These currents present a remarkable spectacle; like rivers of uniform breadth, they cross the sea in different directions, while the adjacent strata of water, which remain undisturbed, form, as it were, the banks of these moving streams. This diffrence between the moving waters and those at rest is most strikingly manifested where long lines of sea-weed, borne onward by the current, enable us to estimate its velocity. In the lower strata of the atmosphere, we may sometimes, during a storm, observe similar phenomena in the limited aerial current, which is indicated by a narrow line of trees, which are often found to be overthrown in the midst of a dense wood.
The general movement of the sea from east to west between p 307 the tropics (termed the equatorial or rotation currnt) is considered to be owing to the propagation of tides and to the trade winds. Its direction is changed by the resistance it experiences from the prominent eastern sh.o.r.es of continents. The results recently obtained by Daussy regarding the velocity of this current, estimated from observations made on the distances traversed by bottles that had purposely been thrown into the sea, agree within one eighteenth with the velocity of motion (10 French nautical miles, 952 toises each, in 24 hours) which I had found from a comparison with earlier experiments.*
[footnote] *Humboldt, "Relat. Hist.", t. i., p. 67; "Nouvelles Annales des Voyages", 1839, p. 255.
Christopher Columbus, during his third voyage, when he was seeking to enter the tropics in the meridian of Teneriffe, wrote in his journal as follows:*
"I regard it as proved that the waters of the sea move from east to west, as do the heavens ("las aguas van con los cielos"), that is to say, like the apparent motion of the sun, moon, and stars."
[footnote] *Humboldt, "Examen Crit. de l"Hist. de la Geogr.", t. iii., p.
100. Columbus adds shortly after (Navarrete, "Coleccion de los Viages y Descubrimientos de los Espanoles", t. i., p. 260), that the movement is strongest in the Caribbean Sea. In fact, Rennell terms this region, "not a current, but a sea in motion". ("Investigation of Currents", p. 23). 66-74.
The narrow currents, or true oceanic rivers which traverse the sea, bring warm water into higher and cold water into lower lat.i.tudes. To the first cla.s.s belongs the celebrated Gulf Stream,* which was known to Anghiera, and more especially to Sir Humphrey Gilbert in the sixteenth century.
[footnote] *Humboldt, "Examen Critique", t. ii., p. 250; "Relat. Hist.", t.
i., p. 66-74.
[footnote] *Petrus Martyr de Anghiera, "De Rebus Oceanicis et Orbe Novo", Bas., 1523, Dec. iii., lib. vi., p. 57. See Humboldt, "Examen Critique", t.
ii., p. 254-257, and t. iii., p. 108.
Its first impulse and origin is to be sought to the south of the Cape of Good Hope; after a long circuit it pours itself from the Caribbean Sea and the Mexican Gulf through the Straits of the Bahamas, and, following a course from south-southwest to north-northeast, continues to recede from the sh.o.r.es of the United States, until, further deflected to the eastward by the Banks of Newfoundland, it approaches the European coasts, frequently throwing a quant.i.ty of tropical seeds ("Mimosa scandens, Guilandina bonduc, Dolichos urens") on the sh.o.r.es of Ireland, the Hebrides, and Norway. The northeastern prolongation tends to mitigate the cold of the ocean, and to ameliorate the climate on the most northern extremity of Scandinavia. At the point where the Gulf Stream p 308 is deflected from the Banks of Newfoundland toward the east, it sends off branches to the south near the Azores.*
[footnote] *Humboldt, "Examen Crit.", t. iii., p. 64-109
This is the situation of the Sarga.s.so Sea, or that great bank of weeds which so vividly occupied the imagination of Christopher Columbus, and which Oviedo calls the sea-weed meadows ("Praderias de yerva"). A host of small marine animals inhabits these tently-moved and evergreen ma.s.ses of "Fucus natans", one of the most generally distributed of the social plants of the sea.
