Cosmos: A Sketch of the Physical Description of the Universe Part 35

[footnote] *'Asie Centrale', t. i., p. 284, 286. The Adriatic Sea likewise follows a direction from S.E. to N.W.

Since grand and lofty mountain chains so strongly excite our imagination by the evidence they afford of great terrestrial revolutions, and when considered as the boundaries of climates, as lines of separation for waters, or as the site of a different form of vegetation, it is the more necessary to demonstrate, by a correct numerical estimation of their volume, how small is the quant.i.ty of their elevated ma.s.s when compared with the area of the adjacent continnents. The ma.s.s of the Pyrenees, for instance, the mean elevation of whose summits, and the real quant.i.ty of whose base have been ascertained by accurate measurements, would if scattered over p 301 the surface of France, only raise its mean level about 115 feet. The ma.s.s of the eastern and western Alps would in like manner only increase the height of Europe about 21 1/2 feet above its present level. I have found by a laborious investigation,* which from its nature, can only give a maximum limit, that the center of gravity of the volume of the land raised above the present level of the sea in Europe and North America is respectively situated at an elevation of 671 and 748 feet, while it is at 1132 and 1152 feet in Asia and South America.

[footnote] *'De la hauteur Moyenne des Continents', in my 'Asie Centrale', t. i., p. 82-90, 165-189. The results which I have obtained are to be regarded as the extreme value ('nombres-limites'). Laplace's estimate of the mean height of continents at 3280 feet is at least three times too high.

The immortal author of the 'Mecanique Celeste' (t. v., p. 14) was led to this conclusion by hypothetical views as to the mean depth of the sea. I have shown ('Asie Centr.', t. i., p. 93) that the old Alexandrian mathematicians, on the testimony of Plutarch ('in Aemilio Paulo', cap. 15), believed this depth to depend on the height of the mountains. The height of the center of gravity of the volume of the continental ma.s.ses is probably subject to slight variations in the course of many centuries.

These numbers show the low level of norther regions. In Asia the vast steppes of Siberia are compensated for by the great elevations of the land (between the Himalaya, the North Thibetian chain of Kuen-lun, and the Celestial Mountains), from 28 degrees 30' to 40 degrees north lat.i.tude. We may, to a certain extent, trace in these numbers the portions of the Earth in which the Plutonic forces were most intensely manifested in the interior by the upheaval of continental ma.s.ses.

There are no reasons why these Plutonic forces may not, in future ages, add new mountain systems to those which Elie de Beaumont has shown to be of such different ages, and inclined in such different directions. Why should the crust of the Earth have lost its property of being elevated in the ridges?

The recently-elevated mountain systems of the Alps and the Cordilleras exhibit in Mont Blanc and Monte Rosa, in Sorata, Illimani, and Chimborazo, colossal elevations which do not favor the a.s.sumption of a decrease in the intensity of the subterranean forces. All geognostic phenomena indicate the periodic alternation of activity and repose;* but the quiet we now enjoy is only apparent.

[footnote] *'Zweiter Geologischer Brief von Elie de Beaumont an Alexander von Humboldt', in Poggendorf's 'Annalen', bd. xxv., s. 1-58.

The tremblings which still agitate the surface under all lat.i.tudes, and in every species of rock, the elevation of Sweden, the appearance of new islands of eruption, are all conclusive as to the unquiet condition of our planet.

p 302 The two envelopes of the solid surface of our planet -- the liquid and the aeriform -- exhibit, owing to the mobility of their particles, their currents, and their atmospheric relations, many a.n.a.logies combined with the contrasts which arise from the great difference in the condition of their aggregation and elasticity. The depths of ocean and of air are alike unknown to us. At some few places under the tropics no bottom has been found with soundings of 276,000 (or more than four miles), while in the air, if, according to Wollaston, we may a.s.sume that it has a limit from which waves of sound may be reverberated, the phenomenon of twilight would incline us to a.s.sume a height at least nine times as great.*

[footnote] *[See Wilson's Paper, 'On Wollaston's Argument from the Limitation of the Atmosphere as to the finite Divisibility of Matter.' -- 'Trans. of the Royal Society of Edinb.', vol. xvi., p. 1, 1845.] -- Tr.

The a?rial ocean rests partly on the solid earth, whose mountain chains and elevated plateaux rise, as we have already seen, like green wooded shoals, and partly on the sea, whose surface forms a moving base, on which rest the lower, denser, and more saturated strata of air.

Proceeding upward and downward from the common limit of the a?rial and liquid oceans, we find that the strata of air and water are subject to determinate laws of decrease of temperature. This decrease is much less rapid in the air than in the sea, which has a tendency under all lat.i.tudes to maintain its temperature in the strata of water most contiguous to the atmosphere, owing to the sinking of the heavier and more cooled particles.

A large series of the most carefully conducted observations on temperature shows us that in the ordinary and mean condition of its surface, the ocean from the equator to the forty-eighth degree of north and south lat.i.tude is somewhat warmer than the adjacent strata of air.*

[footnnote[ *Hamboldt, 'Relation Hist.', t. iii., chap. xxix., p. 514-530.

