascertained to be from about 16,000 feet at
the equator, to _the surface_ at the poles. The plane of the counter-trade
is probably much the same, varying over different localities, from the
varied action between it and the earth which we are considering; and
probably both correspond with the increase of magnetic intensity.
Lieutenant Maury, in an able and original article upon the circulation of
the atmosphere, conceives the bands of comparative calms at the northern
limits of the trades, which he appropriately terms the "_Calms of
Cancer_," to be nodes in the circulation of the atmosphere, and that the
upper or counter-trade here decends and becomes a surface wind from the S.
W., as the N. E. trade is a surface wind; and that an upper current from
the poles approaches and descends at the same node, to make the N. E.
trade. But it is evident he adopted that conclusion too hastily, as he
obviously did the conclusion that the calms of the horse latitudes were a
type of all. We have seen that the latter are increased by a diversion of
the counter-trade, and that they are avoided by making easting. So it may
be observed that our upper current is a S. W. current, and no northerly
upper current is visible, or exists over the country, however it may be in
western Europe and the North Pacific, on the west of the magnetic poles,
where cold, dry northerly and north-easterly winds are found. The origin
and progress of storms withal demonstrates that no such node can exist.
Two points have been made in relation to the course of the counter-trade
in the tropics, and are relied upon to show its progress there to the N.
E., which deserve consideration.
In the first place, it is well known that "rain dust" falls in
considerable quantities on the western coast of Africa, particularly about
the Cape de Verde Islands, and also upon the Mediterranean and
south-western Europe, where it is termed "sirocco dust."
"This dust," says Lieutenant Maury, "when subjected to microscopic
examination, is found to consist of infusoria and organisms, whose
_habitat_ (place of abode) is not Africa, but South America, and in
the S. E. trade-wind region of South America. Professor Ehrenberg has
examined specimens of sea dust, from the Cape de Verdes and the
regions thereabout, from Malta, Genoa, Lyons, and the Tyrol, and he
has found such a similarity among them as would not have been more
striking had these specimens been all taken from the same pile.
"South American forms he recognizes in all of them; indeed, they are
the prevailing form in every specimen he has examined.
"It may, I think, be now regarded as an established fact, that there
is a perpetual upper current of air from South America to north
Africa, and that the volume of air in these upper currents, which
flows to the northward, is nearly equal to the volume which flows to
the southward with the N. E. trade-winds, there can be no doubt,"
etc.
Now, it is doubtless true that this dust is transported in a
counter-trade, and that such dust is found in South America, and is taken
up there by sand-spouts, like those of the ocean in form and action. Both
Humboldt and Gibbon have graphically described them. Yet I do not think
the point well taken. South-eastward of the Cape de Verdes, where the
surface-trades--which, becoming counter-trades, pass over these islands,
and, recurving, pass over the Mediterranean and south-western
Europe--should originate, there is a vast extent of unexplored continent
in the same latitude as the portion of South America where the dust is
found; and the same dry seasons, and the same spouts, in all probability,
exist in both. Until it be shown that such forms have no "_habitat_" in
central and southern and unexplored Africa, upon the same latitudes as in
South America, it may fairly be presumed that the dust is taken up there.
Indeed, the _curve_ upon which this dust is found to fall, in the greatest
quantities, is very remarkable, and corresponds remarkably with the _law
of curvature_ of the counter-trade we have considered, and with the
progress of a storm upon that coast, and over the Mediterranean,
investigated by Colonel Reid. (See Reid, on Storms and Variable Winds, p.
276.) This _curve clearly indicates the origin of the dust in South
Africa_.
The second point is, that ashes from the volcanos of Mexico and Central
America have fallen to the north-east of the place where they were
ejected. Mr. Redfield has grouped these instances of volcanic eruption
usually cited, and I copy from him:
"We learn from Humboldt, that in the great eruption of Jorullo, a
volcano of southern Mexico, which is 2,100 feet above the sea, in
latitude 18 deg. 45', longitude 161 deg. 30', the roofs of the houses in
Queretaro, more than 150 miles north, 37 deg. east from the volcano, were
covered with the volcanic dust. In January, 1845, an eruption took
place in the volcano of Cosiguina, on the Pacific coast of Central
America, in latitude 13 deg. north, and having an elevation of 3,800
feet, the ashes from which fell on the island of Jamaica, distant 730
miles north, 60 deg. east from the volcano. The elevated currents by
which volcanic ashes are thus transported are seldom or never of a
transient or fortuitous character; and these results, therefore,
afford us one of the best indications of their general course. Thus,
the progress of the higher portion of the trade-wind was marked by
the eruption of Tuxtla, latitude 18 deg. 30', longitude 95 deg., which
covered the houses in Vera Cruz with ashes, at the distance of 80
miles north, 55 deg. west, and also at Perote, 160 miles north, 60 deg. west.