The counterpart of this current (which in the Atlantic Ocean, between Africa, America, and Europe, belongs almost exclusively to the northern hemisphere) is to be found in the South Pacific, where a current prevails, the effect of whose low temperature on the climate of the adjacent sh.o.r.es I had an opportunity of observing in the autumn of 1802. It brings the cold waters of the high southern lat.i.tudes to the coast of Chili, follows the sh.o.r.es of this continent and of Peru, first from south to north, and is then deflected from the Bay of Arica onward from south-southeast to north-northwest. At certain seasons of the year the temperature of this cold oceanic current is, in the tropics, only 60 degrees, while the undisturbed adjacent water exhibits a temperature of 81.5 degrees and 83.7 degrees. On that part of the sh.o.r.e of South America south of Payta, which inclines furthest westward, the current is suddenly deflected in the same direction from the sh.o.r.e, turning so sharply to the west that a ship sailing northward pa.s.ses suddenly from cold into warm water.
It is not known to what depth cold and warm oceanic currents propagate their motion; but the deflection experienced by the South African current, from the Lagullas Bank, which is fully from 70 to 80 fathoms deep, would seem to imply the existence of a far-extending propagation. Sand banks and shoals lying beyond the line of these currents may, as was first discovered by the admirable Benjamin Franklin, be recognized by the coldness of the water over them. This depression of the temperature appears to me to depend upon the fact that, by the propagation of the motion of the sea, deep waters rise to the margin of the banks and mix with the upper strata. My lamented friend, Sir Humphrey Davy, ascribed this phenomenon (the knowledge of which is often of great practical utility in securing the safety of the navigator) to the descent of the particles of water that had been cooled by nocturnal radiation p 309 and which remain nearer to the surface, owing to the hinderance placed in the way of their greater descent by the intervention of sand-banks. By his observations Franklin may be said to have converted the thermometer into a sounding line. Mists are frequently found to rest over these depths, owing to the condensation of the vapor of the atmosphere by the cooled waters. I have seen such mists in the south of Jamaica, and also in the Pacific, defining with sharpness and clearness the form of the shoals below them, appearing to the eye as the aerial reflection of the bottom of the sea. A still more striking effect of the cooling produced by shoals is manifested in the higher strata of air, in a somewhat a.n.a.logous manner to that observed in the case of flat coral reefs, or sand islands. In the open sea, far from the land, and when the air is calm, clouds are often observed to rest over the spots where shoals are situated, and their bearing may then be taken by the compa.s.s in the same manner as that of a high mountain or isolated peak.
Although the surface of the ocean is less rich in living forms than that of continents, it is not improbable that, on a further investigation of its depths, its interior may be found to possess a greater richness of organic life than any other portion of our planet. Charles Darwin, in the agreeable narrative of his extensive voyages, justly remarks that our forests do not conceal so many animals as the low woody regions of the ocean, where the sea-weed rooted to the bottom of the shoals, and the severed branches of fuci, loosened by the force of the waves and currents, and swimming free, unfold their delicate foliage, upborne by air-cells.*
[footnote] *[See "Structure and Distribution of Coral Reefs", by Charles Darwin, London, 1842. Also, "Narrative of the Surveying Voyage of H.M.S.
"Fly" in the Eastern Archipelago, during the Years " 1842-1846, by J. B.
Jukes, Naturalist to the expedition, 1847.] -- Tr.