Owing to this decrease of temperature at increasing depths, fishes and other inhabitants of the sea, the nature of whose digestive and respiratory organs fits them for living in deep water, may even, under the tropics, find the low degree of temperature and the coolness of climate characteristic of more temperate and more northern lat.i.tudes. This circ.u.mstance, which is a.n.a.logous to the prevalence of a mild and even cold air on the elevated plains of the torrid zone, exercises a special influence on the migration and geographical distribution of many marine animals. Moreover, the depths at which fishes live, modify, by the increase of pressure, their cutaneous respiration, and the p 303 oxygenous and nitrogenous contents of the swimming bladders.

As fresh and salt water do not attain the maximum of their density at the same degree of temperature, and as the saltness of the sea lowers the thermometrical degree corresponding to this point, we can understand how the water drawn from breat depths of the sea during the voyages of the Kotzebue and Dupet.i.t-Thouars could have been found to have only the temperature of 37 degrees and 36.5 degrees. This icy temperatureof sea water, which is likewise manifested at the depths of tropical seas, first led to a study of the lower polar currents, which move from both poles toward the equator.

Without these submarine currents, the tropical seas at those depths could only have a temperature equal to the local maximum of cold possessed by the falling particles of water at the radiating and cooled surface of the tropical sea. In the Mediterranean, the cause of the absence of such a refrigeration of the lower strata is ingeniously explained by Arago, on the a.s.sumption that the entrance of the deeper polar currents into the Straits of Gibraltar, where the water at the surface flows in from the Atlantic Ocean from west to east, is hindered by the submariine counter-currents which move from east to west, from the Mediterranean into the Atlantic.

The ocean, which acts as a general equalizer and moderator of climates, exhibits a most remarkable uniformity and constancy of temperature, especially between 10 degrees north and 10 degrees south lat.i.tude,* over s.p.a.ces of many thousands of square miles, at a distance from land where it is not penetrated by currents of cold and heated water.

[footnote] *See the series of observations made by me in the South Sea, from 8 degrees 5' to 13 degrees 16' N. lat., in my 'Asie Centrale', t. iii., p. 234.

It has therefore, been justly observed, that an exact and long-continued investigation of these thermic relations of the tropical seas might most easily afford a solution to the great and much-contested problem of the permanence of climates and terrestrial temperatures.*

[footnote] *We might (by means of the temperature of the ocean under the tropics) enter into the consideration of a question which has. .h.i.therto remained unanswered, namely, that of the constancy of terrestrial temperatures, without taking into account the very circ.u.mscribed local influences arising from the diminution of wood in the plains and on mountains, and the drying up of lakes and marshes. Each age might easily transmit to the succeeding one some few data, which would perhaps furnish the most simple, exact, and direct means of deciding whether the sun, which is almost the sole and exclusive source of the heat of our planet, changes its physical const.i.tution and splendor, like the greater number of the stars, or whether, on the contrary, that luminary has attained to a permanent condition." -- Arago, in the 'Comptes Rendus des Seances de l'Acad. des Sciences', t. ii., p. 321, 327.

Great changes in the luminous disk of the sun would, p 304 if they were of long duration, be reflected with more certainty in the mean temperature of the sea than in that of the solid land.

The zones at which occur the maxima of the oceanic temperature and of the density (the saline contents) of its waters, do not correspond with the equator. The two maxima are separated from one another, and the waters of the highest temperature appear to form two nearly parallel lines north and south of the geographical equator. Lenz, in his voyage of circ.u.mnavigation, found in the Pacific the maxima of density in 22 degrees north and 17 degrees south lat.i.tude, while its minimum was situated a few degrees to the south of the equator. In the region of calms the solar heat can exercise but little influence on evaporation, because the stratum of air impregnated with saline aqueous vapor, which rests on the surface of the sea, remains still and unchanged.

The surface of all connected seas must be considered as having a general perfectly equal level with respect to their mean elevation. Local causes (probably prevailing winds and currents) may, however, produce permanent, although trifling changes in the level of some deeply indented bays, as for instance, the Red Sea. The highest level of the water at the Isthmus of Suez is at different hours of the day from 24 to 30 feet above that of the Mediterranean. The form of the Straits of Bab-el-Mandeb, through which the waters appear to find an easier ingress than egress, seems to contribute to this remarkable phenomenon, which was known to the ancients.*

[[footnote] *Humboldt, 'Asie Centrale', t. ii., p. 321, 327.

