[footnote] *In speaking of the greatest depths within the Earth reached by
human labor, we must recollect that there is a difference between the
'absolute depth' (that is to say, the depth below the Earth's surface at
that point) and the 'relative depth' (or that beneath the level of the sea).
The greatest relative depth that man has hitherto reached is probably the
bore at the new salt-works at Minden, in Prussia: in June, 1814, it was
exactly 1993 feet, the absolute depth being 2231 feet. The temperature of
the water at the bottom was 98 degrees F., which assuming the mean
temperature of the air at 49.3 degrees gives an augmentation of temperature
of 1 degree for every 54 feet. The absolute depth of the Artesian well of
Grenelle, near Paris, is only 1795 feet. According to the account of the
missionary Imbert, the fire-springs, "Ho-tsing." of the Chinese, which are
sunk to obtain [carbureted] hydrogen gas for salt-boiling, far exceed our
Artesian springs in depth. In the Chinese province of Szu-tschuan these
fire-springs are very commonly of the depth of more than 2000 feet; indeed,
at Tseu-lieu-tsing (the place of continual flow) there is a Ho-tsing which,
in the year 1812, was found to be 3197 feet deep. (Humboldt, 'Asie
Centrale', t.
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