4 degrees of the
mean winter temperature of Montpellier and Florence.*
[footnote] *Humboldt, 'Sur les Lignes Isothermes', in the 'Memoires de
Physique et de Chimie de la Societe d'Arcueil', t. iii., Paris, 1817, p.
143-165; Knight, in the 'Transactions of the Horticultural Society of
London', vol. i, p. 32; Watson, 'Remarks on the Geographical Distribution of
British Plants', 1835, p. 60; Trevelyan, in Jemieson's 'Edinburgh New Phil.
Journal', No. 18, p. 154; Mahlmann in his admirable German translation of my
'Asie Centrale', th. ii., s. 60.
These observations will suffice to show the important influence exercised on
vegetation and agriculture, on the cultivation of fruit, and on the comfort
of mankind, by differences in the distribution of the same mean annual
temperature, through the different seasons of the year.
The lines which I have termed 'Isochimenal' and 'isotheral' (lines of equal
winter and equal summer temperature) are by no means parallel with the
'isothermal' lines (lines of equal annual temperature). If, for instance,
in countries where myrtles grow wild, and the earth does not remain covered
with snow in the winter, the temperature of the summer and autumn is barely
sufficient to bring apples to perfect ripeness, and if, again, we observe
that the grape rarely attains the ripeness necessary to convert it into
wine, either in islands or in the vicinity of the sea, even when cultivated
on a western coast, the reason must not be sought only in the low degree of
summer heat, indicated, in littoral situations, by the thermometer when
suspended in the shade, but likewise in another cause that has not hitherto
been sufficiently considered, although it exercises an active influence on
many other phenomena (as, for instance, in the inflammation of a mixture of
chlorine and hydrogen), namely the difference between direct and diffused
light, or that which prevails when the sky is clear and when it is overcast
by mist.
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