The admirable geodetic operations of Coraboeuf and Delcrois show that no perceptible difference of level exists between the upper surfaces of the Atlantic and the Mediterranean, along the chain of the Pyrenees, or between the coasts of northern Holland and Ma.r.s.eilles.*

[footnote] *See the numerical results in p. 328-333 of the volume just named. From the geodesical levelings which, at my request, my friend General Bolivar caused to be taken by Lloyd and Falmare, in the years 1828 and 1829, it was ascertained that the level of the Pacific is at the utmost 3 1/2 feet higher than that of the Caribbean Sea; and even that at different hours of the day each of the seas is in turn the higher, according to their respective hours of flood and ebb. If we reflect that in a distance of 64 miles, comprising 933 stations of observation, an error of three feet would be very apt to occur, we may say that in these new operations we have further confirmation of the equilibrium of the waters which communicate round Cape Horn. (Arago, in the 'Annuaire du Bureau des Longitudes pour'

1831, p. 319.) I had inferred from barometrical observations inst.i.tuted in 1799 and 1804, that if there were any difference between the level of the Pacific and the Atlantic (Carribean Sea), it could not exceed three meters (nine feet three inches). See my 'Relat. Hist.', t. iii., p. 555-557, and 'Annales de Chimie', t. i., p. 55-64. The measurements, which appear to establish an excess of height for the waters of the Gulf of Mexico, and for those of the northern part of the Adriatic Sea, obtained by combining the trigonometrical operations of Delcrois and Choppin with those of the Swiss and Austrian engineers, are open to many doubts. Notwithstanding the form of the Adriatic, it is improbable that the level of its waters in its northern portion should be 28 feet higher than that of the Mediterranean at Ma.r.s.eilles, and 25 feet higher than the level of the Atlantic Ocean. See my 'Asie Centrale', t. ii., p. 332.

p 305 Disturbances of equilibrium and consequent movements of the waters are partly irregular and transitory, dependent upon winds, and producing waves which sometimes, at a distance from the sh.o.r.e and during a storm, rise to a height of more than 35 feet; partly regular and periodic, occasioned by the position and attraction of the sun and moon, as the ebb and flow of the tides; and partly permanent, although less intense, occurring as oceanic currents. The phenomena of tides, which prevail in all seas (with the exception of the smaller ones that are completely closed in, and where the ebbing and flowing waves are scarcely or not at all perceptible), have been perfectly explained by the Newtonian doctrine, and thus brought "within the domain of necessary facts." Each of these periodically-recurring oscillations of the waters of the sea has a duration of somewhat more than half a day. Although in the open sea they scarcely attain an elevation of a few feet, they often rise considerably higher where the waves are opposed by the configuration of the sh.o.r.es, as for instance, at St. Malo and in Nova Scotia, where they reach the respective elevation of 50 feet, and of 65 to 70 feet. "It has been shown by the a.n.a.lysis of the great geometrician Laplace, that, supposing the depth to be wholly inconsiderable when compared with the radius of the earth, the stability of the equilibrium of the sea requires that the density of its fluid should be less than that of the earth; and, as we have already seen, the earth's density is in fact five times greater than that of water. The elevated parts of the land can not therefore be overflowed, nor can the remains of marine animals found on the summits of mountains have been conveyed to those localities by any previous high tides.*

[footnote] *Bessel, 'Ueber Fluth und Ebbe', in Schumacher's 'ahrbuch', 1838, s. 225.

It is no slight

This material taken from pages 305-362

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 by Alexander von Humboldt

Translated by E C Otte

from the 1858 Harper & Brothers edition of Cosmos, volume 1 --------------------------------------------------

p 305 [balance of p 305 is in file "09 Humboldt"]

It is no slight p 306 evidence of the importance of a.n.a.lysis, which is too often regarded with contempt among the unscientific, that Laplace's perfect theory of tides has enabled us, in our astronomical ephemerides, to predict the height of spring-tides at the periods of new and full moon, and thus put the inhabitants of the sea-sh.o.r.e on their guard against the increased danger attending these lunar revolutions.

Oceanic currents, which exercise so important an influence on the intercourse of nations and on the climatic relations of adjacent coasts, depend conjointly upon various causes, differing alike in nature and importance. Among these we may reckon the periods at which tides occur in their progress round the earth; the duration and intensity of prevailing winds; the modifications of density and specific gravity which the particles of water undergo in consequence of differences in the temperature and in the relative quant.i.ty of saline contents at different lat.i.tudes and depths;*

and, lastly, the horary variations of the atmospheric pressure, successively propagated from east to west, and occurring with such regularity in the tropics.

[footnote] *The relative density of the particles of water depends simultaneously on the temperature and on the amount of the saline contents -- a circ.u.mstance that is not sufficiently borne in mind in considering the cause of currents. The submarine current, which brings the cold polar water to the equatorial regions, would follow an exactly opposite course, that is to say, from the equator toward the poles, if the difference in saline contents were alone concerned. In this view, the geographical distribution of temperature and of density in the water of the ocean, under the different zones of lat.i.tude and longitude, is of great importance. The numerous observations of Lenz (Poggendorf's 'Annalen', bd. xx., 1830, s. 129), and those of Captain Beechey, collected in his 'Voyage to the Pacific', vol.

ii., p. 727, deserve particular attention. See Humboldt, 'Relat. Hist.', t.

i., p. 74, and 'Asie Centrale', t. iii., p. 346.