The difference alluded to in the text between the true temperature
of the surface of the ground and the indications of a thermometer suspended
in the shade and protected from extraneous influences, is inferred by Dove
from a consideration of the results of fifteen years' observations made at
the Chiswick Gardens. See Dove, in 'Bericht uber die Verhandl. der Berl.
Akad. der Wiss.', August, 1844, s. 285.
At Bordeaux, in the valley of the Garonne (44 degrees 50' lat.), the mean
annual winter, summer, and autumn temperatures are respectively 57 degrees,
43 degrees, 71 degrees, and 58 degrees. In the plains near the
p 325
Baltic (52 degrees 30' lat.), where a wine is produced that can scarcely be
considered potable, these numbers are as follows: 47.5 degrees, 30 degrees,
63.7 degrees, and 47.5 degrees. If it should appear strange that the great
differences indicated by the influence of climate on the production of wine
should not be more clearly manifested by our thermometers, the circumstance
will appear less singular when we remember that a thermometer standing in
the shade, and protected from the effect of direct insolation and nocturnal
radiation can not, at all seasong of the year, and during all periodic
changes of heat, indicate the true superficial temperature of the ground
exposed to the whole effect of the sun's rays.
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