The application of the microscope increases, in the most striking manner, our impression of the rich luxuriance of animal life in the ocean, and reveals to the astonished senses a consciousness of the universality of life. In the oceanic depths, far exceeding the height of our loftiest mountain chains, every stratum of water is animated with polygastric sea-worms, Cyclidiae and Ophrydinae. The waters swarm with countless hosts of small luminiferous animalcules, Mammaria (of the order of Acalephae), Crustacea, Peridinea, and circling Nereides, which when attracted to the surface by peculiar meteorological conditions, convert every wave into a foaming band of flashing light.
p 310 The abundance of those marine animalcules, and the animal matter yielded by their rapid decomposition are so vast that the sea water itself becomes a nutrient fluid to many of the larger animals. However much this richness in animated forms, and this mult.i.tude of the most various and highly-developed microscopic organisms may agreeably excite the fancy, the imagination is even more seriously, and, I might say, more solemnly moved by the impression of boundlessness and immeasureability, which are presented to the mind by every sea voyage. All who possess an ordinary degree of mental activity, and delight to create to themselves an inner world of thought, must be penetrated with the sublime image of the infinite, when gazing around them on the vast and boundless sea, when involuntarily the glance is attracted to the distant horizon, where air and water blend together, and the stars continually rise and set before the eyes of the mariner. This contemplation of the eternal play of the elements is clouded, like every human joy, by a touch of sadness and of longing.
A peculiar predilection for the sea, and a grateful remenbrance of the impression which it has excited in my mind, when I have seen it in the tropics in the calm of nocturnal rest, or in the fury of the tempest, have alone induced me to speak of the individual enjoyment afforded by its aspect before I entered upon the consideration of the favorable influence which the proximity of the ocean has incontrovertibly exercised on the cultivation of the intellect and character of many nations, by the multiplication of those bands which ought to encircle the whole of humanity, by affording additional means of arriving at a knowledge of the configuration of the earth, and furthering the advancement of astronomy, and of all other mathematical and physical sciences. A portion of this influence was at first limited to the Mediterranean and the sh.o.r.es of southwestern Africa, but from the sixteenth century it has widely spread, extending to nations who live at a distance from the sea, in the interior of continents. Since Columbus was sent to "unchain the ocean"* (as the unknown voice whispered to him in a dream when he lay on a sick-bed near p 311 the River Belem), man has ever boldly ventured onward toward the discovery of unknown regions.
[footnote] *The voice addressed him in these words, "Maravillosamente Dios hizo sonar tu nombre en la tierra; de los atamientos de la mar Oceana, que estaban cerrados con cadenas tan fuertes, te di? las llaves" -- "G.o.d will cause thy name to be wonderfully resounded through the earth, and give thee the keys of the gates of the ocean, which are closed with strong chains."
The dream of Columbus is related in the letter to the Catholic monarchs of July the 7th, 1503. (Humboldt, "Examen Critique", t. iii., p. 234.)
The second external and general covering of our planet, the aerial ocean, in the lower strata, and on the shoals of which we live, presents six cla.s.ses of natural phenomena, which manifest the most intimate connection with one another. They are dependent on the chemical composition of the atmosphere, the variations in its transparency, polarization, and color, its density or pressure, its temperature and humidity, and its electricity. The air contains in oxygen the first element of physical animal life, and besides this benefit, it possesses another, which may be said to be of a nearly equally high character, namely, that of conveying sound; a faculty by which it likewise becomes the conveying sound; a faculty by which it likewise becomes the conveyer of speech and the means of communicating thought, and consequently of maintaining social intercourse. If the Earth were deprived of an atmosphere, as we suppose our moon to be, it would present itself to our imagination as a soundless desert.
The relative quant.i.ties of the substances composing the strata of air accessible to us have, since the beginning of the nineteenth century, become the object of investigations, in which Gay-Lussac and myself have taken an active part; it is however, only very recently that the admirable labors of Dumas and Boussingault have, by new and more accurate methods, brought the chemical a.n.a.lysis of the atmosphere to a high degree of perfection.
According to this a.n.a.lysis, a volume of dry air contains 20.8 of oxygen, and 79.2 of nitrogen, besides from two to five thousandth parts of carbonic acid gas, a still smaller quant.i.ty of carbureted hydrogen gas,* and, according to the important experiments of Saussure and Liebig, traces of ammoniacal vapors,** from which plants derive their nitrogenous contents.
