In
tropical climates this invariable stratum is only one foot below the
surface, and this fact has been ingeniously made use of by Boussingault to
obtain a convenient, and as he believes, certain determination of the mean
temperature of the air of different places.*
[footnote] *Boussingault, 'Sur la Profondeus a laquelle se trouve la Couche
de Temperature invariable, entre les Tropiques', in the 'Annales de Chimie
et de Physique', t. liii., 1833, p. 225-247.
This mean temperature of the air at a fixed point, or at a group of
contiguous points on the surface, is to a certain degree the fundamental
element of the climate and agricultural relations of a district; but the
mean temperature of the whole surface is very different from that of the
globe itself. The questions so often agitated, whether the mean temperature
has experienced any considerable differences in the course of centuries,
whether the climate of a country has deteriorated, and whether the winters
have not become milder and the summers cooler, can only be answered by means
of the thermometer; this instrument has, however, scarcely been invented
more than two centuries and a half, and its scientific application hardly
dates back 120 years. The nature and novelty of the means interpose,
therefore, very narrow limits to our investigation regarding the temperature
p 176
of the air.
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