I have felt earthquakes in
clear air and a fresh east wind, as well as in rain and thunder storms. The
regularity of the horary changes in the declination of the magnetic needle
and in the atmospheric pressure remained undisturbed between the tropics on
the days when earthquakes occurred.*
[footnote] *I have given proof that the course of the horary variations of
the barometer is not affected before or after earthquakes, in my 'Relat.
Hist.', t. i., p. 311 and 513.
These facts agree with the observations made by Adolph Erman (in the
temperate zone, on the 8th of March, 1829) on the occasion of an earthquake
at Irkutsk, near the Lake of Baikal. During the violent earthquake of
Cumana, on the 4th of November, 1799, I found the declination and the
intensity of the magnetic force alike unchanged, but, to my surprise, the
inclination of the needle was diminished about 48 degrees.*
[footnonte] *Humboldt, 'Relat. Hist.', t. i., p. 515-517.
There was no ground to suspect an error in the calculation, and yet, in the
many other earthquakes which I have experienced on the elevated plateaux of
Quito and Lima, the inclination as well as the other elements of terrestrial
magnetism remained always unchanged. Although, in general, the processes at
work within the interior of the earth may not be announced by any
meteorological phenomena or any special appearance of the sky, it is, on the
contrary, not improbable, as we shall soon see, that in cases of violent
earthquakes some effect may be imparted to the atmosphere, in consequence of
which they can not always act in a purely dynamic manner.
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