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-shelled infusoria.
As principal 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 latitude, 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 masses (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 quantitative
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
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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 latitude, 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
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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. xxxvii., s. 245-258, and s. 463-486.
If with Kämtz 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 principal 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 Kämtz on the descent of the west wind of the
upper current in high latitudes, and the general phenomena of the direction
of the wind, in his 'Vorlesungen uber µeterologie', 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 entitled
'Witterungsverhältnisse 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
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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 shore 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 assemblage 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. (Lassen, '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 shores of the Malayan
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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 latitudes 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
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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 homogeneous fluid
mass, 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
latitudes. 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 latitude, 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 latitude, 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 shores 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 shores 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 latitude. 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 latitude, we compair Nain, on the coast of
Labrador, with Gottenburg; Halifax with Bordeaus; New
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York with Naples; St. Augustine, in Florida, with Cairo, we find that, under
the same degrees of latitude, 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 latitude 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 shores) is colder than Europe? and how much the mean
annual temperature of Canada and the United States is lower than that of
corresponding latitudes in Europe? are, when thus 'generally expressed',
devoid of meaning. There is a separate difference for each parallel of
latitude, 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 class 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 mass 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 shore 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
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vicinity of an eastern coast in high and middle latitudes; 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 masses, 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
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sinds which prevail in those regions, and which are land winds to eastern
coasts, and sea winds to western coasts, estending over a space which, from
the great mass 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 shore.
Cook's young companion on his second voyage of circumnavigation, 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 latitudes, with that of Western Europe.*
[footnote] *George Forster, 'Klein Schriften', th. iii., 1794, s. 87; Dove,
in Schumacher's 'Jahrbuch fur', s. 289; Kämtz, 'Meteorologie', bd. ii., s.
41, 43, 67, and 96; Arago, in the 'Comptes Rendus', t. i., p. 268.
Even in northern latitudes exact observations show a striking difference
between the 'mean annual temperature' of the east and west coasts of
America. The mean annual temperature of Nain, in (lat. 57 degrees 10'),
is fully 6.8 degrees 'below' the freezing point, while on the northwest
coast, at New Archangel, in Russian America (lat. 57 degrees 3'), it is 12.4
degrees 'above' this point. At the first-named place, the mean summer
temperature hardly amounts to 43 degrees, while at the latter place it is 57
degrees. Pekin (39 degrees 54'), on the eastern coast of Asia, has a mean
annual tempeerature of 52.8 degrees, which is 9 degrees below that of
Naples, situated somewhat further to the north. The mean winter temperature
of Pekin is at least 5.4 degrees below the freezing point, while in Western
Europe, even at Paris (48 degrees 50'), it is nearly 6 degrees above the
freezing point. Pekin has also a mean winter cold which is 4.5 degrees
lower than that of Copenhagen, lying 17 degrees further to the north.