Fogs which have a peculiar smell at some seasons of the year, remind us of these accidental admixtures in the lower strata of the atmosphere. Winds and currents of air caused by the heating of the ground even carry up to a considerable elevation solid substances reduced to a fine powder. The dust which darkens the air for an extended area, and falls on the Cape Verd Islands, to which Darwin has drawn attention, contains, according to Ehrenberg"s discovery, a host of silicious-sh.e.l.led infusoria.
As princ.i.p.al features of a general descriptive picture of the atmosphere, we may enumerate:
1. "Variations of atmospheric pressure": to which belong the horary oscillations, occurring with such regularity in the tropics, where they produce a kind of ebb and flow in the atmosphere, which can not be ascribed to the attraction of the moon,* and which differs so considerably according to geographical lat.i.tude, the seasons of the year, and the elevation above the level of the sea.
[footnote] *Bouvard, by the application of the formulae, in 1827, which Laplace had deposited with the Board of Longitude shortly before his death, found that the portion of the horary oscillations of the pressure of the atmosphere, which depends on the attraction of the moon, can not raise the mercury in the barometer at Paris more than the 0.018 of a millimeter, while eleven years" observations at the same place show the mean barometric oscillation, from 9 A.M. to 3 P.M., to be 0.756 millim., and from 3 P.M. to 9 P.M., 0.373 millim. See "Memoires de l"Acad. des Sciences", t. vii., 1827, p. 267.
2. "Climatic distribution of heat", which depends on the relative position of the transparent and opaque ma.s.ses (the fluid and solid parts of the surface of the earth), and on the hypsometrical configuration of continents; relations which determine the geographical position and curvature of the isothermal lines (or curves of equal mean annual temperature) both in a horizontal and vertical direction, or on a uniform plane, or in different superposed strata of air.
3. "The distribution of the humidity of the atmosphere". The quant.i.tative relations of the humitidy depend on the differences in the solid and oceanic surfaces; on the distance from the equator and the level of the sea; on the form in which the p 314 aqueous vapor is precipitated, and on the connection existing between these deposits and the changes of temperature, and the direction and succession of winds.
4. "The electric condition of the atmosphere". the primary cause of this condition, when the heavens are serene, is still much contested. Under this head we must consider the relation of ascending vapors to the electric charge and the form of the clouds, according to the different periods of the day and year; the difference between the cold and warm zones of the earth, or low and high lands; the frequency or rarity of thunder storms, their periodicity and formation in summer and winter; the causal connection of electricity, with the infrequent occurrence of hail in the night, and with the phenomena of water and sand spouts, so ably investigated by Peltier.
The horary oscillations of the barometer, which in the tropics present two maxima (viz., at 9 or 9 1/4 P.M., and 4 A.M., occurring, therefore, in almost the hottest and coldest hours), have long been the object of my most careful diurnal and nocturnal observations.*
[footnote] *"Observations faites pour constater la Marche des Variations Horaires du Barometre sous les Tropiques", in my "Relation Historique du Voyage aux Regions Equinoxiales", t. iii., p. 270-313.
Their regularity is so great, that, in the daytime especially, the hour may be ascertained from the height of the mercurial column without an error, on the average, of more than fifteen or seventeen minutes. In the torrid zones of the New Continent, on the coasts as well as at elevations of nearly 13,000 feet above the level of the sea, where the mean temperature falls to 44.6 degrees, I have found the regularity of the ebb and flow of the aerial ocean undisturbed by storms, hurricanes, rain, and earthquakes. The amount of the daily oscillations diminishes from 1.32 to 0.18 French lines from the equator to 70 degrees north lat.i.tude, where Bravais made very accurate observations at Bosekop.*
[footnote] *Bravais, in Daemtz and Martins, "Meteorologie", p. 263. At Halle (51 degrees 29" N. lat.), the oscillation still amounts to 0.28 lines.
It would seem that a great many observations will be required in order to obtain results that can be trusted in regard to the hours of the maximum and minimum on mountains in the temperate zone. See the observations of horary variations, collected on the Faulhorn in 1832, 1841, and 1842 (Martins, "Meteorologie", p. 254.)
The supposition that, much nearer the pole, the height of the barometer is really less at 10 A.M. than at 4 P.M., and consequently, that the maximum and minimum influences of these hours p 315 are inverted, is not confirmed by Parry"s observations at Port Bowen (73 degrees 14").
The mean height of the barometer is somewhat less under the equator and in the tropics, owing to the effect of the rising current,* than in the temperate zones, and it appears to attain its maximum in Western Europe between the parallels of 40 degrees and 45 degrees.