The ashes from the volcano, at St. Vincent, which fell at Barbadoes,
and east of that island, in 1812, mark the course of a current from
the westward, which appears there at times, in the region of clouds,
and may, perhaps, be connected with the permanent winds on the
Pacific coast of Mexico."
As to one of the instances cited in the foregoing paragraph, that of
Tuxtla, it may be laid out of the case--the direction conforming
substantially to the assumed course of the counter-trade at that point.
St. Vincent lies W. N. W., or nearly so, of Barbadoes, and a N. W. or
westerly surface-wind, prior to, and during storms, is common in the West
Indies as the N. E. is here--both alike, blowing in opposition to the
progressive course of the storm. There is nothing strange or peculiar,
therefore, respecting that instance, or the existence of variable and
especially S. W. currents, between the trades, with occasional partial
condensation.
The falling of the ashes from Cosiguina, upon Jamaica, has long and often
been cited, as proof that in the West Indies the prevailing upper currents
run from the S. W. But it has been ascertained that, _during the same
eruption, ashes fell 700 miles to the westward, on the deck of the
Conway_, a vessel then upon the Pacific Ocean. That case, therefore, does
not prove the absence of the S. E. counter-trade at the time, but only the
presence of another, and a different current above or below it--and it may
have been either, and transient.
So of the Jorullo instance. Investigation would probably have shown that
ashes fell to the N. W., and that they were carried N. E. by a transient
S. W. wind produced by the existence of a storm to the eastward, or one of
those states of partial condensation of the counter-trade which often
produce currents at greater distances without a storm. Not one of these
cases disproves the existence of a S. E. counter-trade, and the invariable
N. W. progression of the storms of those latitudes demonstrates it.
Occasional anomalous currents, depending upon storm action at considerable
distance, are found in our atmosphere, and doubtless are there also. Thus,
although the N. W. wind is almost invariably a surface wind, I have, in a
few instances, seen a N. W. set at a considerable elevation, converging
toward a peculiarly stormy state of atmosphere far south of us, about the
period of the spring equinox. And so in one or two instances I think I
have seen light cirro-stratus clouds _above_ the counter-trade, when it
ran very low, setting from the N. E., although the usual and almost
invariable location of the N. E. wind is below the counter-trade and the
stratus clouds of the storm. Aeronauts, too, have found these secondary
currents beneath a serene and cloudless sky. Indeed, the S. E.
counter-trade doubtless often induces a thin secondary current of S. W.
wind between itself and the surface-trade, in the same manner that similar
currents are induced with us, and every where.
A question arises here of considerable interest, which, I confess, I can
not answer to my own satisfaction. It is, whether there be, or not, _an
eastern progression of the body of the atmosphere above the machinery of
distribution_. I have thought there was, and that in set fair weather I
had seen a peculiar kind of cirro-cumulus cloud, in patches, the small
cumuli very distinct and rounded, moving due east, which indicated such a
current. But I am not satisfied, from my own observation, that it is so,
nor is it easy to determine the question. The moisture of evaporation
rarely, if ever, ascends to any considerable elevation, and the upper
strata must be very dry. Hence, condensation, if it takes place, is thin,
and perhaps often undiscernable. Investigations upon mountains prove
little, for the winds of the inferior strata rush up their sides and over
them. It is an open question, and future observation may solve it. The
prevailing opinion seems to be that there is. If the theory of Oersted, in
relation to the circular currents of a magnet, be true, there should be
such a progression produced by opposite secondary currents, unless,
indeed, it be also true that those currents are inoperative at so great a
distance, or their influence barely suffices to retain the attenuated
atmosphere in its place. Perhaps the investigations of Ampere conflict
with it. But it is worth while, I think, for philosophers to inquire
whether the transverse position of the needle upon the wire is not the
effect of the central _longitudinal_ currents, conforming to the circular
currents of the wire, and whether it is not owing to the production of the
same currents in a globe by the circular currents of Ampere, that the
globe is magnetized, and the needles made to dip.