Moreover, the thermic condition of the
surface of water, by which two thirds of our planet is covered, is not
favorable to such modes of explanation, when we have reference to an
immediate action and not to an effect of induction in the a?‘rial and
aqueous investment of our terrestrial globe.
In the present condition of our knowledge, it is impossible to afford a
satisfactory reply to all questions regarding the ultimate physical causes
of these phenomena. It is only with reference to that which presents itself
in the triple manifestations of the terrestrial force, as a measurable
relation of space and time, and as a stable element in the midst of change,
that science has recently made such brilliant advances by the aid of the
determination of mean numerical values. From Toronto in Upper Canada to the
Cape of Good Hope and Van Diemen's Land, from Paris to Pekin, the Earth has
been covered, since 1828, with magnetic observatories,* in which every
regular
p 191
or irregular manifestation of the terrestrial force is detected by
uninterrupted and simultaneous observations. A variation
p 192
of 1/40000th of the magnetic intensity is measured, and at certain epochs,
observations are made at intervals of 2 1/2 minutes, and continued for
twenty-four hours consecutively.
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