[footnote] *See the Introduction. This increase of temperature has been
found in the Puits de Grenelle, at Paris, at 58.3 feet; in the boring at the
new salt-works at Minden, almost 53.6; at Pregny, near Geneva, according to
Auguste de la Rive and Marcet, notwithstanding that the mouth of the boring
is 1609 feet above the level of the sea, it is also 53.6 feet. This
coincidence between the results of a method first proposed by Arago in the
year 1821 ('Annuaire du Bureau des Longitudes', 1835, p. 234), for three
different mines, of the absolute depths of 1794, 2231, and 725 feet
respectively, is remarkable. The two points on the Earth, lying at a small
vertical distance from each other, whose annual mean temperatures are most
accurately known, are probably at the spot on which the Paris Observatory
stands, and the Caves de l'Observatoire beneath it; the mean temperature of
the former is 51.5??degrees, and of the latter 53.3??degrees, the difference
being 1.8??degrees for 92 feet, or 1 degree for 51.77 feet. (Poisson,
'Theorie Math. de la Chaleur', p. 415 and 462.) In the course of the last
seventeen years, from causes not yet perfectly understood, but probably not
connected with the actual temperature of the caves, the thermometer standing
there has risen very nearly 0.
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