[footnote] *Humboldt, "Essai sur la Geographie des Plantes", 1807, p. 90; and in "Rel. Hist.", t. iii., p. 313; and on the diminuation of atmospheric pressure in the tropical portions of the Atlantic, in Poggend., "Annalen der Physik", bd. x.x.xvii., s. 245-258, and s. 463-486.
If with K?mtz we connect together by "isobarometric" lines those places which present the same mean difference between the monthly extremes of the barometer, we shall have curves whose geographical position and inflections yield important conclusions regarding the influence exercised by the form of the land and the distribution of seas on the oscillations of the atmosphere.
Hindostan with its high mountain chains and triangular peninsulas, and the eastern coasts of the New Continent, where the warm Gulf Stream turns to the east at the Newfoundland Banks, exhibit greater isobarometric oscillations than do the group of the Antilles and Western Europe. The prevailing winds exercise a princ.i.p.al influence on the diminution of the pressure of the atmosphere, and this, as we have already mentioned, is accompanied, according to Daussey, by an elevation of the mean level of the sea.
[footnote] *Dausay, in the "Comptes Rendus", t. iii., p. 136.
As the most important fluctuations of the pressure of the atmosphere, whether occurring with horary or annual regularity, or accidentally, and then often attended by violence and danger,* are like all the other phenomena of the weather, mainly owing to the heating force of the sun"s rays, it has long been suggested (partly according to the idea of Lambert) that the direction of the wind should be compared with the height of the barometer, alternations of temperature, and the increase and decrease of humidity.
[footnote] *Dove, "Ueber die Sturme", in Poggend., "Annalen", bd. lii., s.
1.
Tables of atmospheric pressure during different winds, termed "barometric windroses", afford a deeper insight into the connection of meteorological phenomena.*
[footnote] *Leopold von Buch, "Barometrische Windrose", in "Abhandl. der Akad. der Wiss. zu Berlin aus den Jahren", 1818-1819, s. 187.
Dove has, with admirable sagacity, recognized, in the "law of rotation" in both hemispheres, which he himself established, the cause of many important processes in the aerial ocean.*
[footnote] *See Dove, "Meteorologishe Untersuchungen", 1837, s. 99-313; and the excellent observations of K?mtz on the descent of the west wind of the upper current in high lat.i.tudes, and the general phenomena of the direction of the wind, in his "Vorlesungen uber eterologie", 1840, s. 58-66, 196-200, 327-336, 353-364; and in Schumacher"s "Jahrbuch fur" 1838, s.
291-302. A very satisfactory and vivid representation of meteorological phenomena is given by Dove, in his small work ent.i.tled "Witterungsverh?ltnisse von Berlin", 1842. On the knowledge of the earlier navigators of the rotation of the wind, see Churruca, "Viage at Magellanes", 1793, p. 15; and on a remarkable expression of Columbus, which his son Don Fernando Colon has presented to us in his "Vida del Almirante", cap. 55, see Humboldt, "Examen Critique de l"Hist. de Geographie", t. iv., p. 253.
The difference of temperature between the p 315 equatorial and polar regions engenders two opposite currents in the upper strata of the atmosphere and on the Earth"s surface. Owing to the difference between the rotatory velocity at the poles and at the equator, the polar current is deflected eastward, and the equatorial current westward. The great phenomena of atmospheric pressure, the warming and cooling of the strata of air, the aqueous deposits, and even, as Dove has correctly represented, the formation and appearance of clouds, alike depend on the opposition of these two currents, on the place where the upper one descends, and on the displacement of the one by the other. Thus the figures of the clouds, which form an animated part of the charms of a landscape, announce the processes at work in the upper regions of the atmosphere, and, when the air is calm, the clouds will often present, on a bright summer sky, the "projected image" of the radiating soil below.
Where this influence of radiation is modified by the relative position of large continental and oceanic surfaces, as between the eastern sh.o.r.e of Africa and the western part of the Indian peninsula, its effects are manifested in the Indian monsoons, which change with the periodic variations in the sun"s declination,* and which were known to the Greek navigators under the name of "Hippalos".
[footnote] *"Monsun" (Malayan "musim", the "hippalos" of the Greeks) is derived from the Arabic word "mausim", a set time or season of the year, the time of the a.s.semblage of pilgrims at Mecca. The word has been applied to the seasons at which certain winds prevail, which are, besides, named from places lying in the direction from whence they come; thus, for instance, there is the "mausim" of Aden, of Guzerat, Malabar, etc. (La.s.sen, "Indische Alterthumskunde", bd. i., 1843, s. 211). On the contrasts between the solid or fluid substrata of the atmosphere, see Dove, in "Der Abhandl. der Akad.
der Wiss. zu Berlin aus dem Jahr" 1842, s. 239.
