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 pa.s.s 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 pa.s.sage 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 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 2"; at Nauta, three hundred and five miles below this, it was 211 3"; at Pebas, one hundred and seventy miles below Nauta, 211 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 below what it was at Santa Cruz, thus giving to Egas an alt.i.tude 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 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 30"
south, almost to 18 south, at Ohuro, making a journey of about 7 30" of lat.i.tude.
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; air 44. 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 c.u.mulus cloud seems to wrap itself around us."
Again, at the Bombam Post-house, in the focus of change from cirrus to c.u.mulus, and stratus, and storm:
"The winds are very gentle, and curl the cirrus or hairy clouds in most graceful shapes about the h.o.a.ry-headed Andes, in rich and delicate cl.u.s.ters; 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, c.u.mulus 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 gra.s.ses 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 pa.s.sed 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 impa.s.sable, and the arrieros put off their journey until the dry season had commenced. After the sun pa.s.sed 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 gra.s.s 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 pa.s.ses 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 impa.s.sable 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 lat.i.tude, while the northern limit of the equatorial belt, nowhere, except upon the mountain ranges and table-lands of Mexico, extends above 25.
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 lat.i.tude 32, 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. In summer, that limit is carried up above Astoria, which is in lat.i.tude 46 11"--about 14--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 lat.i.tude 35 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, 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 pa.s.sing the equatorial belt, would continue on to the north-west until they pa.s.sed the limits of the N. E. trades, and curve in upon the western portion of our continent below 45, 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 pa.s.s 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 pa.s.sage, under canva.s.s, 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 pa.s.sage."
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 pa.s.s 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 lat.i.tudes 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 41"0.31.71.10.90.50.0San Francisco.37 48"1.70.54.42.10.40.0Cant., Far W., Cal.39 02"3.30.66.42.20.90.0Astoria, Oregon.46 11"27.010.96.14.45.92.6Puget"s S"d, Ore.47 07"11.83.94.74.10.80.6Sitka, Russ. Am.57 3"2.59.63.53.31.95.9+-----------------------------------------------------------
---------------------------------------+July.Aug.Sept.Oct.Nov.Dec.Year.--------------------------------0.00.20.00.11.53.49.60.00.00.40.63.05.518.80.00.00.30.13.54.621.90.02.31.96.713.26.287.20.51.31.63.65.96.144.83.710.114.812.77.44.279.5---------------------------------------+
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 north of the southern line of the belt upon that coast. The increased quant.i.ties 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 quant.i.ty 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 lat.i.tudes, in summer, is found every where varying according to the local influences which operate in the particular case. Thus,
------------------------------------------------------------------------ There falls inWinter.Spring.Summer.Aut"mn.Year.
------------------------------------------------------------------- Burlington, Vt., lat. 44 20"5.77.311.49.833.9 Albany, N. Y., lat. 42 39"8.39.812.310.340.7 Minnesota, Iowa, lat. 41 28"7.312.317.411.748.8 St. Peters"g, Russ., lat. 59 56"3.893.205.704.7117.51 Pekin, China, lat. 40.543.3518.802.2925.68 ------------------------------------------------------------------------
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 pa.s.ses 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 to 12 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 of lat.i.tude 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 lat.i.tude 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 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 Fahrenheit, and the hygrometer at 57-1/2. 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 to 47 3" by Fahrenheit"s scale; _but afterward it increased again, and reached to 53 6" at the alt.i.tude 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 9", on the verge of congelation; but it sunk to 14 9" at the enormous alt.i.tude 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 lat.i.tude, though it is undoubtedly elevated by the interposition of a large volume of N. W. air, upon some occasions, to nearly the same alt.i.tude 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 acc.u.mulation of heat at the surface of the earth "_as the day advanced_"--that is, _during the forenoon_, warm currents should ascend, un.o.bserved 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 of Fahrenheit, and to the extent of 6 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 alt.i.tudes, 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 a.s.sociation. 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 princ.i.p.al 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 s.p.a.ce 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 to 12.