[footnote] *Boussingault, "Recherches sur la Composition de l"Atmosphere", in the "Annales de Chimie et de Physique", t. lvii., 1834, p. 171-173; and lxxi. 1839, p. 116. According to Boussingault and Lewy, the proportion of carbonic acid in the atmosphere at Audilly, at a distance, therefore, from the exhalations of a city, varied only between 0.00028 and 0.00031 in volume.
[footnote] **Liebig, in his important work, ent.i.tles "Die Organische Chemie in ihrer Anwendung auf Agricultur und Physiologie", 1840, s. 62-72. On the influence of atmospheric electricity in the production of nitrate of ammonia, which, coming into contact with carbonate of lime, is changed into carbonate of ammonia, see Boussingault"s "Economie Rurale consideree dans ses Rapports avec la Chimie et la Meteorologie", 1844, t. ii., p. 247, 267, and t. i., p. 84.
Some observations of Lewy render it probable that the quant.i.ty of oxygen varies perceptibly p 312 but slightly, over the sea and in the interior of continents, according to local conditions or to the seasons of the year. We may easily conceive that changes in the oxygen held in solution in the sea, produced by microscopic animal organisms, may be attended by alterations in the strata of air in immediate contact with it.*
[footnote] *Lewy, in the "Comptes Rendus de l"Acad. des Sciences", t.
xvii., Part ii., p. 235-248.
The air which Martins collected at Faulhorn at an elevation of 8767 feet, contained as much oxygen as the air at Paris.*
[footnote] *Dumas, in the "Annales de Chimie, 3e Serie", t. iii., 1841, p.
257.
The admixture of carbonate of ammonia in the atmosphere may probably be considered as older than the existence of organic beings on the surface of the earth. The sources from which carbonic acid* may be yielded to the atmosphere are most numerous.
[footnote] *In this enumeration, the exhalation of carbonic acid by plants during the night, while they inhale oxygen, is not taken into account, because the increase of carbonic acid from this source is amply counter-balanced by the respiratory process of plants during the day. See Boussingault"s "Econ. Rurale", t. i., p. 53-68, and Liebig"s "Organische Chemie", s. 16, 21.
In the first place we would mention the respiration of animals, who receive the carbon which they inhale from vegetable food, while vegetables receive it from the atmosphere; in the next place, carbon is supplied from the interior of the earth in the vicinity of exhausted volcanoes and thermal springs, from the decomposition of a small quant.i.ty of carbureted hydrogen gas in the atmosphere, and from the electric discharges of clouds, which are of such frequent occurrence within the tropics. Besides these substances, which we have considered as appertaining to the atmosphere at all heights that are accessible to us, there are others accidentally mixed with them, especially near the ground, which sometimes, in the form of miasmatic and gaseous contagia, exercise a noxious influence on animal organization.
Their chemical nature has not yet been ascertained by direct a.n.a.lysis; but, from the consideration of the processes of decay which are perpetually going on in the animal and vegetable substances with which the surface of our planet is covered, and judging from a.n.a.logies deduced from the comain of pathology, we are led to infer the existence of such noxious local admixtures. Ammoniacal and other nitrogenous vapors, sulphureted hydrogen gas, and compounds a.n.a.logous to the polybasic ternary and quaternary compounds a.n.a.logous to the polybasic ternary and quaternary combinations of the vegetable kingdom, may produce miasmata,*
p 313 which, under various forms, may generate ague and typhus fever (not by any means exclusively on wet, marshy ground, or on coasts covered by putrescent mollusca, and low bushes of "Rhizophora mangle" and Avicennia).
[footnote] *Gay-Lussac, in "Annales de Chimie", t. liii., p. 120; Payen, Mem. sur la Composition Chimique des Vegetaux, p. 36, 42; Liebig, "Org.
Chemie", s. 229-345; Boussingault, "Econ. Rurale", t. i., p. 142-153.