CHAPTER VII.
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° 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° 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 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 Pará, 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 latitude.