The greatest
contrast to these varieties in the relations of the surface of the earth are
manifested in the Steppes of Northern Asia, the grassy plains (savannahs,
llanos, and pampas) of the New Continent, the heath ('Ericeta') of Europe,
and the sandy and stony deserts of Africa.
The law of the decrease of heat with the increase of elevation at different
latitudes is one of the most important subjects involved in the study of
meteorological processes, of the geography of plants, of the theory of
terrestrial refraction, and of the various hypotheses that relate to the
determination of the height of the atmosphere. In the many mountain
journeys which I have undertaken, both within and without the tropics, the
investigation of this law has always formed a special object of my
researches.*
[footnote] *Humboldt, 'Recueil d'Observations Astronomiques', t. i., p.
126-140; 'Relation Historique', t. i., p. 119, 141, 227; Biot, in
'Connaissance des Temps pour l'an' 1841, p. 90-109.
Since we have acquired a more accurate knowledge of the true relations of
the distribution of heat on the surface of the earth, that is to say, of the
inflections of isothermal and isotheral lines, and their unequal distance
apart in the different eastern and western systems of temperature in Asia,
Central Europe, and North America, we can no longer ask the general
question, what fraction of the mean annual or summer temperature corresponds
to the difference of one degree of geographical latitude, taken in the same
meridian? In each system of 'isothermal' lines of equal curvature there
reigns a
p 328
close and necessary connection between three elements, namely, the decrease
of heat in a vertical direction from below upward, the difference of
temperature for every one degree of geographical latitude, and the
uniformity in the mean temperature of a mountain station, and the latitude
of a point situated at the level of the sea.
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