In the knowledge of the monsoons, which undoubtedly dates back thousands of years among the inhabitants of Hindostan and China, of the eastern parts of the Arabian Gulf and of the western sh.o.r.es of the Malayan p 317 Sea, and in the still more ancient and more general acquaintance with land and sea winds, lies concealed, as it were, the germ of that meteorological sciences which is now making such rapid progress. The long chain of "magnetic stations" extending from Moscow to Pekin, across the whole of Northern Asia, will prove of immense importance in determining the "law of the winds", since these stations have also for their object the investigation of general meteorological relations. The comparison of observations made at places lying so many hundred miles apart, will decide, for instance, whether the same east wind blows from the elevated desert of Gobi to the interior of Russia, or whether the direction of the Aerial current first began in the middle of the series of the stations, by the descent of the air from the higher regions. By means of such observations, we may learn, in the strictest sense, "whence" the wind cometh. If we only take the results on which we may depend from those places in which the observations on the direction of the winds have been continued more than twenty years, we shall find (from the most recent and careful calculations of Wilhelm Mahlmann) that in the middle lat.i.tudes of the temperate zone, in both continents, the prevailing aerial current has a west-southwest direction.
Our insight into the "distribution of heat" in the atmosphere has been rendered more clear since the attempt has been made to connect together by lines those places where the mean annual summer and winter temperatures have been ascertain by correct observations. The system of "isothermal, osotheral" and "isochimenal" lines, which I first brought into use in 1817, may, perhaps, if it be gradually perfected by the united efforts of investigators, serve as one of the main foundations of "comparative climatology". Terrestrial magnetism did not acquire a right to be regarded as a science until partial results were graphically connected in a system of lines of "equal declination, equal inclinatiion", and "equal intensity".
The term "climate", taken in its most general sense, indicated all the changes in the atmosphere which sensibly affect our organs, as temperature, humidity, variations in the barometrical pressure, the calm state of the air or the action of opposite winds, the amount of electric tension, the purity of the atmosphere or its admixture with more or less noxious gaseous exhalations, and, finally, the degree of ordinary transparency and clearness of the sky, which is not only important with respect to the increased radiation from the Earth, the organic development of plants, and the ripening of fruits, but p 318 also with reference to its influence on the feelings and mental condition of men.
If the surface of the Earth consisted of one and the same h.o.m.ogeneous fluid ma.s.s, or of strata of rock having the same color, density, smoothness, and power of absorbing heat from the solar rays, and of radiating it in a similar manner through the atmosphere, the isothermal, isotheral, and isochimenal lines would all be parallel to the equator. In this hypothetical condition of the Earth"s surface, the power of absorbing and emitting light and heat would every where be the same under the same lat.i.tudes. The mathematical consideration of climate, which does not exclude the supposition of the existence of currents of heat in the interior, or in the external crust of the earth, nor of the propagation of heat by atmospheric currents, proceeds from this mean, and, as it were, primitive condition. Whatever alters the capacity for absorption and radiation, at places lying under the same parallel of lat.i.tude, gives rise to inflections in the isothermal lines. The nature of these inflections, the angles at which the isothermal, isotheral, or isochimenal lines intersect the parallels of lat.i.tude, their convexity or concavity with respect to the pole of the same hemisphere, are dependent on causes which more or less modify the temperature under different degrees of longitude.
The progress of "Climatology" has been remarkably favored by the extension of European civilization to two opposite coasts, by its transmission from our western sh.o.r.es to a continent which is bounded on the east by the Atlantic Ocean. When, after the ephemeral colonization from Iceland and Greenland, the British laid the foundation of the first permanent settlements on the sh.o.r.es of the United States of America, the emigrants (whose numbers were rapidly increased in consequence either of religious persecution, fanaticism, or love of freedom, and who soon spread over the vast extent of territory lying between the Carolinas, Virginia, and the St.
Lawrence) were astonished to find themselves exposed to an intensity of winter cold far exceeding that which prevailed in Italy, France, and Scotland, situated in corresponding parallels of lat.i.tude. But, however much a consideration of these climatic relations may have awakened attention, it was not attended by any practical results until it could be based on the numerical data of "mean annual temperature". If, between 58 degrees and 30 degrees north lat.i.tude, we compair Nain, on the coast of Labrador, with Gottenburg; Halifax with Bordeaus; New p 319 York with Naples; St. Augustine, in Florida, with Cairo, we find that, under the same degrees of lat.i.tude, the differences of the mean annual temperature between Eastern America and Western Europe, proceeding from north to south, are successively 20.7 degrees, 13.9 degrees, 6.8 degrees, and almost 0 degrees. The gradual decrease of the differences in this series extending over 28 degrees of lat.i.tude is very striking. Further to the south, under the tropics, the isothermal lines are every where parallel to the equator in both hemispheres. We see, from the above examples, that the questions often asked in society, how many degrees America (without distinguishing between the eastern and western sh.o.r.es) is colder than Europe? and how much the mean annual temperature of Canada and the United States is lower than that of corresponding lat.i.tudes in Europe? are, when thus "generally expressed", devoid of meaning. There is a separate difference for each parallel of lat.i.tude, and without a special comparison of the winter and summer temperatures of the opposite coasts, it will be impossible to arrive at a correct idea of climatic relations, in their influence on agriculture and other industrial pursuits, or on the individual comfort or discomfort of manking in general.
