I have already remarked how greatly the study of natural
objects and forces, and the infinite diversity of the sources they open for
our consideration, strengthen the mental activity, and call into action
every manifestation of intellectual progress. These relations require,
however, as little comment as that concatenation of causes by which
particular nations are permitted to enjoy a superiority over others in the
exercise of a material power derived from their command of a portion of
these elementary forces of nature.
If, on the one hand, it were necessary to indicate the difference existing
between the nature of our knowledge of the Earth and of that of the
celestial regions and their contents, I am no less desirous, on the other
hand, to draw attention to the limited boundaries of that portion of
spacefrom which we derive all our knowledge of the heterogeneous character
of matter. This has been somewhat inappropriately termed the Earth's crust;
it includes the strata most contiguous to the upper surface of our planet,
and which have been laid open before us by deep fissure-like valleys, or by
the labors of man, in the bores and shafts formed by miners. These labors*
do not extend beyond a vertical depth of somewhat more than 2000 feet (about
one third of a geographical mile) below the
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level of the sea, and consequently only about 1/9800th of the Earth's
radius.
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