In enumerating the causes which produce disturbances in the form of the isothermal lines, I would distinguish between those which "raise" and those which "lower" the temperature. To the first cla.s.s belong the proximity of a western coast in the temperate zone; the divided configuration of a continent into peninsulas, with deeply-indented bays and inland seas; the aspect of the position of a portion of the land with reference either to a sea of ice spreading far into the polar circle, or to a ma.s.s of continental land of considerable extent, lying in the same meridian, either under the equator, or, at least, within a portion of the tropical zone; the prevalence of southerly or westerly winds on the western sh.o.r.e of a continent in the temperate northern zone; chains of mountains acting as protecting salls against the winds coming from colder regions; the infrequency of swamps, which, in the spring and beginning of summer, long remain covered with ice, and the absence of woods in a dry, sandy soil; finally the constant serenity of the sky in the summer months, and the vicinity of an oceanic current, bringing water which is of a higher temperature than that of the surrounding sea.
Among the causes which tend to "lower" the mean annual temperature I include the following: elevation above the level of the sea, when not forming part of an extended plain; the p 320 vicinity of an eastern coast in high and middle lat.i.tudes; the compact configuration of a continent having no littoral curvatures or bays; the extension of land toward the poles into the region of perpetual ice, without the intervention of a sea remaining open in the winter; a geographical position, in which the equatorial and tropical regions are occupied by the sea, and consequently, the absence, under the same meridian, of a continental tropical land having a strong capacity for the absorption and radiation of heat; mountain chains, whose mural form and direction impede the access of warm winds, the vicinity of isolated peaks, occasioning the descent of cold currents of air down their declivities; extensive woods, which hinder the isolation of the soil by the vital activity of their foliage, which produces great evaporation, owing to the extension of these organs, and increases the surface that is cooled by radiation, acting consequently in a three-fold manner, by shade, evaporation, and radiation; the frequency of swamps or marshes, which in the north form a kind of subterranean glacier in the plains, lasting till the middle of the summer; a cloudy summer sky, which weakens the action of the solar rays; and, finally, a very clear winter sky, favoring the radiation of heat.*
[footnote] *Humboldt, "Recherches sur les Causes des Inflexions des Lignes Isothermes", in "Asie Centr.", t. iii., p. 103-114, 118, 122, 188.
The simultaneous action of these disturbing causes, whether productive of an increase or decrease of heat, determines, as the total effect, the inflection of the isothermal lines, especially with relation to the expansion and configuration of solid continental ma.s.ses, as compared with the liquid oceanic. These perturbations give rise to convex and concave summits of the isothermal curves. There are, however, different orders of disturbing causes, and each one must, therefore, be considered separately, in order that their total effect may afterward be investigated with reference to the motion (direction, local curvature) of the isothermal lines, and the actions by which they are connected together, modified, destroyed, or increased in intensity, as manifested in the contact and intersection of small oscillatory movements. Such is the method by which, I hope, it may some day be possible to connect together, by empirical and numerically expressed laws, vast series of apparently isolated facts, and to exhibit the mutual dependence which must necessarily exist among them.
The trade winds -- easterly winds blowing within the tropics -- give rise, in both temperate zones, to the west, or west-southwest p 321 sinds which prevail in those regions, and which are land winds to eastern coasts, and sea winds to western coasts, estending over a s.p.a.ce which, from the great ma.s.s and the sinking of its cooled particles, is not capable of any considerable degree of cooling, and hence it follows that the east winds of the Continent must be cooler than the west winds, where their temperature is not affected by the occurrence of oceanic currents near the sh.o.r.e.
Cook"s young companion on his second voyage of circ.u.mnavigation, the intelligent George Forster, to whom I am indebted for the lively interest which prompted me to undertake distant travels, was the first who drew attention, in a definite manner, to the climatic differences of temperature existing in the eastern and western coasts of both continents, and to the similarity of temperature of the western coast of North America in the middle lat.i.tudes, with that of Western Europe.*
[footnote] *George Forster, "Klein Schriften", th. iii., 1794, s. 87; Dove, in Schumacher"s "Jahrbuch fur", s. 289; K?mtz, "Meteorologie", bd. ii., s.